Remove underscore if placeholder is in first position

compiler/syntax/src/res_parsetree_viewer.ml

@@ -141,6 +141,55 @@ let rewrite_underscore_apply expr =
     }
   | _ -> expr
 
+(* For pipe RHS: (__x) => f(__x, a, b) -----> f(a, b)
+   Omits the first __x argument only if it's the sole occurrence.
+   If multiple __x exist (e.g., f(__x, __x, b)), keeps all to preserve semantics. *)
+let rewrite_underscore_apply_in_pipe expr =
+  let is_underscore_arg = function
+    | _, {pexp_desc = Pexp_ident {txt = Longident.Lident "__x"}} -> true
+    | _ -> false
+  in
+  let convert_underscore_to_placeholder arg =
+    match arg with
+    | ( lbl,
+        ({pexp_desc = Pexp_ident ({txt = Longident.Lident "__x"} as lid)} as
+         arg_expr) ) ->
+      ( lbl,
+        {
+          arg_expr with
+          pexp_desc = Pexp_ident {lid with txt = Longident.Lident "_"};
+        } )
+    | arg -> arg
+  in
+  match expr.pexp_desc with
+  | Pexp_fun
+      {
+        arg_label = Nolabel;
+        default = None;
+        lhs = {ppat_desc = Ppat_var {txt = "__x"}};
+        rhs = {pexp_desc = Pexp_apply {funct; args}} as e;
+      } -> (
+    match args with
+    | first_arg :: rest_args when is_underscore_arg first_arg ->
+      if List.exists is_underscore_arg rest_args then
+        (* Multiple __x - keep all to preserve semantics *)
+        rewrite_underscore_apply expr
+      else
+        (* Single __x in first position - safe to omit *)
+        {
+          e with
+          pexp_desc =
+            Pexp_apply
+              {
+                funct;
+                args = List.map convert_underscore_to_placeholder rest_args;
+                partial = false;
+                transformed_jsx = false;
+              };
+        }
+    | _ -> rewrite_underscore_apply expr)
+  | _ -> expr
+
 type fun_param_kind =
   | Parameter of {
       attrs: Parsetree.attributes;

compiler/syntax/src/res_parsetree_viewer.mli

@@ -146,6 +146,9 @@ val is_single_pipe_expr : Parsetree.expression -> bool
 (* (__x) => f(a, __x, c) -----> f(a, _, c)  *)
 val rewrite_underscore_apply : Parsetree.expression -> Parsetree.expression
 
+val rewrite_underscore_apply_in_pipe :
+  Parsetree.expression -> Parsetree.expression
+
 (* (__x) => f(a, __x, c) -----> f(a, _, c)  *)
 val is_underscore_apply_sugar : Parsetree.expression -> bool
 

compiler/syntax/src/res_printer.ml

@@ -3985,6 +3985,11 @@ and print_binary_expression ~state (expr : Parsetree.expression) cmt_tbl =
     ->
     let lhs_has_comment_below = has_comment_below cmt_tbl lhs.pexp_loc in
     let lhs_doc = print_operand ~is_lhs:true ~is_multiline:false lhs op in
+    (* For pipe RHS, use pipe-specific rewrite to omit redundant first underscore *)
+    let rhs =
+      if op = "->" then ParsetreeViewer.rewrite_underscore_apply_in_pipe rhs
+      else rhs
+    in
     let rhs_doc = print_operand ~is_lhs:false ~is_multiline:false rhs op in
     Doc.group
       (Doc.concat

scripts/test_syntax.sh

@@ -81,6 +81,7 @@ if [[ $ROUNDTRIP_TEST = 1 ]]; then
     mkdir -p temp/$(dirname $file)
     sexpAst1=temp/$file.sexp
     sexpAst2=temp/$file.2.sexp
+    sexpAst3=temp/$file.3.sexp
     rescript1=temp/$file.res
     rescript2=temp/$file.2.res
 
@@ -89,14 +90,21 @@ if [[ $ROUNDTRIP_TEST = 1 ]]; then
       *.resi ) resIntf=-interface ;;
     esac
 
+    # First pass: original file -> AST1 and text1
     $DUNE_BIN_DIR/res_parser $resIntf -print sexp $file > $sexpAst1
     $DUNE_BIN_DIR/res_parser $resIntf -print res $file > $rescript1
 
+    # Second pass: text1 -> AST2 and text2
     $DUNE_BIN_DIR/res_parser $resIntf -print sexp $rescript1 > $sexpAst2
     $DUNE_BIN_DIR/res_parser $resIntf -print res $rescript1 > $rescript2
 
-    diff --unified $sexpAst1 $sexpAst2
+    # Third pass: text2 -> AST3 (to check idempotency after normalization)
+    $DUNE_BIN_DIR/res_parser $resIntf -print sexp $rescript2 > $sexpAst3
+
+    # Check AST idempotency: AST2 should equal AST3 (allows AST1 != AST2 for canonicalization)
+    diff --unified $sexpAst2 $sexpAst3
     [[ "$?" = 1 ]] && echo 1 > $roundtripTestsResult
+    # Check text idempotency: text1 should equal text2
     diff --unified $rescript1 $rescript2
     [[ "$?" = 1 ]] && echo 1 > $roundtripTestsResult
   } & maybeWait

tests/syntax_tests/data/printer/expr/expected/underscoreApply.res.txt

@@ -12,7 +12,7 @@ let nested2 = (x, y, z) => List.length(_)
 
 let l = [1, 2, 3]->List.map(i => i + 1, _)->List.filter(i => i > 0, _)
 
-let l = (i => i + 1)->List.map(_, [1, 2, 3])
+let l = (i => i + 1)->List.map([1, 2, 3])
 
 let x = List.length(_)
 
@@ -52,8 +52,8 @@ f(a, b, _)[ix] = 2
 
 getDirector(a, b, _).name = "Steve"
 
-filterNone->Array.get(_, 0)
-filterNone->Array.get(_, 0)
+filterNone->Array.get(0)
+filterNone->Array.get(0)
 Array.get(_, 0)
 1 + Array.get(_, 0)
 Array.get(_, 1) + Array.get(_, 0)
@@ -71,3 +71,9 @@ let status =
   json
   ->optional(field("status", string, _), _)
   ->Option.mapOr(Status.Active, Status.fromString)
+
+a->map2(fn)
+
+a->map2(fn)
+
+a->f(_, _, b)

tests/syntax_tests/data/printer/expr/underscoreApply.res

@@ -76,3 +76,9 @@ let status =
     json
     ->optional(field("status", string, _), _)
     ->Option.mapOr(Status.Active, Status.fromString)
+
+a->map2(_, fn)
+
+a->map2(fn)
+
+a->f(_, _, b)