114.08+63

README.md

@@ -1,33 +1,32 @@
-A ppx rewriter for monadic and applicative let bindings and match statements.
+A ppx rewriter for monadic and applicative let bindings, match expressions, and
+if expressions.
 
 Overview
 --------
 
-The aim of this rewriter is to make monadic and applicative code look
+The aim of this rewriter is to make monadic and applicative code look nicer by
-nicer by writing custom binders the same way that we normally bind
+writing custom binders the same way that we normally bind variables. In OCaml,
-variables. In OCaml, the common way to bind the result of a
+the common way to bind the result of a computation to a variable is:
-computation to a variable is:
 
 ```ocaml
 let VAR = EXPR in BODY
 ```
 
-ppx\_let simply adds two new binders: `let%bind` and
+ppx\_let simply adds two new binders: `let%bind` and `let%map`. These are
-`let%map`. These are rewritten into calls to the `bind` and
+rewritten into calls to the `bind` and `map` functions respectively. These
-`map` functions respectively.  These functions are expected to have
+functions are expected to have
 
 ```ocaml
-val map : 'a t -> f:('a -> 'b) -> 'b t
+val map  : 'a t -> f:('a -> 'b)   -> 'b t
-val bind : 'a t -> ('a -> 'b t) -> 'b t
+val bind : 'a t -> f:('a -> 'b t) -> 'b t
 ```
 
 for some type `t`, as one might expect.
 
-These functions are to be provided by the user, and are generally
+These functions are to be provided by the user, and are generally expected to be
-expected to be part of the signatures of monads and applicatives
+part of the signatures of monads and applicatives modules. This is the case for
-modules.  This is the case for all monads and applicatives defined by
+all monads and applicatives defined by the Jane Street's Core suite of
-the Jane Street's Core suite of libraries. (see the section below on
+libraries. (see the section below on getting the right names into scope).
-getting the right names into scope).
 
 ### Parallel bindings
 
@@ -37,9 +36,9 @@ ppx\_let understands parallel bindings as well. i.e.:
 let%bind VAR1 = EXPR1 and VAR2 = EXPR2 and VAR3 = EXPR3 in BODY
 ```
 
-The `and` keyword is seen as a binding combination operator. To do so
+The `and` keyword is seen as a binding combination operator. To do so it expects
-it expects the presence of a `both` function, that lifts the OCaml
+the presence of a `both` function, that lifts the OCaml pair operation to the
-pair operation to the type `t` in question:
+type `t` in question:
 
 ```ocaml
 val both : 'a t -> 'b t -> ('a * 'b) t
@@ -47,47 +46,61 @@ val both : 'a t -> 'b t -> ('a * 'b) t
 
 ### Match statements
 
-We found that this form was quite useful for match statements as
+We found that this form was quite useful for match statements as well. So for
-well. So for convenience ppx\_let also accepts `%bind` and `%map` on
+convenience ppx\_let also accepts `%bind` and `%map` on the `match` keyword.
-the `match` keyword. Morally `match%bind expr with cases` is seen as
+Morally `match%bind expr with cases` is seen as `let%bind x = expr in match x
-`let%bind x = expr in match x with cases`.
+with cases`.
+
+### If statements
+
+As a further convenience, ppx\_let accepts `%bind` and `%map` on the `if`
+keyword. The expression `if%bind expr1 then expr2 else expr3` is morally
+equivalent to `let%bind p = expr1 in if p then expr2 else expr3`.
 
 Syntactic forms and actual rewriting
 ------------------------------------
 
-`ppx_let` adds four syntactic forms
+`ppx_let` adds six syntactic forms
 
 ```ocaml
 let%bind P = M in E
 
-let%map P = M in E
+let%map  P = M in E
 
 match%bind M with P1 -> E1 | P2 -> E2 | ...
 
-match%map M with P1 -> E1 | P2 -> E2 | ...
+match%map  M with P1 -> E1 | P2 -> E2 | ...
+
+if%bind M then E1 else E2
+
+if%map  M then E1 else E2
 ```
 
 that expand into
 
 ```ocaml
-bind M (fun P -> E)
+bind M ~f:(fun P -> E)
 
-map M (fun P -> E)
+map  M ~f:(fun P -> E)
 
-bind M (function P1 -> E1 | P2 -> E2 | ...)
+bind M ~f:(function P1 -> E1 | P2 -> E2 | ...)
 
-map M (function P1 -> E1 | P2 -> E2 | ...)
+map  M ~f:(function P1 -> E1 | P2 -> E2 | ...)
+
+bind M ~f:(function true -> E1 | false -> E2)
+
+map  M ~f:(function true -> E1 | false -> E2)
 ```
 
 respectively.
 
-As with `let`, `let%bind` and `let%map` also support multiple
+As with `let`, `let%bind` and `let%map` also support multiple *parallel*
-*parallel* bindings via the `and` keyword:
+bindings via the `and` keyword:
 
 ```ocaml
 let%bind P1 = M1 and P2 = M2 and P3 = M3 and P4 = M4 in E
 
-let%map P1 = M1 and P2 = M2 and P3 = M3 and P4 = M4 in E
+let%map  P1 = M1 and P2 = M2 and P3 = M3 and P4 = M4 in E
 ```
 
 that expand into
@@ -96,35 +109,34 @@ that expand into
 let x1 = M1 and x2 = M2 and x3 = M3 and x4 = M4 in
 bind
   (both x1 (both x2 (both x3 x4)))
-  (fun (P1, (P2, (P3, P4))) -> E)
+  ~f:(fun (P1, (P2, (P3, P4))) -> E)
 
 let x1 = M1 and x2 = M2 and x3 = M3 and x4 = M4 in
 map
   (both x1 (both x2 (both x3 x4)))
-  (fun (P1, (P2, (P3, P4))) -> E)
+  ~f:(fun (P1, (P2, (P3, P4))) -> E)
 ```
 
-respectively. (Instead of `x1`, `x2`, ... ppx\_let uses
+respectively. (Instead of `x1`, `x2`, ... ppx\_let uses variable names that are
-variable names that are unlikely to clash with other names)
+unlikely to clash with other names)
 
-As with `let`, names introduced by left-hand sides of the let bindings
+As with `let`, names introduced by left-hand sides of the let bindings are not
-are not available in subsequent right-hand sides of the same sequence.
+available in subsequent right-hand sides of the same sequence.
 
 Getting the right names in scope
 --------------------------------
 
-The description of how the `%bind` and `%map` syntax extensions expand
+The description of how the `%bind` and `%map` syntax extensions expand left out
-left out the fact that the names `bind`, `map`, `both`, and `return`
+the fact that the names `bind`, `map`, `both`, and `return` are not used
-are not used directly, but rather qualified by `Let_syntax`.  For
+directly, but rather qualified by `Let_syntax`. For example, we use
-example, we use `Let_syntax.bind` rather than merely `bind`.  This
+`Let_syntax.bind` rather than merely `bind`. This means one just needs to get a
-means one just needs to get a properly loaded `Let_syntax` module in
+properly loaded `Let_syntax` module in scope to use `%bind` and `%map`.
-scope to use `%bind` and `%map`.
 
-For monads, `Core.Std.Monad.Make` produces a submodule `Let_syntax` of
+For monads, `Core.Std.Monad.Make` produces a submodule `Let_syntax` of the
-the appropriate form.
+appropriate form.
 
-For applicatives.  The convention for these modules is to have a
+For applicatives. The convention for these modules is to have a submodule
-submodule `Let_syntax` of the form
+`Let_syntax` of the form
 
 ```ocaml
 module Let_syntax : sig
@@ -135,15 +147,13 @@ module Let_syntax : sig
 end
 ```
 
-The `Open_on_rhs` submodule is used by variants of `%map` and `%bind`
+The `Open_on_rhs` submodule is used by variants of `%map` and `%bind` called
-called `%map_open` and `%bind_open`.  It is locally opened on the
+`%map_open` and `%bind_open`. It is locally opened on the right hand sides of
-right hand sides of the rewritten let bindings in `%map_open` and
+the rewritten let bindings in `%map_open` and `%bind_open` expressions. For
-`%bind_open` expressions.  For `match%map_open` and `match%bind_open`
+`match%map_open` and `match%bind_open` expressions, `Open_on_rhs` is opened for
-expressions, `Open_on_rhs` is opened for the expression being matched
+the expression being matched on.
-on.
 
-`Open_on_rhs` is useful when programming with applicatives, which
+`Open_on_rhs` is useful when programming with applicatives, which operate in a
-operate in a staged manner where the operators used to construct the
+staged manner where the operators used to construct the applicatives are
-applicatives are distinct from the operators used to manipulate the
+distinct from the operators used to manipulate the values those applicatives
-values those applicatives produce.  For monads, `Open_on_rhs` contains
+produce. For monads, `Open_on_rhs` contains `return`.
-`return`.

_oasis

@@ -2,7 +2,7 @@ OASISFormat:           0.4
 OCamlVersion:          >= 4.03.0
 FindlibVersion:        >= 1.3.2
 Name:                  ppx_let
-Version:               114.08+30
+Version:               114.08+63
 Synopsis:              Monadic let-bindings
 Authors:               Jane Street Group, LLC <opensource@janestreet.com>
 Copyrights:            (C) 2015-2016 Jane Street Group LLC <opensource@janestreet.com>

opam

@@ -11,10 +11,11 @@ build: [
 ]
 depends: [
   "ocamlbuild"     {build}
-  "oasis"          {build & >= "0.4" & < "0.4.7"}
+  "oasis"          {build & >= "0.4"}
   "ocamlfind"      {build & >= "1.3.2"}
   "js-build-tools" {build}
   "ppx_core"
   "ppx_driver"
 ]
 available: [ ocaml-version >= "4.03.0" ]
+conclicts: [ "oasis" {= "0.4.7"} ]

src/ppx_let.ml

@@ -93,6 +93,12 @@ let expand_match extension_name ~loc expr cases =
     ~fn:(pexp_function ~loc cases)
 ;;
 
+let expand_if extension_name ~loc expr then_ else_ =
+  expand_match extension_name ~loc expr
+    [ case ~lhs:(pbool ~loc true)  ~guard:None ~rhs:then_
+    ; case ~lhs:(pbool ~loc false) ~guard:None ~rhs:else_
+    ]
+
 let expand ~loc:_ ~path:_ extension_name expr =
   let loc = expr.pexp_loc in
   let expansion =
@@ -110,9 +116,18 @@ let expand ~loc:_ ~path:_ extension_name expr =
         (Extension_name.to_string extension_name)
     | Pexp_match (expr, cases) ->
       expand_match extension_name ~loc expr cases
+    | Pexp_ifthenelse (expr, then_, else_) ->
+      let else_ =
+        match else_ with
+        | Some else_ -> else_
+        | None ->
+          Location.raise_errorf ~loc "'if%%%s' must include an else branch"
+            (Extension_name.to_string extension_name)
+      in
+      expand_if extension_name ~loc expr then_ else_
     | _ ->
       Location.raise_errorf ~loc
-        "'%%%s' can only be used with 'let' and 'match'"
+        "'%%%s' can only be used with 'let', 'match', and 'if'"
         (Extension_name.to_string extension_name)
   in
   { expansion with pexp_attributes = expr.pexp_attributes @ expansion.pexp_attributes }

test/test.ml

@@ -57,6 +57,16 @@ module Monad_example = struct
     match%map a with
     | 0 -> true
     | _ -> false
+
+  let _mif a : _ X.t =
+    if%bind_open a
+    then return true
+    else return false
+
+  let _mif' a : _ X.t =
+    if%map a
+    then true
+    else false
 end
 
 module Applicative_example = struct