Only apply in pipe context

Author: shulhi

compiler/syntax/src/res_parsetree_viewer.ml

@@ -141,6 +141,50 @@ let rewrite_underscore_apply expr =
     }
   | _ -> expr
 
+(* For pipe RHS: (__x) => f(__x, a, b) -----> f(a, b)
+   Omits the first __x argument since the piped value fills that position *)
+let rewrite_underscore_apply_in_pipe expr =
+  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 = call_expr; args}} as e;
+      } -> (
+    match args with
+    | (Nolabel, {pexp_desc = Pexp_ident {txt = Longident.Lident "__x"}})
+      :: rest_args ->
+      (* First arg is __x, skip it and convert remaining __x to _ *)
+      let new_args =
+        List.map
+          (fun 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)
+          rest_args
+      in
+      {
+        e with
+        pexp_desc =
+          Pexp_apply
+            {
+              funct = call_expr;
+              args = new_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 +146,8 @@ 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

@@ -3985,49 +3985,12 @@ 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
-    let rhs_doc =
-      (* For pipe operator, if RHS is (__x) => f(__x, ...), remove the first __x arg
-         so it prints as a->f(...) instead of a->f(_, ...) *)
-      let rhs_to_print =
-        match rhs.pexp_desc with
-        | Pexp_fun
-            {
-              arg_label = Nolabel;
-              default = None;
-              lhs = {ppat_desc = Ppat_var {txt = "__x"}} as pat;
-              rhs =
-                {pexp_desc = Pexp_apply {funct; args; partial; transformed_jsx}}
-                as body;
-              arity;
-              async;
-            } -> (
-          match args with
-          | (Nolabel, {pexp_desc = Pexp_ident {txt = Longident.Lident "__x"}})
-            :: rest_args ->
-            {
-              rhs with
-              pexp_desc =
-                Pexp_fun
-                  {
-                    arg_label = Nolabel;
-                    default = None;
-                    lhs = pat;
-                    rhs =
-                      {
-                        body with
-                        pexp_desc =
-                          Pexp_apply
-                            {funct; args = rest_args; partial; transformed_jsx};
-                      };
-                    arity;
-                    async;
-                  };
-            }
-          | _ -> rhs)
-        | _ -> rhs
-      in
-      print_operand ~is_lhs:false ~is_multiline:false rhs_to_print op
+    (* 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
          [