diff --git a/src/main/compile/dune b/src/main/compile/dune
index 705ed50b9..59b561ef0 100644
--- a/src/main/compile/dune
+++ b/src/main/compile/dune
@@ -8,6 +8,7 @@
     simplify
     ast_simplified
     self_ast_simplified
+    typer_new
     typer
     ast_typed
     transpiler
diff --git a/src/main/run/dune b/src/main/run/dune
index 34f7986af..f39c18a69 100644
--- a/src/main/run/dune
+++ b/src/main/run/dune
@@ -7,6 +7,7 @@
     parser
     simplify
     ast_simplified
+    typer_new
     typer
     ast_typed
     transpiler
diff --git a/src/passes/4-typer-new/dune b/src/passes/4-typer-new/dune
index ec35ab2ce..ef18cd078 100644
--- a/src/passes/4-typer-new/dune
+++ b/src/passes/4-typer-new/dune
@@ -1,6 +1,6 @@
 (library
-  (name typer)
-  (public_name ligo.typer)
+  (name typer_new)
+  (public_name ligo.typer_new)
   (libraries
     simple-utils
     tezos-utils
diff --git a/src/passes/4-typer/dune b/src/passes/4-typer/dune
new file mode 100644
index 000000000..0ee58cc43
--- /dev/null
+++ b/src/passes/4-typer/dune
@@ -0,0 +1,15 @@
+(library
+  (name typer)
+  (public_name ligo.typer)
+  (libraries
+    simple-utils
+    tezos-utils
+    ast_simplified
+    ast_typed
+    operators
+  )
+  (preprocess
+    (pps ppx_let)
+  )
+  (flags (:standard -w +1..62-4-9-44-40-42-48-30@39@33 -open Simple_utils ))
+)
diff --git a/src/passes/4-typer/typer.ml b/src/passes/4-typer/typer.ml
new file mode 100644
index 000000000..2751225eb
--- /dev/null
+++ b/src/passes/4-typer/typer.ml
@@ -0,0 +1,917 @@
+open Trace
+
+module I = Ast_simplified
+module O = Ast_typed
+open O.Combinators
+
+module SMap = O.SMap
+
+module Environment = O.Environment
+
+module Solver = struct
+  type state = Placeholder_for_state_of_new_typer
+  let discard_state (_ : state) = ()
+  let initial_state = Placeholder_for_state_of_new_typer
+end
+
+type environment = Environment.t
+
+module Errors = struct
+  let unbound_type_variable (e:environment) (n:string) () =
+    let title = (thunk "unbound type variable") in
+    let message () = "" in
+    let data = [
+      ("variable" , fun () -> Format.asprintf "%s" n) ;
+      (* TODO: types don't have srclocs for now. *)
+      (* ("location" , fun () -> Format.asprintf "%a" Location.pp (n.location)) ; *)
+      ("in" , fun () -> Format.asprintf "%a" Environment.PP.full_environment e)
+    ] in
+    error ~data title message ()
+
+  let unbound_variable (e:environment) (n:string) (loc:Location.t) () =
+    let title = (thunk "unbound variable") in
+    let message () = "" in
+    let data = [
+      ("variable" , fun () -> Format.asprintf "%s" n) ;
+      ("environment" , fun () -> Format.asprintf "%a" Environment.PP.full_environment e) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let match_empty_variant : type a . a I.matching -> Location.t -> unit -> _ =
+    fun matching loc () ->
+    let title = (thunk "match with no cases") in
+    let message () = "" in
+    let data = [
+      ("variant" , fun () -> Format.asprintf "%a" I.PP.matching_type matching) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let match_missing_case : type a . a I.matching -> Location.t -> unit -> _ =
+    fun matching loc () ->
+    let title = (thunk "missing case in match") in
+    let message () = "" in
+    let data = [
+      ("variant" , fun () -> Format.asprintf "%a" I.PP.matching_type matching) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let match_redundant_case : type a . a I.matching -> Location.t -> unit -> _ =
+    fun matching loc () ->
+    let title = (thunk "missing case in match") in
+    let message () = "" in
+    let data = [
+      ("variant" , fun () -> Format.asprintf "%a" I.PP.matching_type matching) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let unbound_constructor (e:environment) (n:string) (loc:Location.t) () =
+    let title = (thunk "unbound constructor") in
+    let message () = "" in
+    let data = [
+      ("constructor" , fun () -> Format.asprintf "%s" n) ;
+      ("environment" , fun () -> Format.asprintf "%a" Environment.PP.full_environment e) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let unrecognized_constant (n:string) (loc:Location.t) () =
+    let title = (thunk "unrecognized constant") in
+    let message () = "" in
+    let data = [
+      ("constant" , fun () -> Format.asprintf "%s" n) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let wrong_arity (n:string) (expected:int) (actual:int) (loc : Location.t) () =
+    let title () = "wrong arity" in
+    let message () = "" in
+    let data = [
+      ("function" , fun () -> Format.asprintf "%s" n) ;
+      ("expected" , fun () -> Format.asprintf "%d" expected) ;
+      ("actual" , fun () -> Format.asprintf "%d" actual) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let match_tuple_wrong_arity (expected:'a list) (actual:'b list) (loc:Location.t) () =
+    let title () = "matching tuple of different size" in
+    let message () = "" in
+    let data = [
+      ("expected" , fun () -> Format.asprintf "%d" (List.length expected)) ;
+      ("actual" , fun () -> Format.asprintf "%d" (List.length actual)) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  (* TODO: this should be a trace_info? *)
+  let program_error (p:I.program) () =
+    let message () = "" in
+    let title = (thunk "typing program") in
+    let data = [
+      ("program" , fun () -> Format.asprintf "%a" I.PP.program p)
+    ] in
+    error ~data title message ()
+
+  let constant_declaration_error (name:string) (ae:I.expr) (expected: O.type_value option) () =
+    let title = (thunk "typing constant declaration") in
+    let message () = "" in
+    let data = [
+      ("constant" , fun () -> Format.asprintf "%s" name) ;
+      ("expression" , fun () -> Format.asprintf "%a" I.PP.expression ae) ;
+      ("expected" , fun () ->
+          match expected with
+            None -> "(no annotation for the expected type)"
+          | Some expected -> Format.asprintf "%a" O.PP.type_value expected) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp ae.location)
+    ] in
+    error ~data title message ()
+
+  let match_error : type a . ?msg:string -> expected: a I.matching -> actual: O.type_value -> Location.t -> unit -> _ =
+    fun ?(msg = "") ~expected ~actual loc () ->
+    let title = (thunk "typing match") in
+    let message () = msg in
+    let data = [
+      ("expected" , fun () -> Format.asprintf "%a" I.PP.matching_type expected);
+      ("actual" , fun () -> Format.asprintf "%a" O.PP.type_value actual) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let needs_annotation (e : I.expression) (case : string) () =
+    let title = (thunk "this expression must be annotated with its type") in
+    let message () = Format.asprintf "%s needs an annotation" case in
+    let data = [
+      ("expression" , fun () -> Format.asprintf "%a" I.PP.expression e) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp e.location)
+    ] in
+    error ~data title message ()
+
+  let type_error_approximate ?(msg="") ~(expected: string) ~(actual: O.type_value) ~(expression : I.expression) (loc:Location.t) () =
+    let title = (thunk "type error") in
+    let message () = msg in
+    let data = [
+      ("expected"   , fun () -> Format.asprintf "%s" expected);
+      ("actual"     , fun () -> Format.asprintf "%a" O.PP.type_value actual);
+      ("expression" , fun () -> Format.asprintf "%a" I.PP.expression expression) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let type_error ?(msg="") ~(expected: O.type_value) ~(actual: O.type_value) ~(expression : I.expression) (loc:Location.t) () =
+    let title = (thunk "type error") in
+    let message () = msg in
+    let data = [
+      ("expected"   , fun () -> Format.asprintf "%a" O.PP.type_value expected);
+      ("actual"     , fun () -> Format.asprintf "%a" O.PP.type_value actual);
+      ("expression" , fun () -> Format.asprintf "%a" I.PP.expression expression) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let bad_tuple_index (index : int) (ae : I.expression) (t : O.type_value) (loc:Location.t) () =
+    let title = (thunk "invalid tuple index") in
+    let message () = "" in
+    let data = [
+      ("index" , fun () -> Format.asprintf "%d" index) ;
+      ("tuple_value" , fun () -> Format.asprintf "%a" I.PP.expression ae) ;
+      ("tuple_type" , fun () -> Format.asprintf "%a" O.PP.type_value t) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let bad_record_access (field : string) (ae : I.expression) (t : O.type_value) (loc:Location.t) () =
+    let title = (thunk "invalid record field") in
+    let message () = "" in
+    let data = [
+      ("field" , fun () -> Format.asprintf "%s" field) ;
+      ("record_value" , fun () -> Format.asprintf "%a" I.PP.expression ae) ;
+      ("tuple_type" , fun () -> Format.asprintf "%a" O.PP.type_value t) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc)
+    ] in
+    error ~data title message ()
+
+  let not_supported_yet (message : string) (ae : I.expression) () =
+    let title = (thunk "not suported yet") in
+    let message () = message in
+    let data = [
+      ("expression" , fun () -> Format.asprintf "%a"  I.PP.expression ae) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp ae.location)
+    ] in
+    error ~data title message ()
+
+  let not_supported_yet_untranspile (message : string) (ae : O.expression) () =
+    let title = (thunk "not suported yet") in
+    let message () = message in
+    let data = [
+      ("expression" , fun () -> Format.asprintf "%a"  O.PP.expression ae)
+    ] in
+    error ~data title message ()
+
+  let constant_error loc lst tv_opt =
+    let title () = "typing constant" in
+    let message () = "" in
+    let data = [
+      ("location" , fun () -> Format.asprintf "%a" Location.pp loc ) ;
+      ("argument_types" , fun () -> Format.asprintf "%a" PP_helpers.(list_sep Ast_typed.PP.type_value (const " , ")) lst) ;
+      ("type_opt" , fun () -> Format.asprintf "%a" PP_helpers.(option Ast_typed.PP.type_value) tv_opt) ;
+    ] in
+    error ~data title message
+end
+open Errors
+
+let rec type_program (p:I.program) : (O.program * Solver.state) result =
+  let aux (e, acc:(environment * O.declaration Location.wrap list)) (d:I.declaration Location.wrap) =
+    let%bind ed' = (bind_map_location (type_declaration e Solver.Placeholder_for_state_of_new_typer)) d in
+    let loc : 'a . 'a Location.wrap -> _ -> _ = fun x v -> Location.wrap ~loc:x.location v in
+    let (e', _placeholder_for_state_of_new_typer , d') = Location.unwrap ed' in
+    match d' with
+    | None -> ok (e', acc)
+    | Some d' -> ok (e', loc ed' d' :: acc)
+  in
+  let%bind (_, lst) =
+    trace (fun () -> program_error p ()) @@
+    bind_fold_list aux (Environment.full_empty, []) p in
+  ok @@ (List.rev lst , Solver.Placeholder_for_state_of_new_typer)
+
+and type_declaration env (_placeholder_for_state_of_new_typer : Solver.state) : I.declaration -> (environment * Solver.state * O.declaration option) result = function
+  | Declaration_type (type_name , type_expression) ->
+      let%bind tv = evaluate_type env type_expression in
+      let env' = Environment.add_type type_name tv env in
+      ok (env', Solver.Placeholder_for_state_of_new_typer , None)
+  | Declaration_constant (name , tv_opt , expression) -> (
+      let%bind tv'_opt = bind_map_option (evaluate_type env) tv_opt in
+      let%bind ae' =
+        trace (constant_declaration_error name expression tv'_opt) @@
+        type_expression' ?tv_opt:tv'_opt env expression in
+      let env' = Environment.add_ez_ae name ae' env in
+      ok (env', Solver.Placeholder_for_state_of_new_typer , Some (O.Declaration_constant ((make_n_e name ae') , (env , env'))))
+    )
+
+and type_match : type i o . (environment -> i -> o result) -> environment -> O.type_value -> i I.matching -> I.expression -> Location.t -> o O.matching result =
+  fun f e t i ae loc -> match i with
+    | Match_bool {match_true ; match_false} ->
+      let%bind _ =
+        trace_strong (match_error ~expected:i ~actual:t loc)
+        @@ get_t_bool t in
+      let%bind match_true = f e match_true in
+      let%bind match_false = f e match_false in
+      ok (O.Match_bool {match_true ; match_false})
+  | Match_option {match_none ; match_some} ->
+      let%bind t_opt =
+        trace_strong (match_error ~expected:i ~actual:t loc)
+        @@ get_t_option t in
+      let%bind match_none = f e match_none in
+      let (n, b) = match_some in
+      let n' = n, t_opt in
+      let e' = Environment.add_ez_binder n t_opt e in
+      let%bind b' = f e' b in
+      ok (O.Match_option {match_none ; match_some = (n', b')})
+  | Match_list {match_nil ; match_cons} ->
+      let%bind t_list =
+        trace_strong (match_error ~expected:i ~actual:t loc)
+        @@ get_t_list t in
+      let%bind match_nil = f e match_nil in
+      let (hd, tl, b) = match_cons in
+      let e' = Environment.add_ez_binder hd t_list e in
+      let e' = Environment.add_ez_binder tl t e' in
+      let%bind b' = f e' b in
+      ok (O.Match_list {match_nil ; match_cons = (((hd , t_list), (tl , t)), b')})
+  | Match_tuple (lst, b) ->
+      let%bind t_tuple =
+        trace_strong (match_error ~expected:i ~actual:t loc)
+        @@ get_t_tuple t in
+      let%bind lst' =
+        generic_try (match_tuple_wrong_arity t_tuple lst loc)
+        @@ (fun () -> List.combine lst t_tuple) in
+      let aux prev (name, tv) = Environment.add_ez_binder name tv prev in
+      let e' = List.fold_left aux e lst' in
+      let%bind b' = f e' b in
+      ok (O.Match_tuple (lst, b'))
+  | Match_variant lst ->
+      let%bind variant_opt =
+        let aux acc ((constructor_name , _) , _) =
+          let%bind (_ , variant) =
+            trace_option (unbound_constructor e constructor_name loc) @@
+            Environment.get_constructor constructor_name e in
+          let%bind acc = match acc with
+            | None -> ok (Some variant)
+            | Some variant' -> (
+                trace (type_error
+                         ~msg:"in match variant"
+                         ~expected:variant
+                         ~actual:variant'
+                         ~expression:ae
+                         loc
+                      ) @@
+                Ast_typed.assert_type_value_eq (variant , variant') >>? fun () ->
+                ok (Some variant)
+              ) in
+          ok acc in
+        trace (simple_info "in match variant") @@
+        bind_fold_list aux None lst in
+      let%bind variant =
+        trace_option (match_empty_variant i loc) @@
+        variant_opt in
+      let%bind () =
+        let%bind variant_cases' =
+          trace (match_error ~expected:i ~actual:t loc)
+          @@ Ast_typed.Combinators.get_t_sum variant in
+        let variant_cases = List.map fst @@ Map.String.to_kv_list variant_cases' in
+        let match_cases = List.map (Function.compose fst fst) lst in
+        let test_case = fun c ->
+          Assert.assert_true (List.mem c match_cases)
+        in
+        let%bind () =
+          trace_strong (match_missing_case i loc) @@
+          bind_iter_list test_case variant_cases in
+        let%bind () =
+          trace_strong (match_redundant_case i loc) @@
+          Assert.assert_true List.(length variant_cases = length match_cases) in
+        ok ()
+      in
+      let%bind lst' =
+        let aux ((constructor_name , name) , b) =
+          let%bind (constructor , _) =
+            trace_option (unbound_constructor e constructor_name loc) @@
+            Environment.get_constructor constructor_name e in
+          let e' = Environment.add_ez_binder name constructor e in
+          let%bind b' = f e' b in
+          ok ((constructor_name , name) , b')
+        in
+        bind_map_list aux lst in
+      ok (O.Match_variant (lst' , variant))
+
+and evaluate_type (e:environment) (t:I.type_expression) : O.type_value result =
+  let return tv' = ok (make_t tv' (Some t)) in
+  match t with
+  | T_function (a, b) ->
+      let%bind a' = evaluate_type e a in
+      let%bind b' = evaluate_type e b in
+      return (T_function (a', b'))
+  | T_tuple lst ->
+      let%bind lst' = bind_list @@ List.map (evaluate_type e) lst in
+      return (T_tuple lst')
+  | T_sum m ->
+      let aux k v prev =
+        let%bind prev' = prev in
+        let%bind v' = evaluate_type e v in
+        ok @@ SMap.add k v' prev'
+      in
+      let%bind m = SMap.fold aux m (ok SMap.empty) in
+      return (T_sum m)
+  | T_record m ->
+      let aux k v prev =
+        let%bind prev' = prev in
+        let%bind v' = evaluate_type e v in
+        ok @@ SMap.add k v' prev'
+      in
+      let%bind m = SMap.fold aux m (ok SMap.empty) in
+      return (T_record m)
+  | T_variable name ->
+      let%bind tv =
+        trace_option (unbound_type_variable e name)
+        @@ Environment.get_type_opt name e in
+      ok tv
+  | T_constant (cst, lst) ->
+      let%bind lst' = bind_list @@ List.map (evaluate_type e) lst in
+      return (T_constant(Type_name cst, lst'))
+
+and type_expression : environment -> Solver.state -> ?tv_opt:O.type_value -> I.expression -> (O.annotated_expression * Solver.state) result
+  = fun e _placeholder_for_state_of_new_typer ?tv_opt ae ->
+    let%bind res = type_expression' e ?tv_opt ae in
+    ok (res, Solver.Placeholder_for_state_of_new_typer)
+and type_expression' : environment -> ?tv_opt:O.type_value -> I.expression -> O.annotated_expression result = fun e ?tv_opt ae ->
+  let module L = Logger.Stateful() in
+  let return expr tv =
+    let%bind () =
+      match tv_opt with
+      | None -> ok ()
+      | Some tv' -> O.assert_type_value_eq (tv' , tv) in
+    let location = ae.location in
+    ok @@ make_a_e ~location expr tv e in
+  let main_error =
+    let title () = "typing expression" in
+    let content () = "" in
+    let data = [
+      ("expression" , fun () -> Format.asprintf "%a" I.PP.expression ae) ;
+      ("location" , fun () -> Format.asprintf "%a" Location.pp ae.location) ;
+      ("misc" , fun () -> L.get ()) ;
+    ] in
+    error ~data title content in
+  trace main_error @@
+  match ae.expression with
+  (* Basic *)
+  | E_variable name ->
+      let%bind tv' =
+        trace_option (unbound_variable e name ae.location)
+        @@ Environment.get_opt name e in
+      return (E_variable name) tv'.type_value
+  | E_literal (Literal_bool b) ->
+      return (E_literal (Literal_bool b)) (t_bool ())
+  | E_literal Literal_unit | E_skip ->
+      return (E_literal (Literal_unit)) (t_unit ())
+  | E_literal (Literal_string s) ->
+      return (E_literal (Literal_string s)) (t_string ())
+  | E_literal (Literal_bytes s) ->
+      return (E_literal (Literal_bytes s)) (t_bytes ())
+  | E_literal (Literal_int n) ->
+      return (E_literal (Literal_int n)) (t_int ())
+  | E_literal (Literal_nat n) ->
+      return (E_literal (Literal_nat n)) (t_nat ())
+  | E_literal (Literal_timestamp n) ->
+      return (E_literal (Literal_timestamp n)) (t_timestamp ())
+  | E_literal (Literal_mutez n) ->
+      return (E_literal (Literal_mutez n)) (t_mutez ())
+  | E_literal (Literal_address s) ->
+      return (e_address s) (t_address ())
+  | E_literal (Literal_operation op) ->
+      return (e_operation op) (t_operation ())
+  (* Tuple *)
+  | E_tuple lst ->
+      let%bind lst' = bind_list @@ List.map (type_expression' e) lst in
+      let tv_lst = List.map get_type_annotation lst' in
+      return (E_tuple lst') (t_tuple tv_lst ())
+  | E_accessor (ae', path) ->
+      let%bind e' = type_expression' e ae' in
+      let aux (prev:O.annotated_expression) (a:I.access) : O.annotated_expression result =
+        match a with
+        | Access_tuple index -> (
+            let%bind tpl_tv = get_t_tuple prev.type_annotation in
+            let%bind tv =
+              generic_try (bad_tuple_index index ae' prev.type_annotation ae.location)
+              @@ (fun () -> List.nth tpl_tv index) in
+            return (E_tuple_accessor (prev , index)) tv
+          )
+        | Access_record property -> (
+            let%bind r_tv = get_t_record prev.type_annotation in
+            let%bind tv =
+              generic_try (bad_record_access property ae' prev.type_annotation ae.location)
+              @@ (fun () -> SMap.find property r_tv) in
+            return (E_record_accessor (prev , property)) tv
+          )
+        | Access_map ae' -> (
+            let%bind ae'' = type_expression' e ae' in
+            let%bind (k , v) = bind_map_or (get_t_map , get_t_big_map) prev.type_annotation in
+            let%bind () =
+              Ast_typed.assert_type_value_eq (k , get_type_annotation ae'') in
+            return (E_look_up (prev , ae'')) v
+          )
+      in
+      trace (simple_info "accessing") @@
+      bind_fold_list aux e' path
+  (* Sum *)
+  | E_constructor (c, expr) ->
+      let%bind (c_tv, sum_tv) =
+        let error =
+          let title () = "no such constructor" in
+          let content () =
+            Format.asprintf "%s in:\n%a\n"
+              c O.Environment.PP.full_environment e
+          in
+          error title content in
+        trace_option error @@
+        Environment.get_constructor c e in
+      let%bind expr' = type_expression' e expr in
+      let%bind _assert = O.assert_type_value_eq (expr'.type_annotation, c_tv) in
+      return (E_constructor (c , expr')) sum_tv
+  (* Record *)
+  | E_record m ->
+      let aux prev k expr =
+        let%bind expr' = type_expression' e expr in
+        ok (SMap.add k expr' prev)
+      in
+      let%bind m' = bind_fold_smap aux (ok SMap.empty) m in
+      return (E_record m') (t_record (SMap.map get_type_annotation m') ())
+  (* Data-structure *)
+  | E_list lst ->
+      let%bind lst' = bind_map_list (type_expression' e) lst in
+      let%bind tv =
+        let aux opt c =
+          match opt with
+          | None -> ok (Some c)
+          | Some c' ->
+              let%bind _eq = Ast_typed.assert_type_value_eq (c, c') in
+              ok (Some c') in
+        let%bind init = match tv_opt with
+          | None -> ok None
+          | Some ty ->
+              let%bind ty' = get_t_list ty in
+              ok (Some ty') in
+        let%bind ty =
+          let%bind opt = bind_fold_list aux init
+          @@ List.map get_type_annotation lst' in
+          trace_option (needs_annotation ae "empty list") opt in
+        ok (t_list ty ())
+      in
+      return (E_list lst') tv
+  | E_set lst ->
+      let%bind lst' = bind_map_list (type_expression' e) lst in
+      let%bind tv =
+        let aux opt c =
+          match opt with
+          | None -> ok (Some c)
+          | Some c' ->
+              let%bind _eq = Ast_typed.assert_type_value_eq (c, c') in
+              ok (Some c') in
+        let%bind init = match tv_opt with
+          | None -> ok None
+          | Some ty ->
+              let%bind ty' = get_t_set ty in
+              ok (Some ty') in
+        let%bind ty =
+          let%bind opt = bind_fold_list aux init
+          @@ List.map get_type_annotation lst' in
+          trace_option (needs_annotation ae "empty set") opt in
+        ok (t_set ty ())
+      in
+      return (E_set lst') tv
+  | E_map lst ->
+      let%bind lst' = bind_map_list (bind_map_pair (type_expression' e)) lst in
+      let%bind tv =
+        let aux opt c =
+          match opt with
+          | None -> ok (Some c)
+          | Some c' ->
+              let%bind _eq = Ast_typed.assert_type_value_eq (c, c') in
+              ok (Some c') in
+        let%bind key_type =
+          let%bind sub =
+            bind_fold_list aux None
+            @@ List.map get_type_annotation
+            @@ List.map fst lst' in
+          let%bind annot = bind_map_option get_t_map_key tv_opt in
+          trace (simple_info "empty map expression without a type annotation") @@
+          O.merge_annotation annot sub (needs_annotation ae "this map literal")
+        in
+        let%bind value_type =
+          let%bind sub =
+            bind_fold_list aux None
+            @@ List.map get_type_annotation
+            @@ List.map snd lst' in
+          let%bind annot = bind_map_option get_t_map_value tv_opt in
+          trace (simple_info "empty map expression without a type annotation") @@
+          O.merge_annotation annot sub (needs_annotation ae "this map literal")
+        in
+        ok (t_map key_type value_type ())
+      in
+      return (E_map lst') tv
+  | E_big_map lst ->
+      let%bind lst' = bind_map_list (bind_map_pair (type_expression' e)) lst in
+      let%bind tv =
+        let aux opt c =
+          match opt with
+          | None -> ok (Some c)
+          | Some c' ->
+              let%bind _eq = Ast_typed.assert_type_value_eq (c, c') in
+              ok (Some c') in
+        let%bind key_type =
+          let%bind sub =
+            bind_fold_list aux None
+            @@ List.map get_type_annotation
+            @@ List.map fst lst' in
+          let%bind annot = bind_map_option get_t_big_map_key tv_opt in
+          trace (simple_info "empty map expression without a type annotation") @@
+          O.merge_annotation annot sub (needs_annotation ae "this map literal")
+        in
+        let%bind value_type =
+          let%bind sub =
+            bind_fold_list aux None
+            @@ List.map get_type_annotation
+            @@ List.map snd lst' in
+          let%bind annot = bind_map_option get_t_big_map_value tv_opt in
+          trace (simple_info "empty map expression without a type annotation") @@
+          O.merge_annotation annot sub (needs_annotation ae "this map literal")
+        in
+        ok (t_big_map key_type value_type ())
+      in
+      return (E_big_map lst') tv
+  | E_lambda {
+      binder ;
+      input_type ;
+      output_type ;
+      result ;
+    } -> (
+      let%bind input_type =
+        let%bind input_type =
+          (* Hack to take care of let_in introduced by `simplify/ligodity.ml` in ECase's hack *)
+          let default_action e () = fail @@ (needs_annotation e "the returned value") in
+          match input_type with
+          | Some ty -> ok ty
+          | None -> (
+              match result.expression with
+              | I.E_let_in li -> (
+                  match li.rhs.expression with
+                  | I.E_variable name when name = (fst binder) -> (
+                      match snd li.binder with
+                      | Some ty -> ok ty
+                      | None -> default_action li.rhs ()
+                    )
+                  | _ -> default_action li.rhs ()
+                )
+              | _ -> default_action result ()
+            )
+        in
+        evaluate_type e input_type in
+      let%bind output_type =
+        bind_map_option (evaluate_type e) output_type
+      in
+      let e' = Environment.add_ez_binder (fst binder) input_type e in
+      let%bind body = type_expression' ?tv_opt:output_type e' result in
+      let output_type = body.type_annotation in
+      return (E_lambda {binder = fst binder ; body}) (t_function input_type output_type ())
+    )
+  | E_constant ( ("LIST_FOLD"|"MAP_FOLD"|"SET_FOLD") as opname ,
+                  [ collect ; 
+                    init_record ;
+                    ( { expression = (I.E_lambda { binder = (lname, None) ;
+                                                   input_type = None ; 
+                                                   output_type = None ; 
+                                                   result }) ;
+                        location = _ }) as _lambda
+                  ] ) ->
+      (* this special case is here force annotation of the untyped lambda
+         generated by pascaligo's for_collect loop *)
+      let%bind (v_col , v_initr ) = bind_map_pair (type_expression' e) (collect , init_record ) in
+      let tv_col = get_type_annotation v_col   in (* this is the type of the collection  *) 
+      let tv_out = get_type_annotation v_initr in (* this is the output type of the lambda*)
+      let%bind input_type = match tv_col.type_value' with
+        | O.T_constant ( (Type_name "list"|Type_name "set") , t) -> ok @@ t_tuple (tv_out::t) ()
+        | O.T_constant ( Type_name "map" , t) -> ok @@ t_tuple (tv_out::[(t_tuple t ())]) () 
+        | _ ->
+          let wtype = Format.asprintf
+            "Loops over collections expect lists, sets or maps, got type %a" O.PP.type_value tv_col in 
+          fail @@ simple_error wtype in 
+      let e' = Environment.add_ez_binder lname input_type e in
+      let%bind body = type_expression' ?tv_opt:(Some tv_out) e' result in
+      let output_type = body.type_annotation in
+      let lambda' = make_a_e (E_lambda {binder = lname ; body}) (t_function input_type output_type ()) e in
+      let lst' = [v_col; v_initr ; lambda'] in
+      let tv_lst = List.map get_type_annotation lst' in
+      let%bind (opname', tv) =
+        type_constant opname tv_lst tv_opt ae.location in
+      return (E_constant (opname' , lst')) tv
+  | E_constant (name, lst) ->
+      let%bind lst' = bind_list @@ List.map (type_expression' e) lst in
+      let tv_lst = List.map get_type_annotation lst' in
+      let%bind (name', tv) =
+        type_constant name tv_lst tv_opt ae.location in
+      return (E_constant (name' , lst')) tv
+  | E_application (f, arg) ->
+      let%bind f' = type_expression' e f in
+      let%bind arg = type_expression' e arg in
+      let%bind tv = match f'.type_annotation.type_value' with
+        | T_function (param, result) ->
+            let%bind _ = O.assert_type_value_eq (param, arg.type_annotation) in
+            ok result
+        | _ ->
+          fail @@ type_error_approximate
+            ~expected:"should be a function type"
+            ~expression:f
+            ~actual:f'.type_annotation
+            f'.location
+      in
+      return (E_application (f' , arg)) tv
+  | E_look_up dsi ->
+      let%bind (ds, ind) = bind_map_pair (type_expression' e) dsi in
+      let%bind (src, dst) = bind_map_or (get_t_map , get_t_big_map) ds.type_annotation in
+      let%bind _ = O.assert_type_value_eq (ind.type_annotation, src) in
+      return (E_look_up (ds , ind)) (t_option dst ())
+  (* Advanced *)
+  | E_matching (ex, m) -> (
+      let%bind ex' = type_expression' e ex in
+      let%bind m' = type_match (type_expression' ?tv_opt:None) e ex'.type_annotation m ae ae.location in
+      let tvs =
+        let aux (cur:O.value O.matching) =
+          match cur with
+          | Match_bool { match_true ; match_false } -> [ match_true ; match_false ]
+          | Match_list { match_nil ; match_cons = ((_ , _) , match_cons) } -> [ match_nil ; match_cons ]
+          | Match_option { match_none ; match_some = (_ , match_some) } -> [ match_none ; match_some ]
+          | Match_tuple (_ , match_tuple) -> [ match_tuple ]
+          | Match_variant (lst , _) -> List.map snd lst in
+        List.map get_type_annotation @@ aux m' in
+      let aux prec cur =
+        let%bind () =
+          match prec with
+          | None -> ok ()
+          | Some cur' -> Ast_typed.assert_type_value_eq (cur , cur') in
+        ok (Some cur) in
+      let%bind tv_opt = bind_fold_list aux None tvs in
+      let%bind tv =
+        trace_option (match_empty_variant m ae.location) @@
+        tv_opt in
+      return (O.E_matching (ex', m')) tv
+    )
+  | E_sequence (a , b) ->
+    let%bind a' = type_expression' e a in
+    let%bind b' = type_expression' e b in
+    let a'_type_annot = get_type_annotation a' in
+    let%bind () =
+      trace_strong (type_error
+                      ~msg:"first part of the sequence should be of unit type"
+                      ~expected:(O.t_unit ())
+                      ~actual:a'_type_annot
+                      ~expression:a
+                      a'.location) @@
+      Ast_typed.assert_type_value_eq (t_unit () , a'_type_annot) in
+    return (O.E_sequence (a' , b')) (get_type_annotation b')
+  | E_loop (expr , body) ->
+    let%bind expr' = type_expression' e expr in
+    let%bind body' = type_expression' e body in
+    let t_expr' = get_type_annotation expr' in
+    let%bind () =
+      trace_strong (type_error
+                      ~msg:"while condition isn't of type bool"
+                      ~expected:(O.t_bool ())
+                      ~actual:t_expr'
+                      ~expression:expr
+                      expr'.location) @@
+      Ast_typed.assert_type_value_eq (t_bool () , t_expr') in
+    let t_body' = get_type_annotation body' in
+    let%bind () =
+      trace_strong (type_error
+                     ~msg:"while body isn't of unit type"
+                     ~expected:(O.t_unit ())
+                     ~actual:t_body'
+                     ~expression:body
+                     body'.location) @@
+      Ast_typed.assert_type_value_eq (t_unit () , t_body') in
+    return (O.E_loop (expr' , body')) (t_unit ())
+  | E_assign (name , path , expr) ->
+    let%bind typed_name =
+      let%bind ele = Environment.get_trace name e in
+      ok @@ make_n_t name ele.type_value in
+    let%bind (assign_tv , path') =
+      let aux : ((_ * O.access_path) as 'a) -> I.access -> 'a result = fun (prec_tv , prec_path) cur_path ->
+        match cur_path with
+        | Access_tuple index -> (
+            let%bind tpl = get_t_tuple prec_tv in
+            let%bind tv' =
+              trace_option (bad_tuple_index index ae prec_tv ae.location) @@
+              List.nth_opt tpl index in
+            ok (tv' , prec_path @ [O.Access_tuple index])
+          )
+        | Access_record property -> (
+            let%bind m = get_t_record prec_tv in
+            let%bind tv' =
+              trace_option (bad_record_access property ae prec_tv ae.location) @@
+              Map.String.find_opt property m in
+            ok (tv' , prec_path @ [O.Access_record property])
+          )
+        | Access_map _ ->
+          fail @@ not_supported_yet "assign expressions with maps are not supported yet" ae
+      in
+      bind_fold_list aux (typed_name.type_value , []) path in
+    let%bind expr' = type_expression' e ~tv_opt:assign_tv expr in
+    let t_expr' = get_type_annotation expr' in
+    let%bind () =
+      trace_strong (type_error
+                     ~msg:"type of the expression to assign doesn't match left-hand-side"
+                     ~expected:assign_tv
+                     ~actual:t_expr'
+                     ~expression:expr
+                     expr'.location) @@
+      Ast_typed.assert_type_value_eq (assign_tv , t_expr') in
+    return (O.E_assign (typed_name , path' , expr')) (t_unit ())
+  | E_let_in {binder ; rhs ; result} ->
+    let%bind rhs_tv_opt = bind_map_option (evaluate_type e) (snd binder) in
+    let%bind rhs = type_expression' ?tv_opt:rhs_tv_opt e rhs in
+    let e' = Environment.add_ez_declaration (fst binder) rhs e in
+    let%bind result = type_expression' e' result in
+    return (E_let_in {binder = fst binder; rhs; result}) result.type_annotation
+  | E_annotation (expr , te) ->
+    let%bind tv = evaluate_type e te in
+    let%bind expr' = type_expression' ~tv_opt:tv e expr in
+    let%bind type_annotation =
+      O.merge_annotation
+        (Some tv)
+        (Some expr'.type_annotation)
+        (internal_assertion_failure "merge_annotations (Some ...) (Some ...) failed") in
+    ok {expr' with type_annotation}
+
+
+and type_constant (name:string) (lst:O.type_value list) (tv_opt:O.type_value option) (loc : Location.t) : (string * O.type_value) result =
+  (* Constant poorman's polymorphism *)
+  let ct = Operators.Typer.constant_typers in
+  let%bind typer =
+    trace_option (unrecognized_constant name loc) @@
+    Map.String.find_opt name ct in
+  trace (constant_error loc lst tv_opt) @@
+  typer lst tv_opt
+
+let untype_type_value (t:O.type_value) : (I.type_expression) result =
+  match t.simplified with
+  | Some s -> ok s
+  | _ -> fail @@ internal_assertion_failure "trying to untype generated type"
+
+let untype_literal (l:O.literal) : I.literal result =
+  let open I in
+  match l with
+  | Literal_unit -> ok Literal_unit
+  | Literal_bool b -> ok (Literal_bool b)
+  | Literal_nat n -> ok (Literal_nat n)
+  | Literal_timestamp n -> ok (Literal_timestamp n)
+  | Literal_mutez n -> ok (Literal_mutez n)
+  | Literal_int n -> ok (Literal_int n)
+  | Literal_string s -> ok (Literal_string s)
+  | Literal_bytes b -> ok (Literal_bytes b)
+  | Literal_address s -> ok (Literal_address s)
+  | Literal_operation s -> ok (Literal_operation s)
+
+let rec untype_expression (e:O.annotated_expression) : (I.expression) result =
+  let open I in
+  let return e = ok e in
+  match e.expression with
+  | E_literal l ->
+      let%bind l = untype_literal l in
+      return (e_literal l)
+  | E_constant (n, lst) ->
+      let%bind lst' = bind_map_list untype_expression lst in
+      return (e_constant n lst')
+  | E_variable n ->
+      return (e_variable n)
+  | E_application (f, arg) ->
+      let%bind f' = untype_expression f in
+      let%bind arg' = untype_expression arg in
+      return (e_application f' arg')
+  | E_lambda {binder ; body} -> (
+      let%bind io = get_t_function e.type_annotation in
+      let%bind (input_type , output_type) = bind_map_pair untype_type_value io in
+      let%bind result = untype_expression body in
+      return (e_lambda binder (Some input_type) (Some output_type) result)
+    )
+  | E_tuple lst ->
+      let%bind lst' = bind_list
+        @@ List.map untype_expression lst in
+      return (e_tuple lst')
+  | E_tuple_accessor (tpl, ind)  ->
+      let%bind tpl' = untype_expression tpl in
+      return (e_accessor tpl' [Access_tuple ind])
+  | E_constructor (n, p) ->
+      let%bind p' = untype_expression p in
+      return (e_constructor n p')
+  | E_record r ->
+      let%bind r' = bind_smap
+        @@ SMap.map untype_expression r in
+      return (e_record r')
+  | E_record_accessor (r, s) ->
+      let%bind r' = untype_expression r in
+      return (e_accessor r' [Access_record s])
+  | E_map m ->
+      let%bind m' = bind_map_list (bind_map_pair untype_expression) m in
+      return (e_map m')
+  | E_big_map m ->
+      let%bind m' = bind_map_list (bind_map_pair untype_expression) m in
+      return (e_big_map m')
+  | E_list lst ->
+      let%bind lst' = bind_map_list untype_expression lst in
+      return (e_list lst')
+  | E_set lst ->
+      let%bind lst' = bind_map_list untype_expression lst in
+      return (e_set lst')
+  | E_look_up dsi ->
+      let%bind (a , b) = bind_map_pair untype_expression dsi in
+      return (e_look_up a b)
+  | E_matching (ae, m) ->
+      let%bind ae' = untype_expression ae in
+      let%bind m' = untype_matching untype_expression m in
+      return (e_matching ae' m')
+  | E_sequence _
+  | E_loop _
+  | E_assign _ -> fail @@ not_supported_yet_untranspile "not possible to untranspile statements yet" e.expression
+  | E_let_in {binder;rhs;result} ->
+      let%bind tv = untype_type_value rhs.type_annotation in
+      let%bind rhs = untype_expression rhs in
+      let%bind result = untype_expression result in
+      return (e_let_in (binder , (Some tv)) rhs result)
+
+and untype_matching : type o i . (o -> i result) -> o O.matching -> (i I.matching) result = fun f m ->
+  let open I in
+  match m with
+  | Match_bool {match_true ; match_false} ->
+      let%bind match_true = f match_true in
+      let%bind match_false = f match_false in
+      ok @@ Match_bool {match_true ; match_false}
+  | Match_tuple (lst, b) ->
+      let%bind b = f b in
+      ok @@ Match_tuple (lst, b)
+  | Match_option {match_none ; match_some = (v, some)} ->
+      let%bind match_none = f match_none in
+      let%bind some = f some in
+      let match_some = fst v, some in
+      ok @@ Match_option {match_none ; match_some}
+  | Match_list {match_nil ; match_cons = (((hd_name , _) , (tl_name , _)), cons)} ->
+      let%bind match_nil = f match_nil in
+      let%bind cons = f cons in
+      let match_cons = hd_name , tl_name , cons in
+      ok @@ Match_list {match_nil ; match_cons}
+  | Match_variant (lst , _) ->
+      let aux ((a,b),c) =
+        let%bind c' = f c in
+        ok ((a,b),c') in
+      let%bind lst' = bind_map_list aux lst in
+      ok @@ Match_variant lst'
diff --git a/src/passes/4-typer/typer.mli b/src/passes/4-typer/typer.mli
new file mode 100644
index 000000000..ddc2ebb59
--- /dev/null
+++ b/src/passes/4-typer/typer.mli
@@ -0,0 +1,59 @@
+open Trace
+
+module I = Ast_simplified
+module O = Ast_typed
+
+module SMap = O.SMap
+module Environment = O.Environment
+
+module Solver : sig
+  type state = Placeholder_for_state_of_new_typer
+  val discard_state : state -> unit
+  val initial_state : state
+end
+
+type environment = Environment.t
+
+module Errors : sig
+  (*
+  val unbound_type_variable : environment -> string -> unit -> error
+  val unbound_variable : environment -> string -> Location.t -> unit -> error
+  val match_empty_variant : 'a I.matching -> Location.t -> unit -> error
+  val match_missing_case : 'a I.matching -> Location.t -> unit -> error
+  val match_redundant_case : 'a I.matching -> Location.t -> unit -> error
+  val unbound_constructor : environment -> string -> Location.t -> unit -> error
+  val unrecognized_constant : string -> Location.t -> unit -> error
+  *)
+  val wrong_arity : string -> int -> int -> Location.t -> unit -> error
+  (*
+  val match_tuple_wrong_arity : 'a list -> 'b list -> Location.t -> unit -> error
+
+  (* TODO: this should be a trace_info? *)
+  val program_error : I.program -> unit -> error
+  val constant_declaration_error : string -> I.expr -> O.type_value option -> unit -> error
+  val match_error : ?msg:string -> expected:'a I.matching -> actual:O.type_value -> Location.t -> unit -> error
+  val needs_annotation : I.expression -> string -> unit -> error
+  val type_error_approximate : ?msg:string -> expected:string -> actual:O.type_value -> expression:I.expression -> Location.t -> unit -> error
+  val type_error : ?msg:string -> expected:O.type_value -> actual:O.type_value -> expression:I.expression -> Location.t -> unit -> error
+  val bad_tuple_index : int -> I.expression -> O.type_value -> Location.t -> unit -> error
+  val bad_record_access : string -> I.expression -> O.type_value -> Location.t -> unit -> error
+  val not_supported_yet : string -> I.expression -> unit -> error
+  val not_supported_yet_untranspile : string -> O.expression -> unit -> error
+  val constant_error : Location.t -> O.type_value list -> O.type_value option -> unit -> error
+  *)
+end
+
+val type_program : I.program -> (O.program * Solver.state) result
+val type_declaration : environment -> Solver.state -> I.declaration -> (environment * Solver.state * O.declaration option) result
+(* val type_match : (environment -> 'i -> 'o result) -> environment -> O.type_value -> 'i I.matching -> I.expression -> Location.t -> 'o O.matching result *)
+val evaluate_type : environment -> I.type_expression -> O.type_value result
+val type_expression : environment -> Solver.state -> ?tv_opt:O.type_value -> I.expression -> (O.annotated_expression * Solver.state) result
+val type_constant : string -> O.type_value list -> O.type_value option -> Location.t -> (string * O.type_value) result
+(*
+val untype_type_value : O.type_value -> (I.type_expression) result
+val untype_literal : O.literal -> I.literal result
+*)
+val untype_expression : O.annotated_expression -> I.expression result
+(*
+val untype_matching : ('o -> 'i result) -> 'o O.matching -> ('i I.matching) result
+*)