Add surface syntax for partial application of uncurried functions (#6166)

* Add surface syntax for partial application of uncurried functions

Fixes #6165

This PR introduces a new surface syntax for partial application of uncurried functions in ReScript. The syntax allows developers to partially apply uncurried functions more easily and concisely using the ... token in argument lists. The implementation includes changes to the parser, grammar, and printer to handle the new syntax properly.

Example:

res
Copy code
let add = (a, b) => a + b
let ptl1 = add(1, ...)
let result = ptl1(2) // result will be 3
This example demonstrates the new syntax for partial application of the uncurried add function.

* Update CHANGELOG.md

Co-authored-by: Christoph Knittel <[email protected]>

* Update CHANGELOG.md

* Update CHANGELOG.md

---------

Co-authored-by: Christoph Knittel <[email protected]>

Author: cristianoc

CHANGELOG.md

@@ -12,6 +12,9 @@
 
 # 11.0.0-alpha.4 (Unreleased)
 
+#### :rocket: Main New Feature
+- Add surface syntax for partial application of uncurried functions: `foo(1, ...)`. This corresponds to curried application in the old mode. https://github.com/rescript-lang/rescript-compiler/pull/6166
+
 #### :bug: Bug Fix
 
 - Fix broken formatting in uncurried mode for functions with _ placeholder args. https://github.com/rescript-lang/rescript-compiler/pull/6148

res_syntax/src/res_core.ml

@@ -3598,6 +3598,17 @@ and parseCallExpr p funExpr =
     parseCommaDelimitedRegion ~grammar:Grammar.ArgumentList ~closing:Rparen
       ~f:parseArgument p
   in
+  let resPartialAttr =
+    let loc = mkLoc startPos p.prevEndPos in
+    (Location.mkloc "res.partial" loc, Parsetree.PStr [])
+  in
+  let isPartial =
+    match p.token with
+    | DotDotDot when args <> [] ->
+      Parser.next p;
+      true
+    | _ -> false
+  in
   Parser.expect Rparen p;
   let args =
     match args with
@@ -3626,7 +3637,8 @@ and parseCallExpr p funExpr =
          } as expr;
      };
     ]
-      when (not loc.loc_ghost) && p.mode = ParseForTypeChecker ->
+      when (not loc.loc_ghost) && p.mode = ParseForTypeChecker && not isPartial
+      ->
       (*  Since there is no syntax space for arity zero vs arity one,
        *  we expand
        *    `fn(. ())` into
@@ -3670,22 +3682,20 @@ and parseCallExpr p funExpr =
     | [] -> []
   in
   let apply =
-    List.fold_left
-      (fun callBody group ->
+    Ext_list.fold_left args funExpr (fun callBody group ->
         let dotted, args = group in
         let args, wrap = processUnderscoreApplication p args in
         let exp =
           let uncurried =
             p.uncurried_config |> Res_uncurried.fromDotted ~dotted
           in
-          if uncurried then
-            let attrs = [uncurriedAppAttr] in
-            Ast_helper.Exp.apply ~loc ~attrs callBody args
-          else Ast_helper.Exp.apply ~loc callBody args
+          let attrs = if uncurried then [uncurriedAppAttr] else [] in
+          let attrs = if isPartial then resPartialAttr :: attrs else attrs in
+          Ast_helper.Exp.apply ~loc ~attrs callBody args
         in
         wrap exp)
-      funExpr args
   in
+
   Parser.eatBreadcrumb p;
   apply
 

res_syntax/src/res_grammar.ml

@@ -299,7 +299,7 @@ let isListTerminator grammar token =
   | _, Token.Eof
   | ExprList, (Rparen | Forwardslash | Rbracket)
   | ListExpr, Rparen
-  | ArgumentList, Rparen
+  | ArgumentList, (Rparen | DotDotDot)
   | TypExprList, (Rparen | Forwardslash | GreaterThan | Equal)
   | ModExprList, Rparen
   | ( (PatternList | PatternOcamlList | PatternRecord),

res_syntax/src/res_parsetree_viewer.ml

@@ -72,6 +72,15 @@ let processUncurriedAppAttribute attrs =
   in
   process false [] attrs
 
+let processPartialAppAttribute attrs =
+  let rec process partialApp acc attrs =
+    match attrs with
+    | [] -> (partialApp, List.rev acc)
+    | ({Location.txt = "res.partial"}, _) :: rest -> process true acc rest
+    | attr :: rest -> process partialApp (attr :: acc) rest
+  in
+  process false [] attrs
+
 type functionAttributesInfo = {
   async: bool;
   bs: bool;

res_syntax/src/res_parsetree_viewer.mli

@@ -20,6 +20,9 @@ val processBsAttribute : Parsetree.attributes -> bool * Parsetree.attributes
 val processUncurriedAppAttribute :
   Parsetree.attributes -> bool * Parsetree.attributes
 
+val processPartialAppAttribute :
+  Parsetree.attributes -> bool * Parsetree.attributes
+
 type functionAttributesInfo = {
   async: bool;
   bs: bool;

res_syntax/src/res_printer.ml

@@ -3970,6 +3970,13 @@ and printPexpApply ~state expr cmtTbl =
     let uncurried, attrs =
       ParsetreeViewer.processUncurriedAppAttribute expr.pexp_attributes
     in
+    let partial, attrs = ParsetreeViewer.processPartialAppAttribute attrs in
+    let args =
+      if partial then
+        let dummy = Ast_helper.Exp.constant (Ast_helper.Const.int 0) in
+        args @ [(Asttypes.Labelled "...", dummy)]
+      else args
+    in
     let dotted = state.uncurried_config |> Res_uncurried.getDotted ~uncurried in
     let callExprDoc =
       let doc = printExpressionWithComments ~state callExpr cmtTbl in
@@ -4580,7 +4587,7 @@ and printArguments ~state ~dotted
 and printArgument ~state (argLbl, arg) cmtTbl =
   match (argLbl, arg) with
   (* ~a (punned)*)
-  | ( Asttypes.Labelled lbl,
+  | ( Labelled lbl,
       ({
          pexp_desc = Pexp_ident {txt = Longident.Lident name};
          pexp_attributes = [] | [({Location.txt = "res.namedArgLoc"}, _)];
@@ -4594,7 +4601,7 @@ and printArgument ~state (argLbl, arg) cmtTbl =
     let doc = Doc.concat [Doc.tilde; printIdentLike lbl] in
     printComments doc cmtTbl loc
   (* ~a: int (punned)*)
-  | ( Asttypes.Labelled lbl,
+  | ( Labelled lbl,
       {
         pexp_desc =
           Pexp_constraint
@@ -4622,7 +4629,7 @@ and printArgument ~state (argLbl, arg) cmtTbl =
     in
     printComments doc cmtTbl loc
   (* ~a? (optional lbl punned)*)
-  | ( Asttypes.Optional lbl,
+  | ( Optional lbl,
       {
         pexp_desc = Pexp_ident {txt = Longident.Lident name};
         pexp_attributes = [] | [({Location.txt = "res.namedArgLoc"}, _)];
@@ -4642,27 +4649,32 @@ and printArgument ~state (argLbl, arg) cmtTbl =
         (loc, {expr with pexp_attributes = attrs})
       | _ -> (expr.pexp_loc, expr)
     in
-    let printedLbl =
+    let printedLbl, dotdotdot =
       match argLbl with
-      | Asttypes.Nolabel -> Doc.nil
-      | Asttypes.Labelled lbl ->
+      | Nolabel -> (Doc.nil, false)
+      | Labelled "..." ->
+        let doc = Doc.text "..." in
+        (printComments doc cmtTbl argLoc, true)
+      | Labelled lbl ->
         let doc = Doc.concat [Doc.tilde; printIdentLike lbl; Doc.equal] in
-        printComments doc cmtTbl argLoc
-      | Asttypes.Optional lbl ->
+        (printComments doc cmtTbl argLoc, false)
+      | Optional lbl ->
         let doc =
           Doc.concat [Doc.tilde; printIdentLike lbl; Doc.equal; Doc.question]
         in
-        printComments doc cmtTbl argLoc
+        (printComments doc cmtTbl argLoc, false)
     in
     let printedExpr =
       let doc = printExpressionWithComments ~state expr cmtTbl in
       match Parens.expr expr with
-      | Parens.Parenthesized -> addParens doc
+      | Parenthesized -> addParens doc
       | Braced braces -> printBraces doc expr braces
       | Nothing -> doc
     in
     let loc = {argLoc with loc_end = expr.pexp_loc.loc_end} in
-    let doc = Doc.concat [printedLbl; printedExpr] in
+    let doc =
+      if dotdotdot then printedLbl else Doc.concat [printedLbl; printedExpr]
+    in
     printComments doc cmtTbl loc
 
 and printCases ~state (cases : Parsetree.case list) cmtTbl =

res_syntax/tests/printer/expr/UncurriedByDefault.res

@@ -157,3 +157,5 @@ let fnU = (_x): ((unit) => unit) => fooC
 let aU = (() => "foo")->Ok
 
 Ok("_")->Belt.Result.map(concatStrings(_, "foo"))
+
+let ptl1 =  add(1, ...)

res_syntax/tests/printer/expr/expected/UncurriedByDefault.res.txt

@@ -157,3 +157,5 @@ let fnU = (_x): (unit => unit) => fooC
 let aU = (() => "foo")->Ok
 
 Ok("_")->Belt.Result.map(concatStrings(_, "foo"))
+
+let ptl1 = add(1, ...)