diff --git a/src/bin_attacker/attacker_minimal.ml b/src/bin_attacker/attacker_minimal.ml
index cdca9ba6a..18b8ba20c 100644
--- a/src/bin_attacker/attacker_minimal.ml
+++ b/src/bin_attacker/attacker_minimal.ml
@@ -123,7 +123,7 @@ let try_action addr port action =
   | Ok conn ->
       action conn >>=? fun () ->
       P2p_connection.close conn >>= fun () ->
-      return ()
+      return_unit
 
 let replicate n x =
   let rec replicate_acc acc n x =
@@ -226,7 +226,7 @@ let long_chain n conn =
   let prev_ref = ref genesis_block_hashed in
   let rec loop k =
     if k < 1 then
-      return ()
+      return_unit
     else
       let block = signed (block_forged ~prev:!prev_ref []) in
       prev_ref := Block_hash.hash_bytes [block] ;
@@ -238,7 +238,7 @@ let lots_transactions amount fee n conn =
   let signed_op = signed (tx_forged amount fee) in
   let rec loop k =
     if k < 1 then
-      return ()
+      return_unit
     else
       send conn (Operation signed_op) >>=? fun () ->
       loop (k-1) in
diff --git a/src/bin_client/client_protocols_commands.ml b/src/bin_client/client_protocols_commands.ml
index fb35f84ee..d84d81f82 100644
--- a/src/bin_client/client_protocols_commands.ml
+++ b/src/bin_client/client_protocols_commands.ml
@@ -27,7 +27,7 @@ let commands () =
       (fun () (cctxt : #Client_context.full) ->
          Shell_services.Protocol.list cctxt >>=? fun protos ->
          Lwt_list.iter_s (fun ph -> cctxt#message "%a" Protocol_hash.pp ph) protos >>= fun () ->
-         return ()
+         return_unit
       );
 
     command ~group ~desc: "Inject a new protocol into the node."
@@ -42,15 +42,15 @@ let commands () =
               Shell_services.Injection.protocol cctxt proto >>= function
               | Ok hash ->
                   cctxt#message "Injected protocol %a successfully" Protocol_hash.pp_short hash >>= fun () ->
-                  return ()
+                  return_unit
               | Error err ->
                   cctxt#error "Error while injecting protocol from %s: %a"
                     dirname Error_monad.pp_print_error err >>= fun () ->
-                  return ())
+                  return_unit)
            (fun exn ->
               cctxt#error "Error while injecting protocol from %s: %a"
                 dirname Error_monad.pp_print_error [Error_monad.Exn exn] >>= fun () ->
-              return ())
+              return_unit)
       );
 
     command ~group ~desc: "Dump a protocol from the node's record of protocol."
@@ -62,6 +62,6 @@ let commands () =
          Shell_services.Protocol.contents cctxt ph >>=? fun proto ->
          Lwt_utils_unix.Protocol.write_dir (Protocol_hash.to_short_b58check ph) ~hash:ph proto >>=? fun () ->
          cctxt#message "Extracted protocol %a" Protocol_hash.pp_short ph >>= fun () ->
-         return ()
+         return_unit
       ) ;
   ]
diff --git a/src/bin_client/client_rpc_commands.ml b/src/bin_client/client_rpc_commands.ml
index 4987ef6ae..05fad70ac 100644
--- a/src/bin_client/client_rpc_commands.ml
+++ b/src/bin_client/client_rpc_commands.ml
@@ -106,7 +106,7 @@ let fill_in ?(show_optionals=true) input schema =
   element [] (Json_schema.root schema)
 
 let random_fill_in ?(show_optionals=true) schema =
-  let display _ = Lwt.return () in
+  let display _ = Lwt.return_unit in
   let int min max _ _ =
     let max = Int64.of_int max
     and min = Int64.of_int min in
@@ -286,8 +286,8 @@ let list url (cctxt : #Client_context.full) =
   if !collected_args <> [] then begin
     cctxt#message "@,@[<v 2>Dynamic parameter description:@ @ %a@]@."
       (Format.pp_print_list display_arg) !collected_args >>= fun () ->
-    return ()
-  end else return ()
+    return_unit
+  end else return_unit
 
 
 let schema meth url (cctxt : #Client_context.full) =
@@ -299,21 +299,21 @@ let schema meth url (cctxt : #Client_context.full) =
       | exception Not_found ->
           cctxt#message
             "No service found at this URL (but this is a valid prefix)\n%!" >>= fun () ->
-          return ()
+          return_unit
       | { input = Some input ; output } ->
           let json = `O [ "input", Json_schema.to_json (fst input) ;
                           "output", Json_schema.to_json (fst output) ] in
           cctxt#message "%a" Json_repr.(pp (module Ezjsonm)) json >>= fun () ->
-          return ()
+          return_unit
       | { input = None ; output } ->
           let json = `O [ "output", Json_schema.to_json (fst output) ] in
           cctxt#message "%a" Json_repr.(pp (module Ezjsonm)) json >>= fun () ->
-          return ()
+          return_unit
     end
   | _ ->
       cctxt#message
         "No service found at this URL (but this is a valid prefix)\n%!" >>= fun () ->
-      return ()
+      return_unit
 
 let format binary meth url (cctxt : #Client_context.io_rpcs) =
   let args = String.split '/' url in
@@ -329,7 +329,7 @@ let format binary meth url (cctxt : #Client_context.io_rpcs) =
       | exception Not_found ->
           cctxt#message
             "No service found at this URL (but this is a valid prefix)\n%!" >>= fun () ->
-          return ()
+          return_unit
       | { input = Some input ; output } ->
           cctxt#message
             "@[<v 0>\
@@ -338,19 +338,19 @@ let format binary meth url (cctxt : #Client_context.io_rpcs) =
              @]"
             pp input
             pp output >>= fun () ->
-          return ()
+          return_unit
       | { input = None ; output } ->
           cctxt#message
             "@[<v 0>\
              @[<v 2>Output format:@,%a@]@,\
              @]"
             pp output >>= fun () ->
-          return ()
+          return_unit
     end
   | _ ->
       cctxt#message
         "No service found at this URL (but this is a valid prefix)\n%!" >>= fun () ->
-      return ()
+      return_unit
 
 let fill_in ?(show_optionals=true) schema =
   let open Json_schema in
@@ -363,13 +363,13 @@ let display_answer (cctxt : #Client_context.full) = function
   | `Ok json ->
       cctxt#message "%a"
         Json_repr.(pp (module Ezjsonm)) json >>= fun () ->
-      return ()
+      return_unit
   | `Not_found _ ->
       cctxt#message "No service found at this URL\n%!" >>= fun () ->
-      return ()
+      return_unit
   | `Unauthorized _ | `Error _ | `Forbidden _ | `Conflict _ ->
       cctxt#message "Unexpected server answer\n%!" >>= fun () ->
-      return ()
+      return_unit
 
 let call meth raw_url (cctxt : #Client_context.full) =
   let uri = Uri.of_string raw_url in
@@ -381,7 +381,7 @@ let call meth raw_url (cctxt : #Client_context.full) =
           cctxt#message
             "No service found at this URL with this method \
              (but this is a valid prefix)\n%!" >>= fun () ->
-          return ()
+          return_unit
       | { input = None } ->
           cctxt#generic_json_call meth uri >>=?
           display_answer cctxt
@@ -389,14 +389,14 @@ let call meth raw_url (cctxt : #Client_context.full) =
           fill_in ~show_optionals:false (fst input) >>= function
           | Error msg ->
               cctxt#error "%s" msg >>= fun () ->
-              return ()
+              return_unit
           | Ok json ->
               cctxt#generic_json_call meth ~body:json uri >>=?
               display_answer cctxt
     end
   | _ ->
       cctxt#message "No service found at this URL\n%!" >>= fun () ->
-      return ()
+      return_unit
 
 let call_with_json meth raw_url json (cctxt: #Client_context.full) =
   let uri = Uri.of_string raw_url in
diff --git a/src/bin_client/main_client.ml b/src/bin_client/main_client.ml
index 8f622966e..0a19d64e3 100644
--- a/src/bin_client/main_client.ml
+++ b/src/bin_client/main_client.ml
@@ -19,7 +19,7 @@ let display_warning_banner ctxt =
       \                 Use your fundraiser keys @{<warning>AT YOUR OWN RISK@}.@,\
        All transactions happening on the Betanet @{<warning>are expected to be valid in the Mainnet@}.@,\
       \        In doubt, we recommend that you wait for the lunch of the Mainnet.@]@\n@." ;
-    Lwt.return () in
+    Lwt.return_unit in
   Shell_services.P2p.versions ctxt >>= function
   | Error _ -> default ()
   | Ok versions ->
@@ -34,7 +34,7 @@ let display_warning_banner ctxt =
             \               @{<warning>Tezos Zeronet DEVELOPMENT NETWORK@}.@,\
             \         Do @{<warning>NOT@} use your fundraiser keys on this network.@,\
              Zeronet is a testing network, with free tokens and frequent resets.@]@\n@." ;
-          Lwt.return ()
+          Lwt.return_unit
       | "TEZOS" :: "ALPHANET" :: _date :: [] ->
           Format.eprintf
             "@[<v 2>@{<warning>@{<title>Warning@}@}@,@,\
@@ -45,7 +45,7 @@ let display_warning_banner ctxt =
             \             @{<warning>Tezos Alphanet DEVELOPMENT NETWORK.@}@,\
             \        Do @{<warning>NOT@} use your fundraiser keys on this network.@,\
             \        Alphanet is a testing network, with free tokens.@]@\n@." ;
-          Lwt.return ()
+          Lwt.return_unit
       | "TEZOS" :: "BETANET" :: _date :: [] ->
           Format.eprintf
             "@[<v 2>@{<warning>@{<title>Warning@}@}@,@,\
@@ -58,7 +58,7 @@ let display_warning_banner ctxt =
             \            Use your fundraiser keys on this network @{<warning>AT YOUR OWN RISK@}.@,\
             \  All transactions happening on the Betanet @{<warning>are expected to be valid in the Mainnet@}.@,\
             \          If in doubt, we recommend that you wait for the Mainnet lunch.@]@\n@." ;
-          Lwt.return ()
+          Lwt.return_unit
       | "TEZOS" :: _date :: [] ->
           Format.eprintf
             "@[<v 2>@{<warning>@{<title>Warning@}@}@,@,\
@@ -69,7 +69,7 @@ let display_warning_banner ctxt =
             \                  @{<warning>Tezos TEST SANDBOX@}.@,\
             \    Do @{<warning>NOT@} use your fundraiser keys on this network.@,\
              You should not see this message if you are not a developer.@]@\n@." ;
-          Lwt.return ()
+          Lwt.return_unit
       | _ -> default ()
 
 let get_commands_for_version ctxt block protocol =
diff --git a/src/bin_client/test/demo/main.ml b/src/bin_client/test/demo/main.ml
index c4ef9fb5a..9ab66cbc9 100644
--- a/src/bin_client/test/demo/main.ml
+++ b/src/bin_client/test/demo/main.ml
@@ -83,7 +83,7 @@ let precheck_block
     ~ancestor_timestamp:_
     (raw_block: block_header) =
   Fitness.to_int64 raw_block.shell.fitness >>=? fun _ ->
-  return ()
+  return_unit
 
 let begin_application
     ~predecessor_context:context
diff --git a/src/bin_node/node_config_command.ml b/src/bin_node/node_config_command.ml
index 2dc87f128..289f18765 100644
--- a/src/bin_node/node_config_command.ml
+++ b/src/bin_node/node_config_command.ml
@@ -19,7 +19,7 @@ let show (args : Node_shared_arg.t) =
   Node_shared_arg.read_and_patch_config_file args >>=? fun cfg ->
   Node_config_file.check cfg >>= fun () ->
   print_endline @@ Node_config_file.to_string cfg ;
-  return ()
+  return_unit
 
 let reset (args : Node_shared_arg.t) =
   if Sys.file_exists args.config_file then
diff --git a/src/bin_node/node_data_version.ml b/src/bin_node/node_data_version.ml
index 078cee603..f0bcfa4e3 100644
--- a/src/bin_node/node_data_version.ml
+++ b/src/bin_node/node_data_version.ml
@@ -100,7 +100,7 @@ let check_data_dir_version data_dir =
   fail_unless
     (String.equal data_version version)
     (Invalid_data_dir_version (data_version, version)) >>=? fun () ->
-  return ()
+  return_unit
 
 let ensure_data_dir data_dir =
   let write_version () =
diff --git a/src/bin_node/node_identity_command.ml b/src/bin_node/node_identity_command.ml
index d9034cd8f..a38b4923f 100644
--- a/src/bin_node/node_identity_command.ml
+++ b/src/bin_node/node_identity_command.ml
@@ -16,7 +16,7 @@ let identity_file data_dir = data_dir // Node_data_version.default_identity_file
 let show { Node_config_file.data_dir } =
   Node_identity_file.read (identity_file data_dir) >>=? fun id ->
   Format.printf "Peer_id: %a.@." P2p_peer.Id.pp id.peer_id ;
-  return ()
+  return_unit
 
 let generate { Node_config_file.data_dir ; p2p } =
   let identity_file = identity_file data_dir in
@@ -31,7 +31,7 @@ let generate { Node_config_file.data_dir ; p2p } =
     Format.eprintf
       "Stored the new identity (%a) into '%s'.@."
       P2p_peer.Id.pp id.peer_id identity_file ;
-    return ()
+    return_unit
 
 let check { Node_config_file.data_dir ; p2p = { expected_pow } } =
   Node_identity_file.read
@@ -39,7 +39,7 @@ let check { Node_config_file.data_dir ; p2p = { expected_pow } } =
   Format.printf
     "Peer_id: %a. Proof of work is higher than %.2f.@."
     P2p_peer.Id.pp id.peer_id expected_pow ;
-  return ()
+  return_unit
 
 (** Main *)
 
diff --git a/src/bin_node/node_run_command.ml b/src/bin_node/node_run_command.ml
index 9cff5bae0..59df1198c 100644
--- a/src/bin_node/node_run_command.ml
+++ b/src/bin_node/node_run_command.ml
@@ -261,7 +261,7 @@ let run ?verbosity ?sandbox ?checkpoint (config : Node_config_file.t) =
   Lwt_utils.may ~f:RPC_server.shutdown rpc >>= fun () ->
   lwt_log_notice "BYE (%d)" x >>= fun () ->
   Logging_unix.close () >>= fun () ->
-  return ()
+  return_unit
 
 let process sandbox verbosity checkpoint args =
   let verbosity =
@@ -279,8 +279,8 @@ let process sandbox verbosity checkpoint args =
       | Some _ ->
           if config.data_dir = Node_config_file.default_data_dir
           then failwith "Cannot use default data directory while in sandbox mode"
-          else return ()
-      | None -> return ()
+          else return_unit
+      | None -> return_unit
     end >>=? fun () ->
     begin
       match checkpoint with
diff --git a/src/bin_signer/handler.ml b/src/bin_signer/handler.ml
index 246fe5e25..ede757e9c 100644
--- a/src/bin_signer/handler.ml
+++ b/src/bin_signer/handler.ml
@@ -19,12 +19,12 @@ module Authorized_key =
 
 let check_magic_byte magic_bytes data =
   match magic_bytes with
-  | None -> return ()
+  | None -> return_unit
   | Some magic_bytes ->
       let byte = MBytes.get_uint8 data 0 in
       if MBytes.length data > 1
       && (List.mem byte magic_bytes) then
-        return ()
+        return_unit
       else
         failwith "magic byte 0x%02X not allowed" byte
 
@@ -38,7 +38,7 @@ let sign
     (MBytes.get_uint8 data 0) >>= fun () ->
   check_magic_byte magic_bytes data >>=? fun () ->
   begin match require_auth, signature with
-    | false, _ -> return ()
+    | false, _ -> return_unit
     | true, None -> failwith "missing authentication signature field"
     | true, Some signature ->
         let to_sign = Signer_messages.Sign.Request.to_sign ~pkh ~data in
@@ -47,7 +47,7 @@ let sign
             (fun acc (_, key) -> acc || Signature.check key signature to_sign)
             false keys
         then
-          return ()
+          return_unit
         else
           failwith "invalid authentication signature"
   end >>=? fun () ->
diff --git a/src/bin_signer/main_signer.ml b/src/bin_signer/main_signer.ml
index f98afeb43..7adace05f 100644
--- a/src/bin_signer/main_signer.ml
+++ b/src/bin_signer/main_signer.ml
@@ -282,7 +282,7 @@ let main () =
               ~global_options:(global_options ())
               commands cctxt >>=? fun completions ->
             List.iter print_endline completions ;
-            return ()
+            return_unit
         | None ->
             Clic.dispatch commands cctxt remaining
       end
diff --git a/src/bin_signer/socket_daemon.ml b/src/bin_signer/socket_daemon.ml
index ba61a80b0..6c8ebc570 100644
--- a/src/bin_signer/socket_daemon.ml
+++ b/src/bin_signer/socket_daemon.ml
@@ -22,13 +22,13 @@ let run (cctxt : #Client_context.wallet) path ?magic_bytes ~require_auth =
           Handler.sign cctxt req ?magic_bytes ~require_auth >>= fun res ->
           Lwt_utils_unix.Socket.send fd encoding res >>= fun _ ->
           Lwt_unix.close fd >>= fun () ->
-          return ()
+          return_unit
       | Public_key pkh ->
           let encoding = result_encoding Public_key.Response.encoding in
           Handler.public_key cctxt pkh >>= fun res ->
           Lwt_utils_unix.Socket.send fd encoding res >>= fun _ ->
           Lwt_unix.close fd >>= fun () ->
-          return ()
+          return_unit
       | Authorized_keys ->
           let encoding = result_encoding Authorized_keys.Response.encoding in
           begin if require_auth then
@@ -39,7 +39,7 @@ let run (cctxt : #Client_context.wallet) path ?magic_bytes ~require_auth =
           end >>= fun res ->
           Lwt_utils_unix.Socket.send fd encoding res >>= fun _ ->
           Lwt_unix.close fd >>= fun () ->
-          return ()
+          return_unit
     end ;
     loop ()
   in
diff --git a/src/lib_clic/clic.ml b/src/lib_clic/clic.ml
index 8d32ae74f..5ef9e6307 100644
--- a/src/lib_clic/clic.ml
+++ b/src/lib_clic/clic.ml
@@ -607,7 +607,7 @@ let rec parse_args :
   type a ctx. ?command:_ command -> (a, ctx) args -> string list TzString.Map.t -> ctx -> a tzresult Lwt.t =
   fun ?command spec args_dict ctx ->
     match spec with
-    | NoArgs -> return ()
+    | NoArgs -> return_unit
     | AddArg (arg, rest) ->
         parse_arg ?command arg args_dict ctx >>=? fun arg ->
         parse_args ?command rest args_dict ctx >>|? fun rest ->
@@ -637,7 +637,7 @@ type error += Help : 'a command option -> error
 
 let check_help_flag ?command = function
   | ("-help" | "--help") :: _ -> fail (Help command)
-  | _ -> return ()
+  | _ -> return_unit
 
 let add_occurrence long value acc =
   try TzString.Map.add long (TzString.Map.find long acc) acc
@@ -1196,7 +1196,7 @@ let add_manual ~executable_name ~global_options format ppf commands =
                 let commands = List.map (fun c -> Ex c) commands in
                 usage_internal ppf ~executable_name ~global_options ~highlights:keywords commands ;
                 restore_formatter ppf state ;
-                return ()) ]) in
+                return_unit) ]) in
   Lazy.force with_manual
 
 let pp_cli_errors ppf ~executable_name ~global_options ~default errs =
diff --git a/src/lib_client_base/client_aliases.ml b/src/lib_client_base/client_aliases.ml
index 116ffc87c..5a7225154 100644
--- a/src/lib_client_base/client_aliases.ml
+++ b/src/lib_client_base/client_aliases.ml
@@ -138,12 +138,12 @@ module Alias = functor (Entity : Entity) -> struct
     load wallet >>=? fun list ->
     begin
       if force then
-        return ()
+        return_unit
       else
         iter_s (fun (n, v) ->
             if n = name && v = value then begin
               keep := true ;
-              return ()
+              return_unit
             end else if n = name && v <> value then begin
               failwith
                 "another %s is already aliased as %s, \
@@ -155,14 +155,14 @@ module Alias = functor (Entity : Entity) -> struct
                  use --force to insert duplicate"
                 Entity.name n
             end else begin
-              return ()
+              return_unit
             end)
           list
     end >>=? fun () ->
     let list = List.filter (fun (n, _) -> n <> name) list in
     let list = (name, value) :: list in
     if !keep then
-      return ()
+      return_unit
     else
       wallet#write Entity.name list wallet_encoding
 
@@ -199,7 +199,7 @@ module Alias = functor (Entity : Entity) -> struct
   let of_fresh (wallet : #wallet) force (Fresh s) =
     load wallet >>=? fun list ->
     begin if force then
-        return ()
+        return_unit
       else
         iter_s
           (fun (n, v) ->
@@ -212,7 +212,7 @@ module Alias = functor (Entity : Entity) -> struct
                  Entity.name n
                  value
              else
-               return ())
+               return_unit)
           list
     end >>=? fun () ->
     return s
diff --git a/src/lib_client_base/client_confirmations.ml b/src/lib_client_base/client_confirmations.ml
index c691c3af9..c26e24951 100644
--- a/src/lib_client_base/client_confirmations.ml
+++ b/src/lib_client_base/client_confirmations.ml
@@ -14,9 +14,9 @@ let wait_for_bootstrapped (ctxt : #Client_context.full) =
     if not !display then
       ctxt#answer "Waiting for the node to be bootstrapped before injection..." >>= fun () ->
       display := true ;
-      Lwt.return ()
+      Lwt.return_unit
     else
-      Lwt.return ()
+      Lwt.return_unit
   end ;
   Monitor_services.bootstrapped ctxt >>=? fun (stream, _stop) ->
   Lwt_stream.iter_s
@@ -26,10 +26,10 @@ let wait_for_bootstrapped (ctxt : #Client_context.full) =
            Block_hash.pp_short hash
            Time.pp_hum time
            Time.pp_hum (Time.now ())
-       else Lwt.return ()) stream >>= fun () ->
+       else Lwt.return_unit) stream >>= fun () ->
   display := true ;
   ctxt#answer "Node is bootstrapped, ready for injecting operations." >>= fun () ->
-  return ()
+  return_unit
 
 let wait_for_operation_inclusion
     (ctxt : #Client_context.full)
diff --git a/src/lib_client_base/client_keys.ml b/src/lib_client_base/client_keys.ml
index 4e5ffd684..54459dc11 100644
--- a/src/lib_client_base/client_keys.ml
+++ b/src/lib_client_base/client_keys.ml
@@ -210,7 +210,7 @@ let register_key cctxt ?(force=false) (public_key_hash, pk_uri, sk_uri) ?public_
   Public_key.add ~force cctxt name (pk_uri, public_key) >>=? fun () ->
   Secret_key.add ~force cctxt name sk_uri >>=? fun () ->
   Public_key_hash.add ~force cctxt name public_key_hash >>=? fun () ->
-  return ()
+  return_unit
 
 let raw_get_key (cctxt : #Client_context.wallet) pkh =
   begin
diff --git a/src/lib_client_base_unix/client_context_unix.ml b/src/lib_client_base_unix/client_context_unix.ml
index bc4171d68..318820f75 100644
--- a/src/lib_client_base_unix/client_context_unix.ml
+++ b/src/lib_client_base_unix/client_context_unix.ml
@@ -91,10 +91,10 @@ class unix_logger ~base_dir =
   let log channel msg = match channel with
     | "stdout" ->
         print_endline msg ;
-        Lwt.return ()
+        Lwt.return_unit
     | "stderr" ->
         prerr_endline msg ;
-        Lwt.return ()
+        Lwt.return_unit
     | log ->
         let (//) = Filename.concat in
         Lwt_utils_unix.create_dir (base_dir // "logs" // log) >>= fun () ->
diff --git a/src/lib_client_base_unix/client_main_run.ml b/src/lib_client_base_unix/client_main_run.ml
index b3cfe3f2b..edc464e6c 100644
--- a/src/lib_client_base_unix/client_main_run.ml
+++ b/src/lib_client_base_unix/client_main_run.ml
@@ -22,7 +22,7 @@ let builtin_commands =
          Lwt_list.iter_s
            (fun (ver, _) -> cctxt#message "%a" Protocol_hash.pp_short ver)
            (Client_commands.get_versions ()) >>= fun () ->
-         return ()) ;
+         return_unit) ;
   ]
 
 (* Duplicated from the node, here for now since the client still
@@ -156,7 +156,7 @@ let main select_commands =
               ~script ~cur_arg ~prev_arg ~args:original_args ~global_options
               commands client_config >>=? fun completions ->
             List.iter print_endline completions ;
-            return ()
+            return_unit
         | None ->
             Clic.dispatch commands client_config remaining
       end
diff --git a/src/lib_client_commands/client_admin_commands.ml b/src/lib_client_commands/client_admin_commands.ml
index 627a3b52a..e401ddf0b 100644
--- a/src/lib_client_commands/client_admin_commands.ml
+++ b/src/lib_client_commands/client_admin_commands.ml
@@ -24,6 +24,6 @@ let commands () =
               cctxt#message
                 "Block %a no longer marked invalid."
                 Block_hash.pp block >>= fun () ->
-              return ())
+              return_unit)
            blocks) ;
   ]
diff --git a/src/lib_client_commands/client_helpers_commands.ml b/src/lib_client_commands/client_helpers_commands.ml
index a98422656..597bd7d88 100644
--- a/src/lib_client_commands/client_helpers_commands.ml
+++ b/src/lib_client_commands/client_helpers_commands.ml
@@ -33,7 +33,7 @@ let commands () = Clic.[
          | _ :: _ :: _ when unique -> Pervasives.exit 3
          | completions ->
              List.iter print_endline completions ;
-             return ()) ;
+             return_unit) ;
     command
       ~desc: "Wait for the node to be bootstrapped."
       no_options
@@ -48,6 +48,6 @@ let commands () = Clic.[
                 Time.pp_hum time
                 Time.pp_hum (Time.now ())) stream >>= fun () ->
          cctxt#answer "Bootstrapped." >>= fun () ->
-         return ()
+         return_unit
       )
   ]
diff --git a/src/lib_client_commands/client_keys_commands.ml b/src/lib_client_commands/client_keys_commands.ml
index f936ec038..23ba16622 100644
--- a/src/lib_client_commands/client_keys_commands.ml
+++ b/src/lib_client_commands/client_keys_commands.ml
@@ -109,12 +109,12 @@ let gen_keys_containing
                 else begin if attempts mod 25_000 = 0
                   then
                     cctxt#message "Tried %d keys without finding a match" attempts
-                  else Lwt.return () end >>= fun () ->
+                  else Lwt.return_unit end >>= fun () ->
                   loop (attempts + 1) in
               loop 1 >>=? fun key_hash ->
               cctxt#message
                 "Generated '%s' under the name '%s'." key_hash name >>= fun () ->
-              return ()
+              return_unit
             end
 
 let rec input_fundraiser_params (cctxt : #Client_context.io_wallet) =
@@ -234,7 +234,7 @@ let commands () : Client_context.io_wallet Clic.command list =
          Client_keys.neuterize sk_uri >>=? fun pk_uri ->
          begin
            Public_key.find_opt cctxt name >>=? function
-           | None -> return ()
+           | None -> return_unit
            | Some (pk_uri_found, _) ->
                fail_unless (pk_uri = pk_uri_found || force)
                  (failure
@@ -260,7 +260,7 @@ let commands () : Client_context.io_wallet Clic.command list =
          Client_keys.neuterize sk_uri >>=? fun pk_uri ->
          begin
            Public_key.find_opt cctxt name >>=? function
-           | None -> return ()
+           | None -> return_unit
            | Some (pk_uri_found, _) ->
                fail_unless (pk_uri = pk_uri_found || force)
                  (failure
@@ -313,7 +313,7 @@ let commands () : Client_context.io_wallet Clic.command list =
                  cctxt#message "%s: %s (%s sk known)" name v scheme
              | Some _, _ ->
                  cctxt#message "%s: %s (pk known)" name v
-           end >>= fun () -> return ()
+           end >>= fun () -> return_unit
          end l) ;
 
     command ~group ~desc: "Show the keys associated with an implicit account."
@@ -326,24 +326,24 @@ let commands () : Client_context.io_wallet Clic.command list =
          match key_info with
          | None ->
              cctxt#message "No keys found for address" >>= fun () ->
-             return ()
+             return_unit
          | Some (pkh, pk, skloc) ->
              cctxt#message "Hash: %a"
                Signature.Public_key_hash.pp pkh >>= fun () ->
              match pk with
-             | None -> return ()
+             | None -> return_unit
              | Some pk ->
                  cctxt#message "Public Key: %a"
                    Signature.Public_key.pp pk >>= fun () ->
                  if show_private then
                    match skloc with
-                   | None -> return ()
+                   | None -> return_unit
                    | Some skloc ->
                        Secret_key.to_source skloc >>=? fun skloc ->
                        cctxt#message "Secret Key: %s" skloc >>= fun () ->
-                       return ()
+                       return_unit
                  else
-                   return ()) ;
+                   return_unit) ;
 
     command ~group ~desc: "Forget one address."
       (args1 (Clic.switch
diff --git a/src/lib_client_commands/client_p2p_commands.ml b/src/lib_client_commands/client_p2p_commands.ml
index be11879be..2493aa7bb 100644
--- a/src/lib_client_commands/client_p2p_commands.ml
+++ b/src/lib_client_commands/client_p2p_commands.ml
@@ -84,7 +84,7 @@ let commands () =
                   P2p_point.Id.pp p
                   (if pi.trusted then "★" else " ")
       end points >>= fun () ->
-      return ()
+      return_unit
     end ;
 
     command ~group ~desc: "Connect to a new point."
@@ -133,7 +133,7 @@ let commands () =
          cctxt#message
            "The given ip address is %s"
            (if banned then "banned" else "not banned") >>= fun () ->
-         return ()
+         return_unit
       ) ;
 
     command ~group ~desc: "Remove a peer ID from the blacklist and whitelist."
@@ -173,7 +173,7 @@ let commands () =
          cctxt#message
            "The given peer ID is %s"
            (if banned then "banned" else "not banned") >>= fun () ->
-         return ()
+         return_unit
       ) ;
 
     command ~group ~desc: "Clear all ACLs."
diff --git a/src/lib_client_commands/client_report_commands.ml b/src/lib_client_commands/client_report_commands.ml
index eb0b1cc13..4dc137916 100644
--- a/src/lib_client_commands/client_report_commands.ml
+++ b/src/lib_client_commands/client_report_commands.ml
@@ -49,7 +49,7 @@ let commands () =
          Format.fprintf ppf "@[<v>%a@]@."
            (Format.pp_print_list Block_hash.pp)
            (List.concat heads) ;
-         return ()) ;
+         return_unit) ;
     command ~group ~desc: "The blocks that have been marked invalid by the node."
       (args1 output_arg)
       (fixed [ "list" ; "rejected" ; "blocks" ])
@@ -57,10 +57,10 @@ let commands () =
          Shell_services.Invalid_blocks.list cctxt () >>=? function
          | [] ->
              Format.fprintf ppf "No invalid blocks." ;
-             return ()
+             return_unit
          | _ :: _ as invalid ->
              Format.fprintf ppf "@[<v>%a@]@."
                (Format.pp_print_list print_invalid_blocks)
                invalid ;
-             return ()) ;
+             return_unit) ;
   ]
diff --git a/src/lib_error_monad/error_monad.ml b/src/lib_error_monad/error_monad.ml
index c43ada003..97b7743b5 100644
--- a/src/lib_error_monad/error_monad.ml
+++ b/src/lib_error_monad/error_monad.ml
@@ -317,6 +317,8 @@ module Make(Prefix : sig val id : string end) = struct
 
   let return v = Lwt.return (Ok v)
 
+  let return_unit = Lwt.return (Ok ())
+
   let error s = Error [ s ]
 
   let ok v = Ok v
@@ -462,14 +464,14 @@ module Make(Prefix : sig val id : string end) = struct
 
   let rec iter_s f l =
     match l with
-    | [] -> return ()
+    | [] -> return_unit
     | h :: t ->
         f h >>=? fun () ->
         iter_s f t
 
   let rec iter_p f l =
     match l with
-    | [] -> return ()
+    | [] -> return_unit
     | x :: l ->
         let tx = f x and tl = iter_p f l in
         tx >>= fun tx_res ->
@@ -482,7 +484,7 @@ module Make(Prefix : sig val id : string end) = struct
 
   let rec iter2_p f l1 l2 =
     match l1, l2 with
-    | [], [] -> return ()
+    | [], [] -> return_unit
     | [], _ | _, [] -> invalid_arg "Error_monad.iter2_p"
     | x1 :: l1 , x2 :: l2 ->
         let tx = f x1 x2 and tl = iter2_p f l1 l2 in
@@ -497,7 +499,7 @@ module Make(Prefix : sig val id : string end) = struct
   let iteri2_p f l1 l2 =
     let rec iteri2_p i f l1 l2 =
       match l1, l2 with
-      | [], [] -> return ()
+      | [], [] -> return_unit
       | [], _ | _, [] -> invalid_arg "Error_monad.iteri2_p"
       | x1 :: l1 , x2 :: l2 ->
           let tx = f i x1 x2 and tl = iteri2_p (i+1) f l1 l2 in
@@ -526,7 +528,7 @@ module Make(Prefix : sig val id : string end) = struct
         f h acc
 
   let rec join = function
-    | [] -> return ()
+    | [] -> return_unit
     | t :: ts ->
         t >>= function
         | Error _ as err ->
@@ -546,16 +548,16 @@ module Make(Prefix : sig val id : string end) = struct
     | ok -> Lwt.return ok
 
   let fail_unless cond exn =
-    if cond then return () else fail exn
+    if cond then return_unit else fail exn
 
   let fail_when cond exn =
-    if cond then fail exn else return ()
+    if cond then fail exn else return_unit
 
   let unless cond f =
-    if cond then return () else f ()
+    if cond then return_unit else f ()
 
   let _when cond f =
-    if cond then f () else return ()
+    if cond then f () else return_unit
 
   let pp_print_error ppf errors =
     match errors with
@@ -599,7 +601,7 @@ module Make(Prefix : sig val id : string end) = struct
 
   let _assert b loc fmt =
     if b then
-      Format.ikfprintf (fun _ -> return ()) Format.str_formatter fmt
+      Format.ikfprintf (fun _ -> return_unit) Format.str_formatter fmt
     else
       Format.kasprintf (fun msg -> fail (Assert_error (loc, msg))) fmt
 
diff --git a/src/lib_error_monad/error_monad_sig.ml b/src/lib_error_monad/error_monad_sig.ml
index bad2f0f30..d794ee708 100644
--- a/src/lib_error_monad/error_monad_sig.ml
+++ b/src/lib_error_monad/error_monad_sig.ml
@@ -80,6 +80,9 @@ module type S = sig
   (** Sucessful return *)
   val return : 'a -> 'a tzresult Lwt.t
 
+  (** Sucessful return of [()] *)
+  val return_unit : unit tzresult Lwt.t
+
   (** Erroneous result *)
   val error : error -> 'a tzresult
 
diff --git a/src/lib_p2p/p2p.ml b/src/lib_p2p/p2p.ml
index 4f3826e91..6f94ad53e 100644
--- a/src/lib_p2p/p2p.ml
+++ b/src/lib_p2p/p2p.ml
@@ -261,7 +261,7 @@ module Real = struct
         lwt_debug "message sent to %a"
           P2p_peer.Id.pp
           (P2p_pool.Connection.info conn).peer_id >>= fun () ->
-        return ()
+        return_unit
     | Error err ->
         lwt_debug "error sending message from %a: %a"
           P2p_peer.Id.pp
@@ -366,13 +366,13 @@ type ('msg, 'peer_meta, 'conn_meta) net = ('msg, 'peer_meta, 'conn_meta) t
 
 let check_limits =
   let fail_1 v orig =
-    if not (v <= 0.) then return ()
+    if not (v <= 0.) then return_unit
     else
       Error_monad.failwith "value of option %S cannot be negative or null@."
         orig
   in
   let fail_2 v orig =
-    if not (v < 0) then return ()
+    if not (v < 0) then return_unit
     else
       Error_monad.failwith "value of option %S cannot be negative@." orig
   in
@@ -397,10 +397,10 @@ let check_limits =
       "swap-linger" >>=? fun () ->
     begin
       match c.binary_chunks_size with
-      | None -> return ()
+      | None -> return_unit
       | Some size -> P2p_socket.check_binary_chunks_size size
     end >>=? fun () ->
-    return ()
+    return_unit
 
 let create ~config ~limits peer_cfg conn_cfg msg_cfg =
   check_limits limits >>=? fun () ->
@@ -586,7 +586,7 @@ let build_rpc_directory net =
       | None -> failwith "The P2P layer is disabled."
       | Some pool ->
           P2p_pool.connect ~timeout:q#timeout pool point >>=? fun _conn ->
-          return ()
+          return_unit
     end in
 
   (* Network : Connection *)
@@ -821,10 +821,10 @@ let build_rpc_directory net =
     RPC_directory.register dir P2p_services.ACL.S.clear
       begin fun () () () ->
         match net.pool with
-        | None -> return ()
+        | None -> return_unit
         | Some pool ->
             P2p_pool.acl_clear pool ;
-            return ()
+            return_unit
       end in
 
   dir
diff --git a/src/lib_p2p/p2p_io_scheduler.ml b/src/lib_p2p/p2p_io_scheduler.ml
index 995b646a2..b674a3dc5 100644
--- a/src/lib_p2p/p2p_io_scheduler.ml
+++ b/src/lib_p2p/p2p_io_scheduler.ml
@@ -139,14 +139,14 @@ module Scheduler(IO : IO) = struct
             IO.push conn.out_param msg >>= function
             | Ok ()
             | Error [ Canceled ] ->
-                return ()
+                return_unit
             | Error ([P2p_errors.Connection_closed |
                       Exn (Unix.Unix_error (EBADF, _, _) |
                            Lwt_pipe.Closed)] as err) ->
                 lwt_debug "Connection closed (push: %d, %s)"
                   conn.id IO.name >>= fun () ->
                 cancel conn err >>= fun () ->
-                return ()
+                return_unit
             | Error err ->
                 lwt_log_error
                   "@[Unexpected error in connection (push: %d, %s):@ %a@]"
@@ -187,7 +187,7 @@ module Scheduler(IO : IO) = struct
         canceler ;
         in_param ; out_param ;
         current_pop = Lwt.fail Not_found (* dummy *) ;
-        current_push = return () ;
+        current_push = return_unit ;
         counter = Moving_average.create ~init:0 ~alpha ;
         quota = 0 ; last_quota = 0 ;
       } in
@@ -447,7 +447,7 @@ let read_full conn ?pos ?len buf =
   assert (len <= maxlen - pos) ;
   let rec loop pos len =
     if len = 0 then
-      return ()
+      return_unit
     else
       read conn ~pos ~len buf >>=? fun read_len ->
       loop (pos + read_len) (len - read_len) in
diff --git a/src/lib_p2p/p2p_maintenance.ml b/src/lib_p2p/p2p_maintenance.ml
index 9162b46dc..552a9067d 100644
--- a/src/lib_p2p/p2p_maintenance.ml
+++ b/src/lib_p2p/p2p_maintenance.ml
@@ -124,7 +124,7 @@ let rec maintain st =
     (* end of maintenance when enough users have been reached *)
     Lwt_condition.broadcast st.just_maintained () ;
     lwt_debug "Maintenance step ended" >>= fun () ->
-    return ()
+    return_unit
   end
 
 and too_few_connections st n_connected =
@@ -175,7 +175,7 @@ let rec worker_loop st =
         P2p_pool.Pool_event.wait_too_few_connections pool ; (* limits *)
         P2p_pool.Pool_event.wait_too_many_connections pool
       ] >>= fun () ->
-      return ()
+      return_unit
     end >>=? fun () ->
     let n_connected = P2p_pool.active_connections pool in
     if n_connected < st.bounds.min_threshold
@@ -183,7 +183,7 @@ let rec worker_loop st =
       maintain st
     else begin
       P2p_pool.send_swap_request pool ;
-      return ()
+      return_unit
     end
   end >>= function
   | Ok () -> worker_loop st
diff --git a/src/lib_p2p/p2p_pool.ml b/src/lib_p2p/p2p_pool.ml
index 0fa0c6c21..508f9b206 100644
--- a/src/lib_p2p/p2p_pool.ml
+++ b/src/lib_p2p/p2p_pool.ml
@@ -647,13 +647,13 @@ let config { config } = config
 
 let fail_unless_disconnected_point point_info =
   match P2p_point_state.get point_info with
-  | Disconnected -> return ()
+  | Disconnected -> return_unit
   | Requested _ | Accepted _ -> fail P2p_errors.Pending_connection
   | Running _ -> fail P2p_errors.Connected
 
 let fail_unless_disconnected_peer_id peer_info =
   match P2p_peer_state.get peer_info with
-  | Disconnected -> return ()
+  | Disconnected -> return_unit
   | Accepted _ -> fail P2p_errors.Pending_connection
   | Running _ -> fail P2p_errors.Connected
 
@@ -702,7 +702,7 @@ let rec connect ?timeout pool point =
     protect ~canceler begin fun () ->
       log pool (Outgoing_connection point) ;
       Lwt_unix.connect fd uaddr >>= fun () ->
-      return ()
+      return_unit
     end ~on_error: begin fun err ->
       lwt_debug "connect: %a -> disconnect" P2p_point.Id.pp point >>= fun () ->
       P2p_point_state.set_disconnected point_info ;
diff --git a/src/lib_p2p/p2p_socket.ml b/src/lib_p2p/p2p_socket.ml
index ace77b3ac..48b334771 100644
--- a/src/lib_p2p/p2p_socket.ml
+++ b/src/lib_p2p/p2p_socket.ml
@@ -403,7 +403,7 @@ module Writer = struct
 
   let send_message st buf =
     let rec loop = function
-      | [] -> return ()
+      | [] -> return_unit
       | buf :: l ->
           protect ~canceler:st.canceler begin fun () ->
             Crypto.write_chunk st.conn.fd st.conn.cryptobox_data buf
diff --git a/src/lib_p2p/test/test_p2p_banned_peers.ml b/src/lib_p2p/test/test_p2p_banned_peers.ml
index fda991532..db8c72451 100644
--- a/src/lib_p2p/test/test_p2p_banned_peers.ml
+++ b/src/lib_p2p/test/test_p2p_banned_peers.ml
@@ -25,7 +25,7 @@ let test_empty _ =
   List.iter (fun (_peer,addr) ->
       assert_equal_bool ~msg:__LOC__ false (P2p_acl.banned_addr empty addr)
     ) peers ;
-  Lwt.return ()
+  Lwt.return_unit
 ;;
 
 let test_ban _ =
@@ -34,7 +34,7 @@ let test_ban _ =
   List.iter (fun (_,addr) ->
       assert_equal_bool ~msg:__LOC__ true (P2p_acl.banned_addr set addr)
     ) peers ;
-  Lwt.return ()
+  Lwt.return_unit
 ;;
 
 let test_gc _ =
@@ -48,7 +48,7 @@ let test_gc _ =
   List.iter (fun (_peer,addr) ->
       assert_equal_bool ~msg:__LOC__ false (P2p_acl.banned_addr set addr)
     ) peers ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let () =
   let wrap (n, f) =
diff --git a/src/lib_p2p/test/test_p2p_io_scheduler.ml b/src/lib_p2p/test/test_p2p_io_scheduler.ml
index 03aa9f4c7..73c45854e 100644
--- a/src/lib_p2p/test/test_p2p_io_scheduler.ml
+++ b/src/lib_p2p/test/test_p2p_io_scheduler.ml
@@ -72,7 +72,7 @@ let receive conn =
     P2p_io_scheduler.read conn buf >>= function
     | Ok _ -> loop ()
     | Error [P2p_errors.Connection_closed] ->
-        Lwt.return ()
+        Lwt.return_unit
     | Error err -> Lwt.fail (Error err)
   in
   loop ()
@@ -100,7 +100,7 @@ let server
   Lwt.join (List.map receive conns) >>= fun () ->
   iter_p P2p_io_scheduler.close conns >>=? fun () ->
   log_notice "OK %a" P2p_stat.pp (P2p_io_scheduler.global_stat sched) ;
-  return ()
+  return_unit
 
 let max_size ?max_upload_speed () =
   match max_upload_speed with
@@ -131,7 +131,7 @@ let client ?max_upload_speed ?write_queue_size addr port time _n =
   P2p_io_scheduler.close conn >>=? fun () ->
   let stat = P2p_io_scheduler.stat conn in
   lwt_log_notice "Client OK %a" P2p_stat.pp stat >>= fun () ->
-  return ()
+  return_unit
 
 let run
     ?display_client_stat
diff --git a/src/lib_p2p/test/test_p2p_pool.ml b/src/lib_p2p/test/test_p2p_pool.ml
index b7946a248..ebb7c52a3 100644
--- a/src/lib_p2p/test/test_p2p_pool.ml
+++ b/src/lib_p2p/test/test_p2p_pool.ml
@@ -43,7 +43,7 @@ let conn_meta_config : metadata P2p_socket.metadata_config = {
 let sync ch =
   Process.Channel.push ch () >>=? fun () ->
   Process.Channel.pop ch >>=? fun () ->
-  return ()
+  return_unit
 
 let rec sync_nodes nodes =
   iter_p
@@ -57,7 +57,7 @@ let rec sync_nodes nodes =
 let sync_nodes nodes =
   sync_nodes nodes >>= function
   | Ok () | Error (Exn End_of_file :: _) ->
-      return ()
+      return_unit
   | Error _ as err ->
       Lwt.return err
 
@@ -104,7 +104,7 @@ let detach_node f points n =
       P2p_pool.destroy pool >>= fun () ->
       P2p_io_scheduler.shutdown sched >>= fun () ->
       lwt_log_info "Bye." >>= fun () ->
-      return ()
+      return_unit
     end
 
 let detach_nodes run_node points =
@@ -165,7 +165,7 @@ module Simple = struct
     iter_p
       (fun conn ->
          trace Read @@ P2p_pool.read conn >>=? fun Ping ->
-         return ())
+         return_unit)
       conns
 
   let close_all conns =
@@ -183,7 +183,7 @@ module Simple = struct
     sync channel >>=? fun () ->
     close_all conns >>= fun () ->
     lwt_log_info "All connections successfully closed." >>= fun () ->
-    return ()
+    return_unit
 
   let run points = detach_nodes node points
 
@@ -203,12 +203,12 @@ module Random_connections = struct
       if !rem mod total = 0 then
         lwt_log_info "Remaining: %d." (!rem / total)
       else
-        Lwt.return ()
+        Lwt.return_unit
     end >>= fun () ->
     if n > 1 then
       connect_random pool total rem point (pred n)
     else
-      return ()
+      return_unit
 
   let connect_random_all pool points n =
     let total = List.length points in
@@ -219,7 +219,7 @@ module Random_connections = struct
     lwt_log_info "Begin random connections." >>= fun () ->
     connect_random_all pool points repeat >>=? fun () ->
     lwt_log_info "Random connections OK." >>= fun () ->
-    return ()
+    return_unit
 
   let run points repeat = detach_nodes (node repeat) points
 
diff --git a/src/lib_p2p/test/test_p2p_socket.ml b/src/lib_p2p/test/test_p2p_socket.ml
index 902103185..23f8db4fa 100644
--- a/src/lib_p2p/test/test_p2p_socket.ml
+++ b/src/lib_p2p/test/test_p2p_socket.ml
@@ -51,7 +51,7 @@ let rec listen ?port addr =
 let sync ch =
   Process.Channel.push ch () >>=? fun () ->
   Process.Channel.pop ch >>=? fun () ->
-  return ()
+  return_unit
 
 let rec sync_nodes nodes =
   iter_p
@@ -65,7 +65,7 @@ let rec sync_nodes nodes =
 let sync_nodes nodes =
   sync_nodes nodes >>= function
   | Ok () | Error (Exn End_of_file :: _) ->
-      return ()
+      return_unit
   | Error _ as err ->
       Lwt.return err
 
@@ -75,14 +75,14 @@ let run_nodes client server =
     let sched = P2p_io_scheduler.create ~read_buffer_size:(1 lsl 12) () in
     server channel sched main_socket >>=? fun () ->
     P2p_io_scheduler.shutdown sched >>= fun () ->
-    return ()
+    return_unit
   end >>= fun server_node ->
   Process.detach ~prefix:"client: " begin fun channel ->
     Lwt_utils_unix.safe_close main_socket >>= fun () ->
     let sched = P2p_io_scheduler.create ~read_buffer_size:(1 lsl 12) () in
     client channel sched default_addr port >>=? fun () ->
     P2p_io_scheduler.shutdown sched >>= fun () ->
-    return ()
+    return_unit
   end >>= fun client_node ->
   let nodes = [ server_node ; client_node ] in
   Lwt.ignore_result (sync_nodes nodes) ;
@@ -148,13 +148,13 @@ module Low_level = struct
     P2p_io_scheduler.read_full fd msg >>=? fun () ->
     _assert (MBytes.compare simple_msg msg = 0) __LOC__ "" >>=? fun () ->
     P2p_io_scheduler.close fd >>=? fun () ->
-    return ()
+    return_unit
 
   let server _ch sched socket =
     raw_accept sched socket >>= fun (fd, _point) ->
     P2p_io_scheduler.write fd simple_msg >>=? fun () ->
     P2p_io_scheduler.close fd >>=? fun _ ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -177,13 +177,13 @@ module Kick = struct
     _assert (P2p_peer.Id.compare info.peer_id id2.peer_id = 0)
       __LOC__ "" >>=? fun () ->
     P2p_socket.kick auth_fd >>= fun () ->
-    return ()
+    return_unit
 
   let client _ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
     P2p_socket.accept auth_fd encoding >>= fun conn ->
     _assert (is_rejected conn) __LOC__ "" >>=? fun () ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -197,12 +197,12 @@ module Kicked = struct
     accept sched socket >>=? fun (_info, auth_fd) ->
     P2p_socket.accept auth_fd encoding >>= fun conn ->
     _assert (Kick.is_rejected conn) __LOC__ "" >>=? fun () ->
-    return ()
+    return_unit
 
   let client _ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
     P2p_socket.kick auth_fd >>= fun () ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -223,7 +223,7 @@ module Simple_message = struct
     _assert (MBytes.compare simple_msg2 msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let client ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -233,7 +233,7 @@ module Simple_message = struct
     _assert (MBytes.compare simple_msg msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -255,7 +255,7 @@ module Chunked_message = struct
     _assert (MBytes.compare simple_msg2 msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let client ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -266,7 +266,7 @@ module Chunked_message = struct
     _assert (MBytes.compare simple_msg msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -287,7 +287,7 @@ module Oversized_message = struct
     _assert (MBytes.compare simple_msg2 msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let client ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -297,7 +297,7 @@ module Oversized_message = struct
     _assert (MBytes.compare simple_msg msg = 0) __LOC__ "" >>=? fun () ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -314,7 +314,7 @@ module Close_on_read = struct
     P2p_socket.accept auth_fd encoding >>=? fun conn ->
     sync ch >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let client ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -323,7 +323,7 @@ module Close_on_read = struct
     P2p_socket.read conn >>= fun err ->
     _assert (is_connection_closed err) __LOC__ "" >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -340,7 +340,7 @@ module Close_on_write = struct
     P2p_socket.accept auth_fd encoding >>=? fun conn ->
     P2p_socket.close conn >>= fun _stat ->
     sync ch >>=? fun ()->
-    return ()
+    return_unit
 
   let client ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -350,7 +350,7 @@ module Close_on_write = struct
     P2p_socket.write_sync conn simple_msg >>= fun err ->
     _assert (is_connection_closed err) __LOC__ "" >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
@@ -379,7 +379,7 @@ module Garbled_data = struct
     P2p_socket.read conn >>= fun err ->
     _assert (is_connection_closed err) __LOC__ "" >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let client _ch sched addr port =
     connect sched addr port id2 >>=? fun auth_fd ->
@@ -387,7 +387,7 @@ module Garbled_data = struct
     P2p_socket.read conn >>= fun err ->
     _assert (is_decoding_error err) __LOC__ "" >>=? fun () ->
     P2p_socket.close conn >>= fun _stat ->
-    return ()
+    return_unit
 
   let run _dir = run_nodes client server
 
diff --git a/src/lib_protocol_environment/sigs/v1/error_monad.mli b/src/lib_protocol_environment/sigs/v1/error_monad.mli
index 40f32f650..66861ab17 100644
--- a/src/lib_protocol_environment/sigs/v1/error_monad.mli
+++ b/src/lib_protocol_environment/sigs/v1/error_monad.mli
@@ -72,6 +72,9 @@ val ok : 'a -> 'a tzresult
 (** Sucessful return *)
 val return : 'a -> 'a tzresult Lwt.t
 
+(** Sucessful return of [()] *)
+val return_unit : unit tzresult Lwt.t
+
 (** Erroneous result *)
 val error : error -> 'a tzresult
 
diff --git a/src/lib_protocol_environment/test/test_mem_context.ml b/src/lib_protocol_environment/test/test_mem_context.ml
index fb28d2023..b934e10e5 100644
--- a/src/lib_protocol_environment/test/test_mem_context.ml
+++ b/src/lib_protocol_environment/test/test_mem_context.ml
@@ -55,7 +55,7 @@ let test_simple { block2 = ctxt } =
   Assert.equal_string_option (Some "Novembre") (c novembre) ;
   Context.get ctxt ["a";"c"] >>= fun juin ->
   Assert.equal_string_option ~msg:__LOC__ (Some "Juin") (c juin) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let test_continuation { block3a = ctxt } =
   Context.get ctxt ["version"] >>= fun version ->
@@ -66,7 +66,7 @@ let test_continuation { block3a = ctxt } =
   Assert.equal_string_option ~msg:__LOC__ (Some "Juin") (c juin) ;
   Context.get ctxt ["a";"d"] >>= fun mars ->
   Assert.equal_string_option ~msg:__LOC__  (Some "Mars") (c mars) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let test_fork { block3b = ctxt } =
   Context.get ctxt ["version"] >>= fun version ->
@@ -77,7 +77,7 @@ let test_fork { block3b = ctxt } =
   Assert.is_none ~msg:__LOC__ (c juin) ;
   Context.get ctxt ["a";"d"] >>= fun mars ->
   Assert.equal_string_option ~msg:__LOC__ (Some "Février") (c mars) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let test_replay { genesis = ctxt0 }  =
   Context.set ctxt0 ["version"] (MBytes.of_string "0.0") >>= fun ctxt1 ->
@@ -96,7 +96,7 @@ let test_replay { genesis = ctxt0 }  =
   Assert.equal_string_option ~msg:__LOC__ (Some "Novembre") (c novembre) ;
   Context.get ctxt4b ["a";"d"] >>= fun juillet ->
   Assert.equal_string_option ~msg:__LOC__ (Some "Juillet") (c juillet) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let fold_keys s k ~init ~f =
   let rec loop k acc =
@@ -131,7 +131,7 @@ let test_fold { genesis = ctxt } =
   Assert.equal_string_list_list ~msg:__LOC__ [["g";"h"]] l ;
   keys ctxt ["i"] >>= fun l ->
   Assert.equal_string_list_list ~msg:__LOC__ [] l ;
-  Lwt.return ()
+  Lwt.return_unit
 
 (******************************************************************************)
 
diff --git a/src/lib_shell/block_validator.ml b/src/lib_shell/block_validator.ml
index c1b74aaf2..a0c34d163 100644
--- a/src/lib_shell/block_validator.ml
+++ b/src/lib_shell/block_validator.ml
@@ -79,7 +79,7 @@ let check_header
     (invalid_block hash
        (Unexpected_number_of_validation_passes header.shell.validation_passes)
     ) >>=? fun () ->
-  return ()
+  return_unit
 
 let assert_no_duplicate_operations block live_operations operation_hashes =
   fold_left_s (fold_left_s (fun live_operations oph ->
@@ -87,7 +87,7 @@ let assert_no_duplicate_operations block live_operations operation_hashes =
         (invalid_block block @@ Replayed_operation oph) >>=? fun () ->
       return (Operation_hash.Set.add oph live_operations)))
     live_operations operation_hashes >>=? fun _ ->
-  return ()
+  return_unit
 
 let assert_operation_liveness block live_blocks operations =
   iter_s (iter_s (fun op ->
@@ -110,7 +110,7 @@ let check_liveness chain_state pred hash operations_hashes operations =
   assert_no_duplicate_operations
     hash live_operations operations_hashes >>=? fun () ->
   assert_operation_liveness hash live_blocks operations >>=? fun () ->
-  return ()
+  return_unit
 
 let may_patch_protocol
     ~level
@@ -147,7 +147,7 @@ let apply_block
               Oversized_operation
                 { operation = Operation.hash op ;
                   size ; max = Proto.max_operation_data_length })) ops >>=? fun () ->
-       return ())
+       return_unit)
     operations Proto.validation_passes >>=? fun () ->
   let operation_hashes = List.map (List.map Operation.hash) operations in
   check_liveness chain_state pred hash operation_hashes operations >>=? fun () ->
@@ -254,7 +254,7 @@ let check_chain_liveness chain_db hash (header: Block_header.t) =
       Expired_chain { chain_id = State.Chain.id chain_state ;
                       expiration = eol ;
                       timestamp = header.shell.timestamp }
-  | None | Some _ -> return ()
+  | None | Some _ -> return_unit
 
 let get_proto pred hash =
   State.Block.context pred >>= fun pred_context ->
@@ -340,13 +340,13 @@ let on_completion
     | Ok (Some _) ->
         Worker.record_event w
           (Event.Validation_success (Request.view r, st)) ;
-        Lwt.return ()
+        Lwt.return_unit
     | Ok None ->
-        Lwt.return ()
+        Lwt.return_unit
     | Error errs ->
         Worker.record_event w
           (Event.Validation_failure (Request.view r, st, errs)) ;
-        Lwt.return ()
+        Lwt.return_unit
 
 let table = Worker.create_table Queue
 
@@ -355,10 +355,10 @@ let create limits db =
     type self = t
     let on_launch = on_launch
     let on_request = on_request
-    let on_close _ = Lwt.return ()
+    let on_close _ = Lwt.return_unit
     let on_error = on_error
     let on_completion = on_completion
-    let on_no_request _ = return ()
+    let on_no_request _ = return_unit
   end in
   Worker.launch
     table
diff --git a/src/lib_shell/bootstrap_pipeline.ml b/src/lib_shell/bootstrap_pipeline.ml
index 20f05e96d..0782a38c8 100644
--- a/src/lib_shell/bootstrap_pipeline.ml
+++ b/src/lib_shell/bootstrap_pipeline.ml
@@ -53,11 +53,11 @@ let assert_acceptable_header pipeline
       Chain.mem chain_state hash >>= fun in_chain ->
       fail_unless in_chain
         (Checkpoint_error (hash, Some pipeline.peer_id)) >>=? fun () ->
-      return ()
+      return_unit
     else
-      return ()
+      return_unit
   else
-    return ()
+    return_unit
 
 let fetch_step pipeline (step : Block_locator.step)  =
   lwt_log_info "fetching step %a -> %a (%d%s) from peer %a."
@@ -103,7 +103,7 @@ let fetch_step pipeline (step : Block_locator.step)  =
       end
     end
     headers >>=? fun () ->
-  return ()
+  return_unit
 
 let headers_fetch_worker_loop pipeline =
   begin
@@ -113,7 +113,7 @@ let headers_fetch_worker_loop pipeline =
     let seed = {Block_locator.sender_id=pipeline.peer_id; receiver_id=sender_id } in
     let steps = Block_locator.to_steps seed pipeline.locator in
     iter_s (fetch_step pipeline) steps >>=? fun () ->
-    return ()
+    return_unit
   end >>= function
   | Ok () ->
       lwt_log_info "fetched all step from peer %a."
@@ -210,7 +210,7 @@ let rec validation_worker_loop pipeline =
     lwt_log_info "validated block %a from peer %a."
       Block_hash.pp_short hash
       P2p_peer.Id.pp_short pipeline.peer_id >>= fun () ->
-    return ()
+    return_unit
   end >>= function
   | Ok () -> validation_worker_loop pipeline
   | Error [Exn Lwt.Canceled | Canceled | Exn Lwt_pipe.Closed] ->
@@ -286,7 +286,7 @@ let wait_workers pipeline =
 let wait pipeline =
   wait_workers pipeline >>= fun () ->
   match pipeline.errors with
-  | [] -> return ()
+  | [] -> return_unit
   | errors -> Lwt.return_error errors
 
 let cancel pipeline =
diff --git a/src/lib_shell/chain_traversal.ml b/src/lib_shell/chain_traversal.ml
index 46a049337..a3c6e7cd8 100644
--- a/src/lib_shell/chain_traversal.ml
+++ b/src/lib_shell/chain_traversal.ml
@@ -87,7 +87,7 @@ let iter_predecessors ?max ?min_fitness ?min_date heads ~f =
         (fun b -> Time.(min_date <= Block.timestamp b)) in
   let rec loop () =
     match pop () with
-    | None -> Lwt.return ()
+    | None -> Lwt.return_unit
     | Some b ->
         check_count () ;
         f b >>= fun () ->
@@ -97,7 +97,7 @@ let iter_predecessors ?max ?min_fitness ?min_date heads ~f =
             if check_fitness p && check_date p then push p ;
             loop () in
   List.iter push heads ;
-  try loop () with Local.Exit -> Lwt.return ()
+  try loop () with Local.Exit -> Lwt.return_unit
 
 let iter_predecessors ?max ?min_fitness ?min_date heads ~f =
   match heads with
diff --git a/src/lib_shell/chain_validator.ml b/src/lib_shell/chain_validator.ml
index 114b303f5..89d865ce6 100644
--- a/src/lib_shell/chain_validator.ml
+++ b/src/lib_shell/chain_validator.ml
@@ -167,10 +167,10 @@ let may_switch_test_chain w spawn_child block =
         nv.parameters.db chain_state
         nv.parameters.limits (* TODO: different limits main/test ? *) >>= fun child ->
       nv.child <- Some child ;
-      return ()
+      return_unit
     end else begin
       (* Ignoring request... *)
-      return ()
+      return_unit
     end in
 
   let check_child genesis protocol expiration current_time =
@@ -196,7 +196,7 @@ let may_switch_test_chain w spawn_child block =
     else if not activated && not expired then
       create_child genesis protocol expiration
     else
-      return () in
+      return_unit in
 
   begin
     let block_header = State.Block.header block in
@@ -252,7 +252,7 @@ let on_request (type a) w spawn_child (req : a Request.t) : a tzresult Lwt.t =
     begin match nv.prevalidator with
       | Some prevalidator ->
           Prevalidator.flush prevalidator block_hash
-      | None -> return ()
+      | None -> return_unit
     end >>=? fun () ->
     may_switch_test_chain w spawn_child block >>= fun () ->
     Lwt_watcher.notify nv.new_head_input block ;
@@ -267,7 +267,7 @@ let on_completion (type a) w  (req : a Request.t) (update : a) request_status =
   let fitness = State.Block.fitness block in
   let request = State.Block.hash block in
   Worker.record_event w (Processed_block { request ; request_status ; update ; fitness }) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let on_close w =
   let nv = Worker.state w in
@@ -352,7 +352,7 @@ let rec create
     let on_close = on_close
     let on_error _ _ _ errs = Lwt.return (Error errs)
     let on_completion = on_completion
-    let on_no_request _ = return ()
+    let on_no_request _ = return_unit
   end in
   let parameters =
     { max_child_ttl ;
diff --git a/src/lib_shell/injection_directory.ml b/src/lib_shell/injection_directory.ml
index 1d18bbc82..28878faea 100644
--- a/src/lib_shell/injection_directory.ml
+++ b/src/lib_shell/injection_directory.ml
@@ -21,7 +21,7 @@ let inject_block validator ?force ?chain bytes operations =
   read_chain_id validator chain >>= fun chain_id ->
   Validator.validate_block
     validator ?force ?chain_id bytes operations >>=? fun (hash, block) ->
-  return (hash, (block >>=? fun _ -> return ()))
+  return (hash, (block >>=? fun _ -> return_unit))
 
 let inject_operation validator ?chain bytes =
   read_chain_id validator chain >>= fun chain_id ->
@@ -49,7 +49,7 @@ let inject_protocol state ?force:_ proto =
             failwith
               "Previously registered protocol (%a)"
               Protocol_hash.pp_short hash
-        | Some _ -> return ()
+        | Some _ -> return_unit
   in
   Lwt.return (hash, validation)
 
@@ -65,19 +65,19 @@ let build_rpc_directory validator =
   register0 Injection_services.S.block begin fun q (raw, operations) ->
     inject_block validator
       ?chain:q#chain ~force:q#force raw operations >>=? fun (hash, wait) ->
-    (if q#async then return () else wait) >>=? fun () ->
+    (if q#async then return_unit else wait) >>=? fun () ->
     return hash
   end ;
 
   register0 Injection_services.S.operation begin fun q contents ->
     inject_operation validator ?chain:q#chain contents >>= fun (hash, wait) ->
-    (if q#async then return () else wait) >>=? fun () ->
+    (if q#async then return_unit else wait) >>=? fun () ->
     return hash
   end ;
 
   register0 Injection_services.S.protocol begin fun q protocol ->
     inject_protocol state ~force:q#force protocol >>= fun (hash, wait) ->
-    (if q#async then return () else wait) >>=? fun () ->
+    (if q#async then return_unit else wait) >>=? fun () ->
     return hash
   end ;
 
diff --git a/src/lib_shell/peer_validator.ml b/src/lib_shell/peer_validator.ml
index 3e884c916..9d7d667af 100644
--- a/src/lib_shell/peer_validator.ml
+++ b/src/lib_shell/peer_validator.ml
@@ -119,7 +119,7 @@ let bootstrap_new_branch w _ancestor _head unknown_prefix =
   debug w
     "done validating new branch from peer %a."
     P2p_peer.Id.pp_short pv.peer_id ;
-  return ()
+  return_unit
 
 let validate_new_head w hash (header : Block_header.t) =
   let pv = Worker.state w in
@@ -149,7 +149,7 @@ let validate_new_head w hash (header : Block_header.t) =
     Block_hash.pp_short hash
     P2p_peer.Id.pp_short pv.peer_id ;
   set_bootstrapped pv ;
-  return ()
+  return_unit
 
 let only_if_fitness_increases w distant_header cont =
   let pv = Worker.state w in
@@ -164,7 +164,7 @@ let only_if_fitness_increases w distant_header cont =
       Block_hash.pp_short (Block_header.hash distant_header)
       P2p_peer.Id.pp_short pv.peer_id ;
     (* Don't download a branch that cannot beat the current head. *)
-    return ()
+    return_unit
   end else cont ()
 
 let assert_acceptable_head w hash (header: Block_header.t) =
@@ -190,7 +190,7 @@ let may_validate_new_head w hash (header : Block_header.t) =
       P2p_peer.Id.pp_short pv.peer_id ;
     set_bootstrapped pv ;
     pv.last_validated_head <- header ;
-    return ()
+    return_unit
   end else if invalid_block then begin
     debug w
       "ignoring known invalid block %a from peer %a"
@@ -212,7 +212,7 @@ let may_validate_new_head w hash (header : Block_header.t) =
       P2p_peer.Id.pp_short pv.peer_id ;
     Distributed_db.Request.current_branch
       pv.parameters.chain_db ~peer:pv.peer_id () ;
-    return ()
+    return_unit
   end else begin
     only_if_fitness_increases w header @@ fun () ->
     assert_acceptable_head w hash header >>=? fun () ->
@@ -242,7 +242,7 @@ let on_no_request w =
     P2p_peer.Id.pp_short pv.peer_id
     pv.parameters.limits.new_head_request_timeout ;
   Distributed_db.Request.current_head pv.parameters.chain_db ~peer:pv.peer_id () ;
-  return ()
+  return_unit
 
 let on_request (type a) w (req : a Request.t) : a tzresult Lwt.t =
   let pv = Worker.state w in
@@ -262,7 +262,7 @@ let on_request (type a) w (req : a Request.t) : a tzresult Lwt.t =
 
 let on_completion w r _ st =
   Worker.record_event w (Event.Request (Request.view r, st, None )) ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let on_error w r st errs =
   let pv = Worker.state w in
@@ -287,7 +287,7 @@ let on_error w r st errs =
       | Ok _ ->
           Distributed_db.Request.current_head
             pv.parameters.chain_db ~peer:pv.peer_id () ;
-          return ()
+          return_unit
       | Error _ ->
           (* TODO: punish *)
           debug w
@@ -306,7 +306,7 @@ let on_close w =
   let pv = Worker.state w in
   Distributed_db.disconnect pv.parameters.chain_db pv.peer_id >>= fun () ->
   pv.parameters.notify_termination () ;
-  Lwt.return ()
+  Lwt.return_unit
 
 let on_launch _ name parameters =
   let chain_state = Distributed_db.chain_state parameters.chain_db in
@@ -365,7 +365,7 @@ let create
     let on_close = on_close
     let on_error = on_error
     let on_completion = on_completion
-    let on_no_request _ = return ()
+    let on_no_request _ = return_unit
   end in
   Worker.launch table ~timeout: limits.new_head_request_timeout limits.worker_limits
     name parameters
diff --git a/src/lib_shell/prevalidator.ml b/src/lib_shell/prevalidator.ml
index 6c11d8145..d7ded3b57 100644
--- a/src/lib_shell/prevalidator.ml
+++ b/src/lib_shell/prevalidator.ml
@@ -250,10 +250,10 @@ let handle_unprocessed w pv =
                 pv.pending Operation_hash.Map.empty } ;
         pv.pending <-
           Operation_hash.Map.empty ;
-        Lwt.return ()
+        Lwt.return_unit
     | Ok validation_state ->
         match Operation_hash.Map.cardinal pv.pending with
-        | 0 -> Lwt.return ()
+        | 0 -> Lwt.return_unit
         | n -> debug w "processing %d operations" n ;
             Prevalidation.prevalidate validation_state ~sort:true
               (Operation_hash.Map.bindings pv.pending)
@@ -286,7 +286,7 @@ let handle_unprocessed w pv =
               Operation_hash.Map.empty ;
             advertise w pv
               (mempool_of_prevalidation_result validation_result) ;
-            Lwt.return ()
+            Lwt.return_unit
   end >>= fun () ->
   pv.mempool <-
     { Mempool.known_valid =
@@ -301,7 +301,7 @@ let handle_unprocessed w pv =
         Operation_hash.Set.empty } ;
   State.Current_mempool.set (Distributed_db.chain_state pv.chain_db)
     ~head:(State.Block.hash pv.predecessor) pv.mempool >>= fun () ->
-  Lwt.return ()
+  Lwt.return_unit
 
 let fetch_operation w pv ?peer oph =
   debug w
@@ -348,7 +348,7 @@ let on_inject pv op =
           return result
   end >>=? fun result ->
   if List.mem_assoc oph result.applied then
-    return ()
+    return_unit
   else
     let try_in_map map proj or_else =
       try
@@ -413,7 +413,7 @@ let on_flush w pv predecessor =
   pv.validation_state <- validation_state ;
   if not (Protocol_hash.equal old_protocol new_protocol) then
     pv.rpc_directory <- lazy (rpc_directory new_protocol) ;
-  return ()
+  return_unit
 
 let on_advertise pv =
   match pv.advertisement with
@@ -436,15 +436,15 @@ let on_request
           return (() : r)
       | Request.Notify (peer, mempool) ->
           on_notify w pv peer mempool ;
-          return ()
+          return_unit
       | Request.Inject op ->
           on_inject pv op
       | Request.Arrived (oph, op) ->
           on_operation_arrived pv oph op ;
-          return ()
+          return_unit
       | Request.Advertise ->
           on_advertise pv ;
-          return ()
+          return_unit
     end >>=? fun r ->
     handle_unprocessed w pv >>= fun () ->
     return r
@@ -498,12 +498,12 @@ let on_launch w _ (limits, chain_db) =
 let on_error w r st errs =
   Worker.record_event w (Event.Request (r, st, Some errs)) ;
   match r with
-  | Request.(View (Inject _)) -> return ()
+  | Request.(View (Inject _)) -> return_unit
   | _ -> Lwt.return (Error errs)
 
 let on_completion w r _ st =
-  Worker.record_event w (Event.Request (Request.view r, st, None )) ;
-  Lwt.return ()
+  Worker.record_event w (Event.Request (Request.view r, st, None)) ;
+  Lwt.return_unit
 
 let table = Worker.create_table Queue
 
@@ -516,7 +516,7 @@ let create limits chain_db =
     let on_close = on_close
     let on_error = on_error
     let on_completion = on_completion
-    let on_no_request _ = return ()
+    let on_no_request _ = return_unit
   end in
   Worker.launch table limits.worker_limits
     (State.Chain.id chain_state)
diff --git a/src/lib_shell/protocol_validator.ml b/src/lib_shell/protocol_validator.ml
index b374bd507..19e2b2598 100644
--- a/src/lib_shell/protocol_validator.ml
+++ b/src/lib_shell/protocol_validator.ml
@@ -47,7 +47,7 @@ let rec worker_loop bv =
         end >>=? fun _ ->
         match wakener with
         | None ->
-            return ()
+            return_unit
         | Some wakener ->
             if valid then
               match Registered_protocol.get hash with
@@ -63,7 +63,7 @@ let rec worker_loop bv =
                 (Error
                    [Invalid_protocol { hash ;
                                        error = Compilation_failed }]) ;
-            return ()
+            return_unit
   end >>= function
   | Ok () ->
       worker_loop bv
@@ -137,17 +137,17 @@ let fetch_and_compile_protocols pv ?peer ?timeout (block: State.Block.t) =
   let protocol =
     Context.get_protocol context >>= fun protocol_hash ->
     fetch_and_compile_protocol pv ?peer ?timeout protocol_hash >>=? fun _ ->
-    return ()
+    return_unit
   and test_protocol =
     Context.get_test_chain context >>= function
-    | Not_running -> return ()
+    | Not_running -> return_unit
     | Forking { protocol }
     | Running { protocol } ->
         fetch_and_compile_protocol pv ?peer ?timeout protocol  >>=? fun _ ->
-        return () in
+        return_unit in
   protocol >>=? fun () ->
   test_protocol >>=? fun () ->
-  return ()
+  return_unit
 
 let prefetch_and_compile_protocols pv ?peer ?timeout block =
   try ignore (fetch_and_compile_protocols pv ?peer ?timeout block) with _ -> ()
diff --git a/src/lib_shell/state.ml b/src/lib_shell/state.ml
index 5d3600686..498f7181c 100644
--- a/src/lib_shell/state.ml
+++ b/src/lib_shell/state.ml
@@ -489,7 +489,7 @@ module Chain = struct
       (fun id ->
          locked_read global_state data id >>=? fun chain ->
          Chain_id.Table.add data.chains id chain ;
-         return ())
+         return_unit)
       ids
 
   let read_all state =
diff --git a/src/lib_shell/test/test_locator.ml b/src/lib_shell/test/test_locator.ml
index 5683ce695..b7dbae13c 100644
--- a/src/lib_shell/test/test_locator.ml
+++ b/src/lib_shell/test/test_locator.ml
@@ -306,7 +306,7 @@ let test_locator base_dir =
          then
            Assert.fail_msg "Invalid locator %i" size)
       l_exp l_lin;
-    return ()
+    return_unit
   in
   let stop = locator_limit + 20 in
   let rec loop size =
@@ -314,7 +314,7 @@ let test_locator base_dir =
       check_locator size >>=? fun _ ->
       loop (size+5)
     )
-    else return ()
+    else return_unit
   in
   loop 1
 
diff --git a/src/lib_shell/test/test_state.ml b/src/lib_shell/test/test_state.ml
index 26920bc10..0467798bd 100644
--- a/src/lib_shell/test/test_state.ml
+++ b/src/lib_shell/test/test_state.ml
@@ -120,7 +120,7 @@ let build_valid_chain state vtbl pred names =
        attempt None)
     pred
     names >>= fun _ ->
-  Lwt.return ()
+  Lwt.return_unit
 
 let build_example_tree chain =
   let vtbl = Hashtbl.create 23 in
@@ -159,11 +159,11 @@ let wrap_state_init f base_dir =
       genesis >>=? fun (state, chain) ->
     build_example_tree chain >>= fun vblock ->
     f { state ; chain ; vblock } >>=? fun () ->
-    return ()
+    return_unit
   end
 
 let test_init (_ : state) =
-  return ()
+  return_unit
 
 
 
@@ -181,7 +181,7 @@ let test_read_block (s: state) =
           (* FIXME COMPARE read operations ??? *)
           Lwt.return_unit
     ) (vblocks s) >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -209,7 +209,7 @@ let test_path (s: state) =
   check_path "A2" "A6" ["A3"; "A4"; "A5"; "A6"] >>= fun () ->
   check_path "B2" "B6" ["B3"; "B4"; "B5"; "B6"] >>= fun () ->
   check_path "A1" "B3" ["A2"; "A3"; "B1"; "B2"; "B3"] >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -237,7 +237,7 @@ let test_ancestor s =
   check_ancestor "B1" "A3" (vblock s "A3") >>= fun () ->
   check_ancestor "A2" "B1" (vblock s "A2") >>= fun () ->
   check_ancestor "B1" "A2" (vblock s "A2") >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -269,7 +269,7 @@ let test_locator s =
   check_locator 4 "B8" ["B7";"B6";"B5";"B4"] >>= fun () ->
   check_locator 0 "A5" [] >>= fun () ->
   check_locator 100 "A5" ["A4";"A3";"A2";"A1";"Genesis"] >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -291,7 +291,7 @@ let compare s name heads l =
 let test_known_heads s =
   Chain.known_heads s.chain >>= fun heads ->
   compare s "initial" heads ["A8";"B8"] ;
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -306,7 +306,7 @@ let test_head s =
   Chain.head s.chain >>= fun head ->
   if not (Block_hash.equal (State.Block.hash head) (State.Block.hash @@ vblock s "A6")) then
     Assert.fail_msg "unexpected head" ;
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -360,7 +360,7 @@ let test_mem s =
   test_mem s "B1" >>= fun () ->
   test_mem s "B6" >>= fun () ->
   test_mem s "B8" >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
@@ -388,7 +388,7 @@ let test_new_blocks s =
   test s "A6" "A6" "A6" [] >>= fun () ->
   test s "A8" "A6" "A6" ["A7";"A8"] >>= fun () ->
   test s "A8" "B7" "A3" ["A4";"A5";"A6";"A7";"A8"] >>= fun () ->
-  return ()
+  return_unit
 
 
 (****************************************************************************)
diff --git a/src/lib_shell/worker.ml b/src/lib_shell/worker.ml
index 6102b526c..6dff472c1 100644
--- a/src/lib_shell/worker.ml
+++ b/src/lib_shell/worker.ml
@@ -265,7 +265,7 @@ module Make
          List.iter (fun (_, ring) -> Ring.clear ring) w.event_log ;
          Lwt_unix.sleep (w.limits.zombie_lifetime -. w.limits.zombie_memory) >>= fun () ->
          Hashtbl.remove w.table.zombies w.id ;
-         Lwt.return ()) ;
+         Lwt.return_unit) ;
       Lwt.return_unit in
     let rec loop () =
       begin
@@ -286,7 +286,7 @@ module Make
                 w.current_request <- None ;
                 Handlers.on_completion w
                   request res Worker_types.{ pushed ; treated ; completed } >>= fun () ->
-                return ()
+                return_unit
             | Some u ->
                 Handlers.on_request w request >>= fun res ->
                 Lwt.wakeup_later u res ;
@@ -295,7 +295,7 @@ module Make
                 w.current_request <- None ;
                 Handlers.on_completion w
                   request res Worker_types.{ pushed ; treated ; completed } >>= fun () ->
-                return ()
+                return_unit
       end >>= function
       | Ok () ->
           loop ()
diff --git a/src/lib_signer_backends/encrypted.ml b/src/lib_signer_backends/encrypted.ml
index 27e0e1c65..2c8a8620c 100644
--- a/src/lib_signer_backends/encrypted.ml
+++ b/src/lib_signer_backends/encrypted.ml
@@ -94,10 +94,10 @@ let decrypt_all (cctxt : #Client_context.io_wallet) =
   Secret_key.load cctxt >>=? fun sks ->
   iter_s begin fun (name, sk_uri) ->
     if Uri.scheme (sk_uri : sk_uri :> Uri.t) <> Some scheme then
-      return ()
+      return_unit
     else
       decrypt cctxt ~name sk_uri >>=? fun _ ->
-      return ()
+      return_unit
   end sks
 
 let rec read_passphrase (cctxt : #Client_context.io) =
diff --git a/src/lib_signer_backends/ledger.ml b/src/lib_signer_backends/ledger.ml
index dd07c9803..2276fe35f 100644
--- a/src/lib_signer_backends/ledger.ml
+++ b/src/lib_signer_backends/ledger.ml
@@ -254,7 +254,7 @@ let commands =
            | [] ->
                cctxt#message "No device found." >>= fun () ->
                cctxt#message "Make sure a Ledger Nano S is connected and in the Tezos Wallet app." >>= fun () ->
-               return ()
+               return_unit
            | ledgers ->
                iter_s begin fun { Ledger.device_info = { Hidapi.path ;
                                                          manufacturer_string ;
@@ -303,7 +303,7 @@ let commands =
                             | Ledgerwallet_tezos.Secp256r1 -> "p2")
                            Signature.Public_key_hash.pp pkh))
                    of_curve >>= fun () ->
-                 return ()
+                 return_unit
                end ledgers) ;
 
       Clic.command ~group
@@ -341,7 +341,7 @@ let commands =
                       Corresponding full public key: %a@]"
                      Signature.Public_key_hash.pp pkh
                      Signature.Public_key.pp pk >>= fun () ->
-                   return ()
+                   return_unit
         )
     ]
 
diff --git a/src/lib_stdlib/lwt_canceler.ml b/src/lib_stdlib/lwt_canceler.ml
index 0c46bcfee..74dc977a8 100644
--- a/src/lib_stdlib/lwt_canceler.ml
+++ b/src/lib_stdlib/lwt_canceler.ml
@@ -21,14 +21,14 @@ let create () =
   let cancelation = Lwt_condition.create () in
   let cancelation_complete = Lwt_condition.create () in
   { cancelation ; cancelation_complete ;
-    cancel_hook = (fun () -> Lwt.return ()) ;
+    cancel_hook = (fun () -> Lwt.return_unit) ;
     canceling = false ;
     canceled = false ;
   }
 
 let cancel st =
   if st.canceled then
-    Lwt.return ()
+    Lwt.return_unit
   else if st.canceling then
     Lwt_condition.wait st.cancelation_complete
   else begin
@@ -39,7 +39,7 @@ let cancel st =
       (fun () ->
          st.canceled <- true ;
          Lwt_condition.broadcast st.cancelation_complete () ;
-         Lwt.return ())
+         Lwt.return_unit)
   end
 
 let on_cancel st cb =
@@ -47,7 +47,7 @@ let on_cancel st cb =
   st.cancel_hook <- (fun () -> hook () >>= cb)
 
 let cancelation st =
-  if st.canceling then Lwt.return ()
+  if st.canceling then Lwt.return_unit
   else Lwt_condition.wait st.cancelation
 
 let canceled st = st.canceling
diff --git a/src/lib_stdlib/lwt_idle_waiter.ml b/src/lib_stdlib/lwt_idle_waiter.ml
index 08162fc7e..0d7938644 100644
--- a/src/lib_stdlib/lwt_idle_waiter.ml
+++ b/src/lib_stdlib/lwt_idle_waiter.ml
@@ -39,7 +39,7 @@ let rec may_run_idle_tasks w =
             w.pending_tasks <- [] ;
             List.iter (fun u -> Lwt.wakeup u ()) pending_tasks ;
             may_run_idle_tasks w ;
-            Lwt.return ())
+            Lwt.return_unit)
 
 let wrap_error f =
   Lwt.catch
@@ -73,11 +73,11 @@ let when_idle w f =
   Lwt.on_cancel t (fun () -> canceled := true) ;
   let f () =
     if !canceled then
-      Lwt.return ()
+      Lwt.return_unit
     else
       wrap_error f >>= fun res ->
       wakeup_error u res ;
-      Lwt.return () in
+      Lwt.return_unit in
   w.pending_idle <- f :: w.pending_idle ;
   may_run_idle_tasks w ;
   t
diff --git a/src/lib_stdlib/lwt_utils.ml b/src/lib_stdlib/lwt_utils.ml
index 634a6f7ba..aa6268ec1 100644
--- a/src/lib_stdlib/lwt_utils.ml
+++ b/src/lib_stdlib/lwt_utils.ml
@@ -60,10 +60,10 @@ let worker name ~run ~cancel =
     (fun () ->
        Lwt.catch run fail >>= fun () ->
        LC.signal stop ();
-       Lwt.return ()) ;
+       Lwt.return_unit) ;
   waiter >>= fun () ->
   log_info "%s worker ended" name ;
-  Lwt.return ()
+  Lwt.return_unit
 
 
 let rec chop k l =
@@ -162,6 +162,6 @@ let stable_sort cmp l =
 let sort = stable_sort
 
 let unless cond f =
-  if cond then Lwt.return () else f ()
+  if cond then Lwt.return_unit else f ()
 
 
diff --git a/src/lib_stdlib_unix/lwt_lock_file.ml b/src/lib_stdlib_unix/lwt_lock_file.ml
index df0999d39..3db0ce5c5 100644
--- a/src/lib_stdlib_unix/lwt_lock_file.ml
+++ b/src/lib_stdlib_unix/lwt_lock_file.ml
@@ -21,7 +21,7 @@ let create_inner
       Lwt_main.at_exit (fun () -> Lwt_unix.unlink fn) ;
     let pid_str = string_of_int @@ Unix.getpid () in
     Lwt_unix.write_string fd pid_str 0 (String.length pid_str) >>= fun _ ->
-    return ()
+    return_unit
   end
 
 let create = create_inner Unix.F_TLOCK
diff --git a/src/lib_stdlib_unix/lwt_utils_unix.ml b/src/lib_stdlib_unix/lwt_utils_unix.ml
index dbfe9ce50..1124d1554 100644
--- a/src/lib_stdlib_unix/lwt_utils_unix.ml
+++ b/src/lib_stdlib_unix/lwt_utils_unix.ml
@@ -75,7 +75,7 @@ let remove_dir dir =
     Lwt_stream.iter_s
       (fun file ->
          if file = "." || file = ".." then
-           Lwt.return ()
+           Lwt.return_unit
          else begin
            let file = Filename.concat dir file in
            if Sys.is_directory file
@@ -87,7 +87,7 @@ let remove_dir dir =
   if Sys.file_exists dir && Sys.is_directory dir then
     remove dir
   else
-    Lwt.return ()
+    Lwt.return_unit
 
 let rec create_dir ?(perm = 0o755) dir =
   Lwt_unix.file_exists dir >>= function
@@ -167,7 +167,7 @@ module Json = struct
       Lwt_io.with_file ~mode:Output file begin fun chan ->
         let str = Data_encoding.Json.to_string ~minify:false json in
         Lwt_io.write chan str >>= fun _ ->
-        return ()
+        return_unit
       end
     end
 
@@ -363,7 +363,7 @@ module Socket = struct
         (* we set the beginning of the buf with the length of what is next *)
         MBytes.set_int16 buf 0 encoded_message_len ;
         write_mbytes fd buf >>= fun () ->
-        return ()
+        return_unit
 
   let recv fd encoding =
     let header_buf = MBytes.create message_len_size in
@@ -383,7 +383,7 @@ module Socket = struct
 end
 
 
-let rec retry ?(log=(fun _ -> Lwt.return ())) ?(n=5) ?(sleep=1.) f =
+let rec retry ?(log=(fun _ -> Lwt.return_unit)) ?(n=5) ?(sleep=1.) f =
   f () >>= function
   | Ok r -> Lwt.return (Ok r)
   | (Error error) as x ->
diff --git a/src/lib_storage/test/test_context.ml b/src/lib_storage/test/test_context.ml
index 4a40861ec..1b8730db6 100644
--- a/src/lib_storage/test/test_context.ml
+++ b/src/lib_storage/test/test_context.ml
@@ -116,7 +116,7 @@ let test_simple { idx ; block2 } =
       Assert.equal_string_option (Some "Novembre") (c novembre) ;
       get ctxt ["a";"c"] >>= fun juin ->
       Assert.equal_string_option ~msg:__LOC__ (Some "Juin") (c juin) ;
-      Lwt.return ()
+      Lwt.return_unit
 
 let test_continuation { idx ; block3a } =
   checkout idx block3a >>= function
@@ -131,7 +131,7 @@ let test_continuation { idx ; block3a } =
       Assert.equal_string_option ~msg:__LOC__ (Some "Juin") (c juin) ;
       get ctxt ["a";"d"] >>= fun mars ->
       Assert.equal_string_option ~msg:__LOC__  (Some "Mars") (c mars) ;
-      Lwt.return ()
+      Lwt.return_unit
 
 let test_fork { idx ; block3b } =
   checkout idx block3b >>= function
@@ -146,7 +146,7 @@ let test_fork { idx ; block3b } =
       Assert.is_none ~msg:__LOC__ (c juin) ;
       get ctxt ["a";"d"] >>= fun mars ->
       Assert.equal_string_option ~msg:__LOC__ (Some "Février") (c mars) ;
-      Lwt.return ()
+      Lwt.return_unit
 
 let test_replay { idx ; genesis }  =
   checkout idx genesis >>= function
@@ -169,7 +169,7 @@ let test_replay { idx ; genesis }  =
       Assert.equal_string_option ~msg:__LOC__ (Some "Novembre") (c novembre) ;
       get ctxt4b ["a";"d"] >>= fun juillet ->
       Assert.equal_string_option ~msg:__LOC__ (Some "Juillet") (c juillet) ;
-      Lwt.return ()
+      Lwt.return_unit
 
 let fold_keys s k ~init ~f =
   let rec loop k acc =
@@ -208,7 +208,7 @@ let test_fold { idx ; genesis } =
       Assert.equal_string_list_list ~msg:__LOC__ [["g";"h"]] l ;
       keys ctxt ["i"] >>= fun l ->
       Assert.equal_string_list_list ~msg:__LOC__ [] l ;
-      Lwt.return ()
+      Lwt.return_unit
 
 (******************************************************************************)
 
diff --git a/src/proto_alpha/lib_client/client_proto_context.ml b/src/proto_alpha/lib_client/client_proto_context.ml
index 28c6d1a1e..afe6f80d1 100644
--- a/src/proto_alpha/lib_client/client_proto_context.ml
+++ b/src/proto_alpha/lib_client/client_proto_context.ml
@@ -189,7 +189,7 @@ let source_to_keys (wallet : #Proto_alpha.full) ~chain ~block source =
 let save_contract ~force cctxt alias_name contract =
   RawContractAlias.add ~force cctxt alias_name contract >>=? fun () ->
   message_added_contract cctxt alias_name >>= fun () ->
-  return ()
+  return_unit
 
 let originate_contract
     (cctxt : #Proto_alpha.full)
@@ -305,7 +305,7 @@ let inject_activate_operation
   begin
     match confirmations with
     | None ->
-        return ()
+        return_unit
     | Some _confirmations ->
         Alpha_services.Contract.balance
           cctxt (`Main, `Head 0)
@@ -315,7 +315,7 @@ let inject_activate_operation
           Ed25519.Public_key_hash.pp pkh
           Client_proto_args.tez_sym
           Tez.pp balance >>= fun () ->
-        return ()
+        return_unit
   end >>=? fun () ->
   match Apply_operation_result.pack_contents_list op result with
   | Apply_operation_result.Single_and_result
diff --git a/src/proto_alpha/lib_client/client_proto_contracts.ml b/src/proto_alpha/lib_client/client_proto_contracts.ml
index fee3345ec..a2b7fcbea 100644
--- a/src/proto_alpha/lib_client/client_proto_contracts.ml
+++ b/src/proto_alpha/lib_client/client_proto_contracts.ml
@@ -141,4 +141,4 @@ let may_check_key sourcePubKey sourcePubKeyHash =
            (Ed25519.Public_key.hash sourcePubKey) sourcePubKeyHash)
         (failure "Invalid public key in `client_proto_endorsement`")
   | None ->
-      return ()
+      return_unit
diff --git a/src/proto_alpha/lib_client/client_proto_programs.ml b/src/proto_alpha/lib_client/client_proto_programs.ml
index 54e1e6d3e..2a7cc84e4 100644
--- a/src/proto_alpha/lib_client/client_proto_programs.ml
+++ b/src/proto_alpha/lib_client/client_proto_programs.ml
@@ -33,7 +33,7 @@ let print_errors (cctxt : #Client_context.printer) errs ~show_source ~parsed =
        ~show_source
        ~parsed) errs >>= fun () ->
   cctxt#error "error running script" >>= fun () ->
-  return ()
+  return_unit
 
 let print_big_map_diff ppf = function
   | None -> ()
@@ -60,7 +60,7 @@ let print_run_result (cctxt : #Client_context.printer) ~show_source ~parsed = fu
         print_expr storage
         (Format.pp_print_list Operation_result.pp_internal_operation) operations
         print_big_map_diff maybe_diff >>= fun () ->
-      return ()
+      return_unit
   | Error errs ->
       print_errors cctxt errs ~show_source ~parsed
 
@@ -74,7 +74,7 @@ let print_trace_result (cctxt : #Client_context.printer) ~show_source ~parsed =
         (Format.pp_print_list Operation_result.pp_internal_operation) operations
         print_big_map_diff maybe_big_map_diff
         print_execution_trace trace >>= fun () ->
-      return ()
+      return_unit
   | Error errs ->
       print_errors cctxt errs ~show_source ~parsed
 
@@ -140,7 +140,7 @@ let print_typecheck_result
       "(@[<v 0>(types . %a)@ (errors . %a)@])"
       Michelson_v1_emacs.print_type_map (program, type_map)
       Michelson_v1_emacs.report_errors (program, errs) >>= fun () ->
-    return ()
+    return_unit
   else
     match res with
     | Ok (type_map, gas) ->
@@ -149,8 +149,8 @@ let print_typecheck_result
           Gas.pp gas >>= fun () ->
         if show_types then
           cctxt#message "%a" Micheline_printer.print_expr program >>= fun () ->
-          return ()
-        else return ()
+          return_unit
+        else return_unit
     | Error errs ->
         cctxt#warning "%a"
           (Michelson_v1_error_reporter.report_errors
diff --git a/src/proto_alpha/lib_client/injection.ml b/src/proto_alpha/lib_client/injection.ml
index 7c4a4351b..22972df66 100644
--- a/src/proto_alpha/lib_client/injection.ml
+++ b/src/proto_alpha/lib_client/injection.ml
@@ -321,13 +321,13 @@ let may_patch_limits
   | Some contents ->
       simulate cctxt ~chain ~block ?branch contents >>=? fun (_, _, result) ->
       begin match detect_script_failure result with
-        | Ok () -> return ()
+        | Ok () -> return_unit
         | Error _ ->
             cctxt#message
               "@[<v 2>This simulation failed:@,%a@]"
               Operation_result.pp_operation_result
               (contents, result.contents) >>= fun () ->
-            return ()
+            return_unit
       end >>=? fun () ->
       let res = pack_contents_list contents result.contents in
       patch_list res
@@ -345,7 +345,7 @@ let inject_operation
   preapply cctxt ~chain ~block
     ?branch ?src_sk contents >>=? fun (_oph, op, result) ->
   begin match detect_script_failure result with
-    | Ok () -> return ()
+    | Ok () -> return_unit
     | Error _ as res ->
         cctxt#message
           "@[<v 2>This simulation failed:@,%a@]"
diff --git a/src/proto_alpha/lib_client_commands/client_proto_context_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_context_commands.ml
index f6d82947d..00e6e459a 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_context_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_context_commands.ml
@@ -73,7 +73,7 @@ let commands () =
           then cctxt#message "%Ld" (Time.to_seconds v)
           else cctxt#message "%s" (Time.to_notation v)
         end >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc: "Lists all non empty contracts of the block."
@@ -85,7 +85,7 @@ let commands () =
         Lwt_list.iter_s
           (fun (alias, hash, kind) -> cctxt#message "%s%s%s" hash kind alias)
           contracts >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc: "Get the balance of a contract."
@@ -98,7 +98,7 @@ let commands () =
           ~chain:`Main ~block:cctxt#block
           contract >>=? fun amount ->
         cctxt#answer "%a %s" Tez.pp amount Client_proto_args.tez_sym >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc: "Get the storage of a contract."
@@ -114,7 +114,7 @@ let commands () =
             cctxt#error "This is not a smart contract."
         | Some storage ->
             cctxt#answer "%a" Michelson_v1_printer.print_expr_unwrapped storage >>= fun () ->
-            return ()
+            return_unit
       end ;
 
     command ~group ~desc: "Get the manager of a contract."
@@ -130,7 +130,7 @@ let commands () =
         Public_key_hash.to_source manager >>=? fun m ->
         cctxt#message "%s (%s)" m
           (match mn with None -> "unknown" | Some n -> "known as " ^ n) >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc: "Get the delegate of a contract."
@@ -144,13 +144,13 @@ let commands () =
           contract >>=? function
         | None ->
             cctxt#message "none" >>= fun () ->
-            return ()
+            return_unit
         | Some delegate ->
             Public_key_hash.rev_find cctxt delegate >>=? fun mn ->
             Public_key_hash.to_source delegate >>=? fun m ->
             cctxt#message "%s (%s)" m
               (match mn with None -> "unknown" | Some n -> "known as " ^ n) >>= fun () ->
-            return ()
+            return_unit
       end ;
 
     command ~group ~desc: "Set the delegate of a contract."
@@ -169,7 +169,7 @@ let commands () =
           ~chain:`Main ~block:cctxt#block ?confirmations:cctxt#confirmations
           ~dry_run
           contract (Some delegate) ~fee ~src_pk ~manager_sk >>=? fun _ ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc: "Withdraw the delegate from a contract."
@@ -185,7 +185,7 @@ let commands () =
           ~chain:`Main ~block:cctxt#block ?confirmations:cctxt#confirmations
           ~dry_run
           contract None ~fee ~src_pk ~manager_sk >>=? fun _ ->
-        return ()
+        return_unit
       end ;
 
     command ~group ~desc:"Open a new account."
@@ -215,10 +215,10 @@ let commands () =
           ~fee ?delegate ~delegatable ~manager_pkh ~balance
           ~source ~src_pk ~src_sk () >>=? fun (_res, contract) ->
         if dry_run then
-          return ()
+          return_unit
         else
           save_contract ~force cctxt alias_name contract >>=? fun () ->
-          return ()
+          return_unit
       end ;
 
     command ~group ~desc: "Launch a smart contract on the blockchain."
@@ -256,13 +256,13 @@ let commands () =
           ~fee ?gas_limit ?storage_limit ~delegate ~delegatable ~spendable ~initial_storage
           ~manager ~balance ~source ~src_pk ~src_sk ~code () >>= fun errors ->
         report_michelson_errors ~no_print_source ~msg:"origination simulation failed" cctxt errors >>= function
-        | None -> return ()
+        | None -> return_unit
         | Some (_res, contract) ->
             if dry_run then
-              return ()
+              return_unit
             else
               save_contract ~force cctxt alias_name contract >>=? fun () ->
-              return ()
+              return_unit
       end ;
 
     command ~group ~desc: "Transfer tokens / call a smart contract."
@@ -286,9 +286,9 @@ let commands () =
           ~dry_run
           ~source ~fee ~src_pk ~src_sk ~destination ~arg ~amount ?gas_limit ?storage_limit () >>=
         report_michelson_errors ~no_print_source ~msg:"transfer simulation failed" cctxt >>= function
-        | None -> return ()
+        | None -> return_unit
         | Some (_res, _contracts) ->
-            return ()
+            return_unit
       end;
 
     command ~group ~desc: "Reveal the public key of the contract manager."
@@ -304,7 +304,7 @@ let commands () =
         reveal cctxt
           ~chain:`Main ~block:cctxt#block ?confirmations:cctxt#confirmations
           ~source ~fee ~src_pk ~src_sk () >>=? fun _res ->
-        return ()
+        return_unit
       end;
 
     command ~group ~desc: "Register the public key hash as a delegate."
@@ -320,7 +320,7 @@ let commands () =
           ~chain:`Main ~block:cctxt#block ?confirmations:cctxt#confirmations
           ~dry_run
           ~fee ~manager_sk:src_sk src_pk >>=? fun _res ->
-        return ()
+        return_unit
       end;
 
     command ~group ~desc:"Register and activate an Alphanet/Zeronet faucet account."
@@ -349,7 +349,7 @@ let commands () =
              activate_account cctxt
                ~chain:`Main ~block:cctxt#block ?confirmations:cctxt#confirmations
                ~encrypted ~force key name >>=? fun _res ->
-             return ()
+             return_unit
       );
 
     command ~group ~desc:"Activate a fundraiser account."
@@ -368,7 +368,7 @@ let commands () =
            ~block:cctxt#block ?confirmations:cctxt#confirmations
            ~dry_run
            name code >>=? fun _res ->
-         return ()
+         return_unit
       );
 
     command ~desc:"Wait until an operation is included in a block"
@@ -408,7 +408,7 @@ let commands () =
           (failure "check-previous cannot be negative") >>=? fun () ->
         Client_confirmations.wait_for_operation_inclusion ctxt
           ~chain:`Main ~confirmations ~predecessors operation_hash >>=? fun _ ->
-        return ()
+        return_unit
       end ;
 
     command ~group:binary_description ~desc:"Describe unsigned block header"
@@ -419,7 +419,7 @@ let commands () =
           Data_encoding.Binary_schema.pp
           (Data_encoding.Binary.describe
              (Alpha_context.Block_header.unsigned_encoding)) >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~group:binary_description ~desc:"Describe unsigned block header"
@@ -430,7 +430,7 @@ let commands () =
           Data_encoding.Binary_schema.pp
           (Data_encoding.Binary.describe
              Alpha_context.Operation.unsigned_encoding) >>= fun () ->
-        return ()
+        return_unit
       end
 
   ]
diff --git a/src/proto_alpha/lib_client_commands/client_proto_contracts_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_contracts_commands.ml
index 42e1043cb..686c0a05e 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_contracts_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_contracts_commands.ml
@@ -64,6 +64,6 @@ let commands  () =
        @@ stop)
       (fun () (_, contract) (cctxt : Proto_alpha.full) ->
          cctxt#message "%a\n%!" Contract.pp contract >>= fun () ->
-         return ()) ;
+         return_unit) ;
 
   ]
diff --git a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
index 9e9680fa7..cf9e35c07 100644
--- a/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
+++ b/src/proto_alpha/lib_client_commands/client_proto_programs_commands.ml
@@ -91,7 +91,7 @@ let commands () =
       (fun () (cctxt : Proto_alpha.full) ->
          Program.load cctxt >>=? fun list ->
          Lwt_list.iter_s (fun (n, _) -> cctxt#message "%s" n) list >>= fun () ->
-         return ()) ;
+         return_unit) ;
 
     command ~group ~desc: "Add a script to the library."
       (args1 (Program.force_switch ()))
@@ -118,7 +118,7 @@ let commands () =
       (fun () (_, program) (cctxt : Proto_alpha.full) ->
          Program.to_source program >>=? fun source ->
          cctxt#message "%s\n" source >>= fun () ->
-         return ()) ;
+         return_unit) ;
 
     command ~group ~desc: "Ask the node to run a script."
       (args3 trace_stack_switch amount_arg no_print_source_flag)
@@ -161,7 +161,7 @@ let commands () =
              cctxt#message
                "(@[<v 0>(types . ())@ (errors . %a)@])"
                Michelson_v1_emacs.report_errors res_with_errors >>= fun () ->
-             return ()
+             return_unit
          | (parsed, errors) ->
              cctxt#message "%a"
                (fun ppf () ->
@@ -188,7 +188,7 @@ let commands () =
          | Ok gas ->
              cctxt#message "@[<v 0>Well typed@,Gas remaining: %a@]"
                Proto_alpha.Alpha_context.Gas.pp gas >>= fun () ->
-             return ()
+             return_unit
          | Error errs ->
              cctxt#warning "%a"
                (Michelson_v1_error_reporter.report_errors
@@ -230,7 +230,7 @@ let commands () =
                MBytes.pp_hex (Alpha_environment.Raw_hashes.sha256 bytes)
                MBytes.pp_hex (Alpha_environment.Raw_hashes.sha512 bytes)
                Proto_alpha.Alpha_context.Gas.pp remaining_gas >>= fun () ->
-             return ()
+             return_unit
          | Error errs ->
              cctxt#warning "%a"
                (Michelson_v1_error_reporter.report_errors
@@ -253,7 +253,7 @@ let commands () =
       (fun () bytes sk cctxt ->
          Client_keys.sign cctxt sk bytes >>=? fun signature ->
          cctxt#message "Signature: %a" Signature.pp signature >>= fun () ->
-         return ()) ;
+         return_unit) ;
 
     command ~group
       ~desc: "Check the signature of a byte sequence as per Michelson \
@@ -273,10 +273,10 @@ let commands () =
          | false -> cctxt#error "invalid signature"
          | true ->
              if quiet then
-               return ()
+               return_unit
              else
                cctxt#message "Signature check successfull." >>= fun () ->
-               return ()
+               return_unit
       ) ;
 
   ]
diff --git a/src/proto_alpha/lib_delegate/client_baking_denunciation.ml b/src/proto_alpha/lib_delegate/client_baking_denunciation.ml
index 0883e60cf..5f983789e 100644
--- a/src/proto_alpha/lib_delegate/client_baking_denunciation.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_denunciation.ml
@@ -94,9 +94,9 @@ let process_endorsements (cctxt : #Proto_alpha.full) state ~chain
       | _ ->
           lwt_log_error "Inconsistent endorsement found %a"
             Operation_hash.pp hash >>= fun () ->
-          return ()
+          return_unit
     ) endorsements >>=? fun () ->
-  return ()
+  return_unit
 
 let process_block (cctxt : #Proto_alpha.full) state ~chain (header : Alpha_block_services.block_info) =
   let { Alpha_block_services.hash ; metadata = { protocol_data = { baker ; level = { level } } } } = header in
@@ -167,7 +167,7 @@ let endorsements_index = 0
 let process_new_block (cctxt : #Proto_alpha.full) state { hash ; chain_id ; level ; protocol ; next_protocol } =
   if Protocol_hash.(protocol <> next_protocol) then
     lwt_log_error "Protocol changing detected. Skipping the block." >>= fun () ->
-    return ()
+    return_unit
   else 
     lwt_debug "Block level : %a" Raw_level.pp level >>= fun () ->
     let chain = `Hash chain_id in
@@ -182,7 +182,7 @@ let process_new_block (cctxt : #Proto_alpha.full) state { hash ; chain_id ; leve
           lwt_log_error "Error while fetching operations in block %a@\n%a"
             Block_hash.pp_short hash
             pp_print_error errs >>= fun () ->
-          return ()
+          return_unit
     end >>=? fun () ->
     (* Processing endorsements *)
     begin Alpha_block_services.Operations.operations cctxt ~chain ~block () >>= function
@@ -190,15 +190,15 @@ let process_new_block (cctxt : #Proto_alpha.full) state { hash ; chain_id ; leve
           if List.length operations > endorsements_index then
             let endorsements = List.nth operations endorsements_index in
             process_endorsements cctxt state ~chain endorsements level
-          else return ()
+          else return_unit
       | Error errs ->
           lwt_log_error "Error while fetching operations in block %a@\n%a"
             Block_hash.pp_short hash
             pp_print_error errs >>= fun () ->
-          return ()
+          return_unit
     end >>=? fun () ->
     cleanup_old_operations state ;
-    return ()
+    return_unit
 
 let create (cctxt : #Proto_alpha.full) ~preserved_levels valid_blocks_stream =
 
@@ -225,7 +225,7 @@ let create (cctxt : #Proto_alpha.full) ~preserved_levels valid_blocks_stream =
     ~cctxt
     ~stream:valid_blocks_stream
     ~state_maker
-    ~pre_loop:(fun _ _ _ -> return ())
+    ~pre_loop:(fun _ _ _ -> return_unit)
     ~compute_timeout:(fun _ -> Lwt_utils.never_ending ())
-    ~timeout_k:(fun _ _ () -> return ())
+    ~timeout_k:(fun _ _ () -> return_unit)
     ~event_k:process_block
diff --git a/src/proto_alpha/lib_delegate/client_baking_endorsement.ml b/src/proto_alpha/lib_delegate/client_baking_endorsement.ml
index 19005456e..74cf3557d 100644
--- a/src/proto_alpha/lib_delegate/client_baking_endorsement.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_endorsement.ml
@@ -39,12 +39,12 @@ let inject_endorsement
 
 let check_endorsement cctxt level pkh =
   State.get cctxt pkh >>=? function
-  | None -> return ()
+  | None -> return_unit
   | Some recorded_level ->
       if Raw_level.(level = recorded_level) then
         Error_monad.failwith "Level %a already endorsed" Raw_level.pp recorded_level
       else
-        return ()
+        return_unit
 
 let previously_endorsed_level cctxt pkh new_lvl  =
   State.get cctxt pkh >>=? function
@@ -116,7 +116,7 @@ let endorse_for_delegate cctxt block delegate =
     Raw_level.pp level
     name
     Operation_hash.pp_short oph >>= fun () ->
-  return ()
+  return_unit
 
 let allowed_to_endorse cctxt bi delegate  =
   Client_keys.Public_key_hash.name cctxt delegate >>=? fun name ->
@@ -144,7 +144,7 @@ let prepare_endorsement ~(max_past:int64) () (cctxt : #Proto_alpha.full) state b
   if Time.diff (Time.now ()) bi.Client_baking_blocks.timestamp > max_past then
     lwt_log_info "Ignore block %a: forged too far the past"
       Block_hash.pp_short bi.hash >>= fun () ->
-    return ()
+    return_unit
   else
     lwt_log_info "Received new block %a"
       Block_hash.pp_short bi.hash >>= fun () ->
@@ -158,7 +158,7 @@ let prepare_endorsement ~(max_past:int64) () (cctxt : #Proto_alpha.full) state b
         block = bi ;
         delegates ;
       } ;
-    return ()
+    return_unit
 
 let compute_timeout state =
   match state.pending with
diff --git a/src/proto_alpha/lib_delegate/client_baking_forge.ml b/src/proto_alpha/lib_delegate/client_baking_forge.ml
index 673902bfb..b66a8ed31 100644
--- a/src/proto_alpha/lib_delegate/client_baking_forge.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_forge.ml
@@ -409,7 +409,7 @@ let compute_timeout { future_slots } =
   | (timestamp, _) :: _ ->
       match Client_baking_scheduling.sleep_until timestamp with
       | None ->
-          Lwt.return ()
+          Lwt.return_unit
       | Some timeout ->
           timeout
 
@@ -470,7 +470,7 @@ let insert_block
   | [] ->
       lwt_debug
         "Can't compute slots for %a" Block_hash.pp_short bi.hash >>= fun () ->
-      return ()
+      return_unit
   | (_ :: _) as slots ->
       iter_p
         (fun ((timestamp, (_, _, delegate)) as slot) ->
@@ -480,7 +480,7 @@ let insert_block
              name
              Block_hash.pp_short bi.hash >>= fun () ->
            state.future_slots <- insert_baking_slot slot state.future_slots ;
-           return ()
+           return_unit
         )
         slots
 
@@ -657,7 +657,7 @@ let record_nonce_hash cctxt block_hash seed_nonce seed_nonce_hash =
     Client_baking_nonces.add cctxt block_hash seed_nonce
     |> trace_exn (Failure "Error while recording block")
   else
-    return ()
+    return_unit
 
 let pp_operation_list_list =
   Format.pp_print_list
@@ -724,12 +724,12 @@ let bake
             Raw_level.pp level priority
             Fitness.pp shell_header.fitness
             pp_operation_list_list operations >>= fun () ->
-          return ()
+          return_unit
     end
 
   | _ -> (* no candidates, or none fit-enough *)
       lwt_debug "No valid candidates." >>= fun () ->
-      return ()
+      return_unit
 
 
 
diff --git a/src/proto_alpha/lib_delegate/client_baking_lib.ml b/src/proto_alpha/lib_delegate/client_baking_lib.ml
index 3269ba233..faf906f48 100644
--- a/src/proto_alpha/lib_delegate/client_baking_lib.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_lib.ml
@@ -47,13 +47,13 @@ let bake_block (cctxt : #Proto_alpha.full)
   let src_pkh = Signature.Public_key.hash src_pk in
   Client_baking_forge.State.record cctxt src_pkh level.level >>=? fun () ->
   begin match seed_nonce with
-    | None -> return ()
+    | None -> return_unit
     | Some seed_nonce ->
         Client_baking_nonces.add cctxt block_hash seed_nonce
         |> trace_exn (Failure "Error while recording block")
   end >>=? fun () ->
   cctxt#message "Injected block %a" Block_hash.pp_short block_hash >>= fun () ->
-  return ()
+  return_unit
 
 let endorse_block cctxt delegate =
   Client_keys.get_key cctxt delegate >>=? fun (_src_name, src_pk, src_sk) ->
@@ -61,7 +61,7 @@ let endorse_block cctxt delegate =
     cctxt#block ~src_sk src_pk >>=? fun oph ->
   cctxt#answer "Operation successfully injected in the node." >>= fun () ->
   cctxt#answer "Operation hash is '%a'." Operation_hash.pp oph >>= fun () ->
-  return ()
+  return_unit
 
 let get_predecessor_cycle (cctxt : #Client_context.printer) cycle =
   match Cycle.pred cycle with
@@ -79,7 +79,7 @@ let do_reveal cctxt block blocks =
   Client_baking_revelation.forge_seed_nonce_revelation cctxt
     block nonces >>=? fun () ->
   Client_baking_nonces.dels cctxt (List.map fst blocks) >>=? fun () ->
-  return ()
+  return_unit
 
 let reveal_block_nonces (cctxt : #Proto_alpha.full) block_hashes =
   Lwt_list.filter_map_p
diff --git a/src/proto_alpha/lib_delegate/client_baking_revelation.ml b/src/proto_alpha/lib_delegate/client_baking_revelation.ml
index c7ab41b54..6f97aac2e 100644
--- a/src/proto_alpha/lib_delegate/client_baking_revelation.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_revelation.ml
@@ -29,7 +29,7 @@ let forge_seed_nonce_revelation
   | [] ->
       cctxt#message "No nonce to reveal for block %a"
         Block_hash.pp_short hash >>= fun () ->
-      return ()
+      return_unit
   | _ ->
       inject_seed_nonce_revelation cctxt ~chain block nonces >>=? fun oph ->
       cctxt#answer
@@ -38,4 +38,4 @@ let forge_seed_nonce_revelation
         Block_hash.pp_short hash >>= fun () ->
       cctxt#answer "@[<v 2>Operation hash are:@ %a@]"
         (Format.pp_print_list Operation_hash.pp_short) oph >>= fun () ->
-      return ()
+      return_unit
diff --git a/src/proto_alpha/lib_delegate/client_baking_simulator.ml b/src/proto_alpha/lib_delegate/client_baking_simulator.ml
index 2b36529bf..f2c4e43f1 100644
--- a/src/proto_alpha/lib_delegate/client_baking_simulator.ml
+++ b/src/proto_alpha/lib_delegate/client_baking_simulator.ml
@@ -77,4 +77,4 @@ let add_operation st ( op : Operation.packed ) =
   return { st with state ; rev_operations = op :: st.rev_operations }
 
 let finalize_construction inc =
-  Main.finalize_block inc.state >>=? fun _ -> return ()
+  Main.finalize_block inc.state >>=? fun _ -> return_unit
diff --git a/src/proto_alpha/lib_delegate/client_daemon.ml b/src/proto_alpha/lib_delegate/client_daemon.ml
index 811d55359..8e8e2fa6f 100644
--- a/src/proto_alpha/lib_delegate/client_daemon.ml
+++ b/src/proto_alpha/lib_delegate/client_daemon.ml
@@ -14,7 +14,7 @@ module Endorser = struct
       cctxt `Main >>=? fun block_stream ->
     Client_baking_endorsement.create cctxt ~delay delegates block_stream >>=? fun () ->
     ignore min_date;
-    return ()
+    return_unit
 
 end
 
@@ -26,7 +26,7 @@ module Baker = struct
     Client_baking_forge.create cctxt
       ?threshold ?max_priority ~context_path delegates block_stream >>=? fun () ->
     ignore min_date;
-    return ()
+    return_unit
 
 end
 
@@ -35,6 +35,6 @@ module Accuser = struct
   let run (cctxt : #Proto_alpha.full) ~preserved_levels =
     Client_baking_blocks.monitor_valid_blocks cctxt ~chains:[ `Main ] () >>=? fun valid_blocks_stream ->
     Client_baking_denunciation.create cctxt ~preserved_levels valid_blocks_stream >>=? fun () ->
-    return ()
+    return_unit
 
 end
diff --git a/src/proto_alpha/lib_delegate/test/proto_alpha_helpers.ml b/src/proto_alpha/lib_delegate/test/proto_alpha_helpers.ml
index a02a0609f..279e94025 100644
--- a/src/proto_alpha/lib_delegate/test/proto_alpha_helpers.ml
+++ b/src/proto_alpha/lib_delegate/test/proto_alpha_helpers.ml
@@ -36,7 +36,7 @@ let no_write_context ?(block = `Head 0) config : #Client_context.full = object
   method write : type a. string ->
     a ->
     a Data_encoding.encoding -> unit Error_monad.tzresult Lwt.t =
-    fun _ _ _ -> return ()
+    fun _ _ _ -> return_unit
   method with_lock : type a. (unit -> a Lwt.t) -> a Lwt.t = fun f -> f ()
   method block = block
   method confirmations = None
@@ -614,7 +614,7 @@ let display_level block =
   Alpha_block_services.metadata
     !rpc_ctxt ~chain:`Main ~block () >>=? fun { protocol_data = { level } } ->
   Format.eprintf "Level: %a@." Level.pp_full level ;
-  return ()
+  return_unit
 
 let endorsement_security_deposit block =
   Constants_services.all !rpc_ctxt (`Main, block) >>=? fun c ->
diff --git a/src/proto_alpha/lib_delegate/test/test_michelson_parser.ml b/src/proto_alpha/lib_delegate/test/test_michelson_parser.ml
index 48aebdfe0..ffbe96d9d 100644
--- a/src/proto_alpha/lib_delegate/test/test_michelson_parser.ml
+++ b/src/proto_alpha/lib_delegate/test/test_michelson_parser.ml
@@ -65,7 +65,7 @@ let test_known_tez_litterals () =
        let vs = Tez_repr.of_string s in
        Assert.is_none ~msg:("Unexpected successful parsing of " ^ s) vs)
     known_bad_tez_litterals ;
-  return ()
+  return_unit
 
 let test_random_tez_litterals () =
   for _ = 0 to 100_000 do
@@ -91,7 +91,7 @@ let test_random_tez_litterals () =
           Assert.equal ~prn:Int64.to_string ~msg:(Tez_repr.to_string vv) v rev
     end
   done ;
-  return ()
+  return_unit
 
 open Tezos_micheline
 open Micheline
diff --git a/src/proto_alpha/lib_delegate/test/test_rpc.ml b/src/proto_alpha/lib_delegate/test/test_rpc.ml
index 4f72ed8a1..3f91fb4eb 100644
--- a/src/proto_alpha/lib_delegate/test/test_rpc.ml
+++ b/src/proto_alpha/lib_delegate/test/test_rpc.ml
@@ -50,10 +50,10 @@ let run blkid =
       let res = predicate result in
       Format.eprintf "/%s (%d) -> %B@." (String.concat "/" path) depth res ;
       success := !success  && res ;
-      return ()
+      return_unit
     ) tests >>=? fun () ->
   if !success then
-    return ()
+    return_unit
   else
     failwith "Error!"
 
diff --git a/src/proto_alpha/lib_delegate/test/test_vote.ml b/src/proto_alpha/lib_delegate/test/test_vote.ml
index dc1d832df..528e57b85 100644
--- a/src/proto_alpha/lib_delegate/test/test_vote.ml
+++ b/src/proto_alpha/lib_delegate/test/test_vote.ml
@@ -90,7 +90,7 @@ let rpc_port = try int_of_string Sys.argv.(2) with _ -> 18400
 let change_to_demo_proto () =
   init ~exe ~vote:true ~rpc_port () >>=? fun (_node_pid, hash) ->
   run_change_to_demo_proto (`Hash (hash, 0)) Account.bootstrap_accounts >>=? fun _blkh ->
-  return ()
+  return_unit
 
 let tests = [
   "change_to_demo_proto", (fun _ -> change_to_demo_proto ()) ;
diff --git a/src/proto_alpha/lib_protocol/src/apply.ml b/src/proto_alpha/lib_protocol/src/apply.ml
index 9f210297c..007fb0e8b 100644
--- a/src/proto_alpha/lib_protocol/src/apply.ml
+++ b/src/proto_alpha/lib_protocol/src/apply.ml
@@ -350,13 +350,13 @@ let apply_manager_operation_content :
         match script with
         | None -> begin
             match parameters with
-            | None -> return ()
+            | None -> return_unit
             | Some arg ->
                 Lwt.return (Script.force_decode arg) >>=? fun arg ->
                 match Micheline.root arg with
                 | Prim (_, D_Unit, [], _) ->
                     (* Allow [Unit] parameter to non-scripted contracts. *)
-                    return ()
+                    return_unit
                 | _ -> fail (Script_interpreter.Bad_contract_parameter destination)
           end >>=? fun () ->
             let result =
diff --git a/src/proto_alpha/lib_protocol/src/baking.ml b/src/proto_alpha/lib_protocol/src/baking.ml
index 0f2e2af1b..c4d5b5910 100644
--- a/src/proto_alpha/lib_protocol/src/baking.ml
+++ b/src/proto_alpha/lib_protocol/src/baking.ml
@@ -237,7 +237,7 @@ let check_proof_of_work_stamp ctxt block =
       block.Block_header.shell
       block.protocol_data.contents
       proof_of_work_threshold then
-    return ()
+    return_unit
   else
     fail Invalid_stamp
 
@@ -250,7 +250,7 @@ let check_signature block key =
         (shell, contents) in
     Signature.check ~watermark:Block_header key signature unsigned_header in
   if check_signature key block then
-    return ()
+    return_unit
   else
     fail (Invalid_block_signature (Block_header.hash block,
                                    Signature.Public_key.hash key))
@@ -266,7 +266,7 @@ let check_fitness_gap ctxt (block : Block_header.t) =
   if Compare.Int64.(gap <= 0L || max_fitness_gap ctxt < gap) then
     fail (Invalid_fitness_gap (max_fitness_gap ctxt, gap))
   else
-    return ()
+    return_unit
 
 let last_of_a_cycle ctxt l =
   Compare.Int32.(Int32.succ l.Level.cycle_position =
diff --git a/src/proto_alpha/lib_protocol/src/contract_storage.ml b/src/proto_alpha/lib_protocol/src/contract_storage.ml
index 918739a71..2af9e8dec 100644
--- a/src/proto_alpha/lib_protocol/src/contract_storage.ml
+++ b/src/proto_alpha/lib_protocol/src/contract_storage.ml
@@ -285,12 +285,12 @@ let exists c contract =
 
 let must_exist c contract =
   exists c contract >>=? function
-  | true -> return ()
+  | true -> return_unit
   | false -> fail (Non_existing_contract contract)
 
 let must_be_allocated c contract =
   allocated c contract >>=? function
-  | true -> return ()
+  | true -> return_unit
   | false ->
       match Contract_repr.is_implicit contract with
       | Some pkh -> fail (Empty_implicit_contract pkh)
@@ -315,7 +315,7 @@ let check_counter_increment c contract counter =
   Storage.Contract.Counter.get c contract >>=? fun contract_counter ->
   let expected = Z.succ contract_counter in
   if Compare.Z.(expected = counter)
-  then return ()
+  then return_unit
   else if Compare.Z.(expected > counter) then
     fail (Counter_in_the_past (contract, expected, counter))
   else
diff --git a/src/proto_alpha/lib_protocol/src/delegate_storage.ml b/src/proto_alpha/lib_protocol/src/delegate_storage.ml
index 3c69d5971..c71adb5b0 100644
--- a/src/proto_alpha/lib_protocol/src/delegate_storage.ml
+++ b/src/proto_alpha/lib_protocol/src/delegate_storage.ml
@@ -195,11 +195,11 @@ let set_base c is_delegatable contract delegate =
             when Signature.Public_key_hash.equal delegate current_delegate ->
               if self_delegation then
                 Storage.Contract.Inactive_delegate.mem c contract >>= function
-                | true -> return ()
+                | true -> return_unit
                 | false -> fail Active_delegate
               else
                 fail Current_delegate
-          | None | Some _ -> return ()
+          | None | Some _ -> return_unit
         end >>=? fun () ->
         Storage.Contract.Balance.mem c contract >>= fun exists ->
         fail_when
diff --git a/src/proto_alpha/lib_protocol/src/helpers_services.ml b/src/proto_alpha/lib_protocol/src/helpers_services.ml
index b60ca4135..17aef6430 100644
--- a/src/proto_alpha/lib_protocol/src/helpers_services.ml
+++ b/src/proto_alpha/lib_protocol/src/helpers_services.ml
@@ -491,10 +491,10 @@ module Parse = struct
         Lwt.return (parse_operation raw) >>=? fun op ->
         begin match check with
           | Some true ->
-              return () (* FIXME *)
+              return_unit (* FIXME *)
           (* I.check_signature ctxt *)
           (* op.protocol_data.signature op.shell op.protocol_data.contents *)
-          | Some false | None -> return ()
+          | Some false | None -> return_unit
         end >>|? fun () -> op
       end operations
     end ;
diff --git a/src/proto_alpha/lib_protocol/src/raw_context.ml b/src/proto_alpha/lib_protocol/src/raw_context.ml
index 6d63c60d0..f2fe667a6 100644
--- a/src/proto_alpha/lib_protocol/src/raw_context.ml
+++ b/src/proto_alpha/lib_protocol/src/raw_context.ml
@@ -398,7 +398,7 @@ let check_inited ctxt =
   | Some bytes ->
       let s = MBytes.to_string bytes in
       if Compare.String.(s = version_value) then
-        return ()
+        return_unit
       else
         storage_error (Incompatible_protocol_version s)
 
@@ -432,13 +432,13 @@ let prepare ~level ~timestamp ~fitness ctxt =
 
 let check_first_block ctxt =
   Context.get ctxt version_key >>= function
-  | None -> return ()
+  | None -> return_unit
   | Some bytes ->
       let s = MBytes.to_string bytes in
       if Compare.String.(s = version_value) then
         failwith "Internal error: previously initialized context."
       else if Compare.String.(s = "genesis") then
-        return ()
+        return_unit
       else
         storage_error (Incompatible_protocol_version s)
 
diff --git a/src/proto_alpha/lib_protocol/src/script_interpreter.ml b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
index 8d487fdcc..f5972a3ad 100644
--- a/src/proto_alpha/lib_protocol/src/script_interpreter.ml
+++ b/src/proto_alpha/lib_protocol/src/script_interpreter.ml
@@ -730,11 +730,11 @@ let rec interp
             logged_return (Item (amount, rest), ctxt) in
     let stack = (Item (arg, Empty)) in
     begin match log with
-      | None -> return ()
+      | None -> return_unit
       | Some log ->
           unparse_stack ctxt (stack, code.bef) >>=? fun stack ->
           log := (code.loc, Gas.level ctxt, stack) :: !log ;
-          return ()
+          return_unit
     end >>=? fun () ->
     step ctxt code stack >>=? fun (Item (ret, Empty), ctxt) ->
     return (ret, ctxt)
diff --git a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
index c26d2d96f..3595dd044 100644
--- a/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
+++ b/src/proto_alpha/lib_protocol/src/script_ir_translator.ml
@@ -363,7 +363,7 @@ let unexpected expr exp_kinds exp_ns exp_prims =
 let check_kind kinds expr =
   let kind = kind expr in
   if List.mem kind kinds then
-    return ()
+    return_unit
   else
     let loc = location expr in
     fail (Invalid_kind (loc, kinds, kind))
@@ -1147,8 +1147,8 @@ let rec parse_data
                        if Compare.Int.(0 = (compare_comparable key_type value k))
                        then fail (Duplicate_map_keys (loc, strip_locations expr))
                        else fail (Unordered_map_keys (loc, strip_locations expr))
-                     else return ()
-                 | None -> return ()
+                     else return_unit
+                 | None -> return_unit
                end >>=? fun () ->
                return (Some k, map_update k (Some (item_wrapper v)) map, ctxt)
            | Prim (loc, D_Elt, l, _) ->
@@ -1403,8 +1403,8 @@ let rec parse_data
                      if Compare.Int.(0 = (compare_comparable t value v))
                      then fail (Duplicate_set_values (loc, strip_locations expr))
                      else fail (Unordered_set_values (loc, strip_locations expr))
-                   else return ()
-               | None -> return ()
+                   else return_unit
+               | None -> return_unit
              end >>=? fun () ->
              Lwt.return (Gas.consume ctxt (Michelson_v1_gas.Cost_of.set_update v false set)) >>=? fun ctxt ->
              return (Some v, set_update v true set, ctxt))
diff --git a/src/proto_alpha/lib_protocol/src/storage_description.ml b/src/proto_alpha/lib_protocol/src/storage_description.ml
index c74b8b127..df46401e4 100644
--- a/src/proto_alpha/lib_protocol/src/storage_description.ml
+++ b/src/proto_alpha/lib_protocol/src/storage_description.ml
@@ -173,7 +173,7 @@ type _ opt_handler =
 
 let rec combine_object = function
   | [] -> Handler { encoding = Data_encoding.unit ;
-                    get = fun _ _ -> return () }
+                    get = fun _ _ -> return_unit }
   | (name, Opt_handler handler) :: fields ->
       let Handler handlers = combine_object fields in
       Handler { encoding =
diff --git a/src/proto_alpha/lib_protocol/test/activation.ml b/src/proto_alpha/lib_protocol/test/activation.ml
index fa5807b72..8bf377d6d 100644
--- a/src/proto_alpha/lib_protocol/test/activation.ml
+++ b/src/proto_alpha/lib_protocol/test/activation.ml
@@ -168,7 +168,7 @@ let activation_init () =
 let simple_init_with_commitments () =
   activation_init () >>=? fun (blk, _contracts, _secrets) ->
   Block.bake blk >>=? fun _ ->
-  return ()
+  return_unit
 
 (** A single activation *)
 let single_activation () =
@@ -196,7 +196,7 @@ let multi_activation_1 () =
 
       return blk
     ) blk secrets >>=? fun _ ->
-  return ()
+  return_unit
 
 (** All in one bake *)
 let multi_activation_2 () =
diff --git a/src/proto_alpha/lib_protocol/test/delegation.ml b/src/proto_alpha/lib_protocol/test/delegation.ml
index 44a1e4eee..a8e7a19b0 100644
--- a/src/proto_alpha/lib_protocol/test/delegation.ml
+++ b/src/proto_alpha/lib_protocol/test/delegation.ml
@@ -19,7 +19,7 @@ open Test_tez
    interpret the other tests that use them. *)
 
 let expect_error err = function
-  | err0 :: _ when err = err0 -> return ()
+  | err0 :: _ when err = err0 -> return_unit
   | _ -> failwith "Unexpected successful result"
 
 let expect_alpha_error err =
@@ -27,13 +27,13 @@ let expect_alpha_error err =
 
 let expect_non_delegatable_contract = function
   | Alpha_environment.Ecoproto_error (Delegate_storage.Non_delegatable_contract _) :: _ ->
-      return ()
+      return_unit
   | _ ->
       failwith "Contract is not delegatable and operation should fail."
 
 let expect_no_deletion_pkh pkh = function
   | Alpha_environment.Ecoproto_error (Delegate_storage.No_deletion pkh0) :: _ when pkh0 = pkh ->
-      return ()
+      return_unit
   | _ ->
       failwith "Delegate can not be deleted and operation should fail."
 
@@ -115,7 +115,7 @@ let bootstrap_manager_already_registered_delegate ~fee () =
     begin
       Incremental.add_operation ~expect_failure:(function
           |  Alpha_environment.Ecoproto_error Delegate_storage.Active_delegate :: _ ->
-              return ()
+              return_unit
           | _ ->
               failwith "Delegate is already active and operation should fail.")
         i sec_reg >>=? fun i ->
@@ -147,7 +147,7 @@ let delegate_to_bootstrap_by_origination ~fee () =
     begin
       Incremental.add_operation i ~expect_failure:(function
           |  Alpha_environment.Ecoproto_error Contract.Balance_too_low _ :: _ ->
-              return ()
+              return_unit
           | _ ->
               failwith "Not enough balance for origination burn: operation should fail.")
         op >>=? fun i ->
@@ -307,7 +307,7 @@ Not credited:
 
 let expect_unregistered_key pkh = function
   | Alpha_environment.Ecoproto_error Roll_storage.Unregistered_delegate pkh0 :: _
-    when pkh = pkh0 -> return ()
+    when pkh = pkh0 -> return_unit
   | _ -> failwith "Delegate key is not registered: operation should fail."
 
 (* A1: no self-delegation *)
@@ -893,7 +893,7 @@ let double_registration () =
   (* credit 1μꜩ+ check balance *)
   Op.transaction (I i) bootstrap impl_contract (Tez.one_mutez) >>=? fun create_contract ->
   Incremental.add_operation i create_contract >>=? fun i ->
-  (* return () *)
+  (* return_unit *)
   (* self-delegation *)
   Op.delegation (I i) impl_contract (Some pkh) >>=? fun self_delegation ->
   Incremental.add_operation i self_delegation >>=? fun i ->
diff --git a/src/proto_alpha/lib_protocol/test/double_baking.ml b/src/proto_alpha/lib_protocol/test/double_baking.ml
index 2f559f44b..939fb6abe 100644
--- a/src/proto_alpha/lib_protocol/test/double_baking.ml
+++ b/src/proto_alpha/lib_protocol/test/double_baking.ml
@@ -91,7 +91,7 @@ let same_blocks () =
   Assert.proto_error ~loc:__LOC__ res begin function
     | Apply.Invalid_double_baking_evidence _ -> true
     | _ -> false end >>=? fun () ->
-  return ()
+  return_unit
 
 (** Check that a double baking operation exposing two blocks with
     different levels fails *)
diff --git a/src/proto_alpha/lib_protocol/test/helpers/assert.ml b/src/proto_alpha/lib_protocol/test/helpers/assert.ml
index 5e70ed738..9fbb162f5 100644
--- a/src/proto_alpha/lib_protocol/test/helpers/assert.ml
+++ b/src/proto_alpha/lib_protocol/test/helpers/assert.ml
@@ -12,7 +12,7 @@ open Proto_alpha
 let error ~loc v f =
   match v with
   | Error err when List.exists f err ->
-      return ()
+      return_unit
   | Ok _ ->
       failwith "Unexpected successful result (%s)" loc
   | Error err ->
@@ -28,13 +28,13 @@ let equal ~loc (cmp : 'a -> 'a -> bool) msg pp a b  =
   if not (cmp a b) then
     failwith "@[@[[%s]@] - @[%s : %a is not equal to %a@]@]" loc msg pp a pp b
   else
-    return ()
+    return_unit
 
 let not_equal ~loc (cmp : 'a -> 'a -> bool) msg pp a b  =
   if cmp a b then
     failwith "@[@[[%s]@] - @[%s : %a is equal to %a@]@]" loc msg pp a pp b
   else
-    return ()
+    return_unit
 
 (* tez *)
 let equal_tez ~loc (a:Alpha_context.Tez.t) (b:Alpha_context.Tez.t) =
diff --git a/src/proto_alpha/lib_protocol/test/helpers/block.ml b/src/proto_alpha/lib_protocol/test/helpers/block.ml
index 930d9cc4d..73bb10eca 100644
--- a/src/proto_alpha/lib_protocol/test/helpers/block.ml
+++ b/src/proto_alpha/lib_protocol/test/helpers/block.ml
@@ -252,7 +252,7 @@ let genesis
           else return acc
         ) Tez_repr.zero initial_accounts >>=? fun _ ->
       failwith "Insufficient tokens in initial accounts to create one roll"
-    with Exit -> return ()
+    with Exit -> return_unit
   end >>=? fun () ->
 
   let constants : Constants_repr.parametric = {
diff --git a/src/proto_alpha/lib_protocol/test/origination.ml b/src/proto_alpha/lib_protocol/test/origination.ml
index e18829641..9ae4a9e8f 100644
--- a/src/proto_alpha/lib_protocol/test/origination.ml
+++ b/src/proto_alpha/lib_protocol/test/origination.ml
@@ -132,7 +132,7 @@ let regular () =
   Context.get_endorser (B b) >>=? fun (account, _slots) ->
   Op.delegation (B b) new_contract (Some account) >>=? fun operation ->
   Block.bake ~operation b >>=? fun _ ->
-  return ()
+  return_unit
 
 (*******************)
 (** ask source contract to pay a fee when originating a contract *)
@@ -141,7 +141,7 @@ let regular () =
 let pay_fee () =
   register_origination ~credit:(Tez.of_int 2) ~fee:ten_tez () >>=? fun (b, contract, new_contract) ->
   transfer_and_check_balances b new_contract contract (Tez.of_int 2) >>=? fun _ ->
-  return ()
+  return_unit
 
 (******************************************************)
 (** Errors *)
@@ -189,7 +189,7 @@ let undelegatable fee () =
     begin
       let expect_failure = function
         | Alpha_environment.Ecoproto_error (Delegate_storage.Non_delegatable_contract _) :: _ ->
-            return ()
+            return_unit
         | _ ->
             failwith "The contract is not delegatable, it fails!"
       in
@@ -225,7 +225,7 @@ let credit fee () =
     begin
       let not_enough_money = function
         | Alpha_environment.Ecoproto_error (Proto_alpha.Contract_storage.Balance_too_low _) :: _ ->
-            return ()
+            return_unit
         | _ -> failwith "The contract does not have enough money, it fails!"
       in
       Incremental.add_operation ~expect_failure:not_enough_money i operation >>=? fun i ->
@@ -259,7 +259,7 @@ let origination_contract_from_origination_contract_not_enough_fund fee () =
   Op.origination ~fee (I inc) ~credit:amount contract >>=? fun (operation, orig_contract) ->
   let expect_failure = function
     | Alpha_environment.Ecoproto_error (Contract_storage.Balance_too_low _) :: _ ->
-        return ()
+        return_unit
     | _ ->
         failwith "The contract has not enough funds, it fails!"
   in
@@ -378,7 +378,6 @@ let origination_contract_from_origination_contract () =
   Context.Contract.balance (B b) orig_contract >>=? fun credit0 ->
   Assert.equal_tez ~loc:__LOC__ credit0 credit
 
-
 (******************************************************)
 
 let tests = [
diff --git a/src/proto_alpha/lib_protocol/test/transfer.ml b/src/proto_alpha/lib_protocol/test/transfer.ml
index 6832912ba..e007784ea 100644
--- a/src/proto_alpha/lib_protocol/test/transfer.ml
+++ b/src/proto_alpha/lib_protocol/test/transfer.ml
@@ -68,7 +68,7 @@ let single_transfer ?fee ?expect_failure amount =
   transfer_and_check_balances ~loc:__LOC__ ?fee ?expect_failure
     b contract_1 contract_2 amount >>=? fun (b,_) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** single transfer without fee *)
 let block_with_a_single_transfer () =
@@ -79,7 +79,7 @@ let transfer_zero_tez () =
   single_transfer ~expect_failure:(
     function
     | Alpha_environment.Ecoproto_error (Contract_storage.Empty_transaction _) :: _ ->
-        return ()
+        return_unit
     | _ ->
         failwith "Empty transaction should fail")
     Tez.zero
@@ -101,7 +101,7 @@ let block_originate_and_transfer_with_fee () =
   Incremental.add_operation b operation >>=? fun b ->
   transfer_and_check_balances ~loc:__LOC__ b ~fee:ten_tez contract new_contract ten_tez >>=? fun (b, _) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block, and two contracts;
     2- Add a transfer from a current balance of a source contract
@@ -113,7 +113,7 @@ let block_transfer_from_contract_balance () =
   Context.Contract.balance (I b) contract_1 >>=? fun balance ->
   transfer_and_check_balances ~loc:__LOC__ b contract_1 contract_2 balance >>=? fun (b,_) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block and a single contract;
     2- Add a transfer to a contract itself without fee into this block;
@@ -128,7 +128,7 @@ let block_transfers_without_with_fee_to_self () =
   transfer_to_itself_and_check_balances ~loc:__LOC__ b ~fee:ten_tez contract ten_tez
   >>=? fun (b, _) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block, two contracts;
     2- Add three transfers into the block;
@@ -140,7 +140,7 @@ let four_transfers_bake_three_transfers () =
   n_transactions 3 b contract_1 contract_2 ten_tez >>=? fun b ->
   Op.transaction (I b) contract_1 contract_2 ten_tez >>=? fun _ ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a contract from a bootstrap contract;
     2- Create two implicit contracts;
@@ -163,7 +163,7 @@ let transfer_from_implicit_to_implicit_contract () =
   transfer_and_check_balances ~loc:__LOC__ ~fee:(Tez.of_int 3) b
     src dest ten_tez >>=? fun (b, _) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block, contract from bootstrap accounts, contract from originate;
     2- Add a transfer from the bootstract contract into the implicit contract;
@@ -183,7 +183,7 @@ let transfer_from_implicit_to_originated_contract () =
   transfer_and_check_balances ~loc:__LOC__ b src new_contract Alpha_context.Tez.one
   >>=? fun (b, _) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block with 2 contracts;
     2- Originate 2 contracts from the previous ones;
@@ -199,7 +199,7 @@ let transfer_from_originated_to_originated () =
   transfer_and_check_balances ~loc:__LOC__ b
     orig_contract_1 orig_contract_2 Alpha_context.Tez.one >>=? fun (b,_) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block, an originate contract, an impicit contract, a contract
        from bootstrap;
@@ -216,7 +216,7 @@ let transfer_from_originated_to_implicit () =
   transfer_and_check_balances ~loc:__LOC__ b new_contract src Alpha_context.Tez.one
   >>=? fun (b, _) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** Checking that the sender of a transaction is the actual
     manager of the contract.
@@ -231,7 +231,7 @@ let ownership_sender () =
   transfer_and_check_balances ~loc:__LOC__ b imcontract_1 contract_2 Tez.one
   >>=? fun (b,_) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (* Slow tests case *)
 
@@ -240,7 +240,7 @@ let multiple_transfer n ?fee amount =
   Incremental.begin_construction b >>=? fun b ->
   n_transactions n b ?fee contract_1 contract_2 amount >>=? fun b ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block with two contracts;
     2- Apply 100 transfers. *)
@@ -280,7 +280,7 @@ let block_with_multiple_transfers_with_without_fee () =
   n_transactions 20  ~fee:ten b contracts.(1) contracts.(0) twenty >>=? fun b ->
 
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** 1- Create a block with two contracts;
     2- Bake 10 blocks with a transfer each time. *)
@@ -293,7 +293,7 @@ let build_a_chain () =
       >>=? fun (b, _) ->
       Incremental.finalize_block b
     ) b (1 -- 10) >>=? fun _ ->
-  return ()
+  return_unit
 
 (*********************************************************************)
 (* Expected error test cases                                         *)
@@ -314,13 +314,13 @@ let balance_too_low fee () =
   Op.transaction ~fee (I i) contract_1 contract_2 Tez.max_tez >>=? fun op ->
   let expect_failure = function
     | Alpha_environment.Ecoproto_error (Contract_storage.Balance_too_low _) :: _ ->
-        return ()
+        return_unit
     | _ ->
         failwith "balance too low should fail"
   in
   if fee > balance1 then begin
     Incremental.add_operation ~expect_failure i op >>= fun _res ->
-    return ()
+    return_unit
   end
   else begin
     Incremental.add_operation ~expect_failure i op >>=? fun i ->
@@ -352,7 +352,7 @@ let balance_too_low_two_transfers fee () =
     two_third_of_balance >>=? fun operation ->
   let expect_failure = function
     | Alpha_environment.Ecoproto_error (Contract_storage.Balance_too_low _) :: _ ->
-        return ()
+        return_unit
     | _ ->
         failwith "balance too low should fail"
   in
@@ -446,7 +446,7 @@ let random_transfer () =
       transfer_and_check_balances ~loc:__LOC__ b source dest amount
   end >>=? fun (b,_) ->
   Incremental.finalize_block b >>=? fun _ ->
-  return ()
+  return_unit
 
 (** Transfer random transactions *)
 let random_multi_transactions () =
diff --git a/src/proto_demo/lib_client/client_proto_main.ml b/src/proto_demo/lib_client/client_proto_main.ml
index 08f39ce9a..0055ed3e1 100644
--- a/src/proto_demo/lib_client/client_proto_main.ml
+++ b/src/proto_demo/lib_client/client_proto_main.ml
@@ -21,17 +21,17 @@ let demo cctxt =
     cctxt.message "Calling the 'failing' RPC." >>= fun () ->
     Client_proto_rpcs.failing cctxt.rpc_config block 3 >>= function
     | Error [Environment.Ecoproto_error [Error.Demo_error 3]] ->
-        return ()
+        return_unit
     | _ -> failwith "..."
   end >>=? fun () ->
   cctxt.message "Direct call to `demo_error`." >>= fun () ->
   begin Error.demo_error 101010 >|= Environment.wrap_error >>= function
     | Error [Environment.Ecoproto_error [Error.Demo_error 101010]] ->
-        return ()
+        return_unit
     | _ -> failwith "...."
   end >>=? fun () ->
   cctxt.answer "All good!" >>= fun () ->
-  return ()
+  return_unit
 
 let bake cctxt =
   Client_node_rpcs.Blocks.info cctxt.rpc_config block >>=? fun bi ->
@@ -56,7 +56,7 @@ let bake cctxt =
       proto = MBytes.create 0 } >>=? fun bytes ->
   Client_node_rpcs.inject_block cctxt.rpc_config ~chain_id:bi.chain_id bytes [] >>=? fun hash ->
   cctxt.answer "Injected %a" Block_hash.pp_short hash >>= fun () ->
-  return ()
+  return_unit
 
 let handle_error cctxt = function
   | Ok res ->
diff --git a/src/proto_genesis/lib_client/client_proto_main.ml b/src/proto_genesis/lib_client/client_proto_main.ml
index ba4d896c8..f69dc91f0 100644
--- a/src/proto_genesis/lib_client/client_proto_main.ml
+++ b/src/proto_genesis/lib_client/client_proto_main.ml
@@ -77,7 +77,7 @@ let commands () =
           (Activate { protocol = hash ; fitness ; protocol_parameters })
           sk >>=? fun hash ->
         cctxt#answer "Injected %a" Block_hash.pp_short hash >>= fun () ->
-        return ()
+        return_unit
       end ;
 
     command ~desc: "Fork a test protocol"
@@ -94,7 +94,7 @@ let commands () =
                                 delay = Int64.mul 24L 3600L })
           sk >>=? fun hash ->
         cctxt#answer "Injected %a" Block_hash.pp_short hash >>= fun () ->
-        return ()
+        return_unit
       end ;
 
   ]
diff --git a/src/proto_genesis/lib_protocol/src/data.ml b/src/proto_genesis/lib_protocol/src/data.ml
index d24e9e78a..2877f1d9d 100644
--- a/src/proto_genesis/lib_protocol/src/data.ml
+++ b/src/proto_genesis/lib_protocol/src/data.ml
@@ -128,7 +128,7 @@ module Init = struct
     | Some version ->
         if Compare.String.(version_value <> MBytes.to_string version) then
           failwith "Internal error: incompatible protocol version" ;
-        return ()
+        return_unit
 
   let tag_first_block ctxt =
     Context.get ctxt version_key >>= function