Parse arity1 as f(. ignore())

The use of `ignore()` prevents possible naming conflicts with the user putting magic binding names in scope.

Author: Iwan

src/res_ast_conversion.ml

@@ -413,6 +413,8 @@ let normalize =
           ]
         ) when hasUncurriedAttribute expr.pexp_attributes
         ->
+        (* In Reason both f(.) and f(. ()) express arity0. In ReScript only f(.)
+         * expresses arity0. We tweak the Reason version to always match the f(.) on the Reason side. *)
         {expr with
           pexp_attributes = mapper.attributes mapper expr.pexp_attributes;
           pexp_desc = Pexp_apply (

src/res_core.ml

@@ -3423,12 +3423,12 @@ and parseCallExpr p funExpr =
         pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, None);
         pexp_loc = loc;
         pexp_attributes = []
-      } as expr)
+      } as unitExpr)
     ] when (not loc.loc_ghost) && p.mode = ParseForTypeChecker ->
       (*  Since there is no syntax space for arity zero vs arity one,
        *  we expand
        *    `fn(. ())` into
-       *    `fn(. {let __res_unit = (); __res_unit})`
+       *    `fn(. ignore())`
        *  when the parsetree is intended for type checking
        *
        *  Note:
@@ -3440,12 +3440,12 @@ and parseCallExpr p funExpr =
    [
      true,
      Asttypes.Nolabel,
-     Ast_helper.Exp.let_
-      Asttypes.Nonrecursive
-      [Ast_helper.Vb.mk
-        (Ast_helper.Pat.var (Location.mknoloc "__res_unit"))
-        expr]
-      (Ast_helper.Exp.ident (Location.mknoloc (Longident.Lident "__res_unit")))
+     Ast_helper.Exp.apply
+       (Ast_helper.Exp.ident (Location.mknoloc (Longident.Lident "ignore")))
+       [
+         Asttypes.Nolabel,
+         unitExpr
+       ]
    ]
   | args -> args
   in

tests/conversion/reason/__snapshots__/render.spec.js.snap

@@ -1599,6 +1599,12 @@ let _ =
       ),
     )
   )
+
+let () = {
+  resolve(.)
+  resolve(.)
+  resolve(. ignore())
+}
 "
 `;
 

tests/conversion/reason/uncurrried.re

@@ -75,3 +75,9 @@ let _ =
         ),
       )
     });
+
+let () = {
+  resolve(.)
+  resolve(. ())
+  resolve(. ignore())
+}

tests/parsing/grammar/expressions/__snapshots__/parse.spec.js.snap

@@ -12,8 +12,7 @@ let unitUncurried = ((apply ())[@bs ])
 `;
 
 exports[`argument.js 1`] = `
-"let foo ~a:((a)[@ns.namedArgLoc ])  =
-  ((a (let __res_unit = () in __res_unit))[@bs ]) +. 1.
+"let foo ~a:((a)[@ns.namedArgLoc ])  = ((a (ignore ()))[@bs ]) +. 1.
 let a = ((fun () -> 2)[@bs ])
 let bar = foo ~a:((a)[@ns.namedArgLoc ])
 let comparisonResult =
@@ -24,7 +23,7 @@ let comparisonResult =
       (elementProps ~onClick:((fun _ -> Js.log \\"hello world\\")
          [@ns.namedArgLoc ])))[@bs ])
 ;;((resolve ())[@bs ])
-;;((resolve (let __res_unit = () in __res_unit))[@bs ])"
+;;((resolve (ignore ()))[@bs ])"
 `;
 
 exports[`array.js 1`] = `