implement syntax for arity zero vs arity one in uncurried application

Since there is no syntax space for arity zero vs arity one,
we parse
  `fn(. ())` into
  `fn(. {let __res_unit = (); __res_unit})`
  when the parsetree is intended for type checking

`fn(.)` is treated as zero arity application

Author: Iwan

src/res_core.ml

@@ -3267,14 +3267,13 @@ and parseArgument p =
     match p.Parser.token with
     | Dot ->
       let uncurried = true in
-      let startPos = p.Parser.startPos in
       Parser.next(p);
       begin match p.token with
         (* apply(.) *)
         | Rparen ->
-          let loc = mkLoc startPos p.prevEndPos in
-          let unitExpr = Ast_helper.Exp.construct ~loc
-            (Location.mkloc (Longident.Lident "()") loc) None
+          let unitExpr = Ast_helper.Exp.construct
+            (Location.mknoloc (Longident.Lident "()"))
+            None
           in
           Some (uncurried, Asttypes.Nolabel, unitExpr)
         | _ ->
@@ -3368,6 +3367,37 @@ and parseCallExpr p funExpr =
       Ast_helper.Exp.construct
         ~loc (Location.mkloc (Longident.Lident "()") loc) None
     ]
+  | [
+      true,
+      Asttypes.Nolabel,
+      ({
+        pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, None);
+        pexp_loc = loc;
+        pexp_attributes = []
+      } as expr)
+    ] 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})`
+       *  when the parsetree is intended for type checking
+       *
+       *  Note:
+       *    `fn(.)` is treated as zero arity application.
+       *  The invisible unit expression here has loc_ghost === true
+       *
+       *  Related: https://github.com/rescript-lang/syntax/issues/138
+       *)
+   [
+     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")))
+   ]
   | args -> args
   in
   let loc = {funExpr.pexp_loc with loc_end = p.prevEndPos} in

src/res_printer.ml

@@ -4013,8 +4013,15 @@ and printArgumentsWithCallbackInLastPosition ~uncurried args cmtTbl =
 
 and printArguments ~uncurried (args : (Asttypes.arg_label * Parsetree.expression) list) cmtTbl =
   match args with
-  | [Nolabel, {pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, _)}] ->
-    if uncurried then Doc.text "(.)" else Doc.text "()"
+  | [Nolabel, {pexp_desc = Pexp_construct ({txt = Longident.Lident "()"}, _); pexp_loc = loc}] ->
+    (* See "parseCallExpr", ghost unit expression is used the implement
+     * arity zero vs arity one syntax.
+     * Related: https://github.com/rescript-lang/syntax/issues/138 *)
+    begin match uncurried, loc.loc_ghost with
+    | true, true -> Doc.text "(.)" (* arity zero *)
+    | true, false -> Doc.text "(. ())" (* arity one *)
+    | _ -> Doc.text "()"
+    end
   | [(Nolabel, arg)] when ParsetreeViewer.isHuggableExpression arg ->
     let argDoc =
       let doc = printExpressionWithComments arg cmtTbl in

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

@@ -12,7 +12,8 @@ let unitUncurried = ((apply ())[@bs ])
 `;
 
 exports[`argument.js 1`] = `
-"let foo ~a:((a)[@ns.namedArgLoc ])  = ((a ())[@bs ]) +. 1.
+"let foo ~a:((a)[@ns.namedArgLoc ])  =
+  ((a (let __res_unit = () in __res_unit))[@bs ]) +. 1.
 let a = ((fun () -> 2)[@bs ])
 let bar = foo ~a:((a)[@ns.namedArgLoc ])
 let comparisonResult =
@@ -21,7 +22,9 @@ let comparisonResult =
 ;;((callback firstNode ~y:((y)[@ns.namedArgLoc ]))[@bs ])
 ;;((document.createElementWithOptions \\"div\\"
       (elementProps ~onClick:((fun _ -> Js.log \\"hello world\\")
-         [@ns.namedArgLoc ])))[@bs ])"
+         [@ns.namedArgLoc ])))[@bs ])
+;;((resolve ())[@bs ])
+;;((resolve (let __res_unit = () in __res_unit))[@bs ])"
 `;
 
 exports[`array.js 1`] = `

tests/parsing/grammar/expressions/argument.js

@@ -7,3 +7,7 @@ callback(. firstNode, ~y)
 document.createElementWithOptions(. "div", elementProps(~onClick=_ =>
     Js.log("hello world")
   ))
+
+
+resolve(.)
+resolve(. ())

tests/printer/expr/__snapshots__/render.spec.js.snap

@@ -73,6 +73,9 @@ document.createElementWithOptions(.
   \\"div\\",
   elementProps(~onClick=_ => Js.log(\\"hello world\\")),
 )
+
+resolve(.)
+resolve(. ())
 "
 `;
 

tests/printer/expr/apply.js

@@ -46,3 +46,6 @@ call(~\"let": int)
 document.createElementWithOptions(. "div", elementProps(~onClick=_ =>
     Js.log("hello world")
   ))
+
+resolve(.)
+resolve(. ())