Fix idempotency issue with comments in apps on lambda expressions

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+* Idempotency problem with comments in applications on lambda expressions. [#3128](https://github.com/fsprojects/fantomas/issues/3128)
+
 ## 7.0.1 - 2025-02-21
 
 ### Fixed

src/Fantomas.Core.Tests/LambdaTests.fs

@@ -1430,3 +1430,22 @@ MailboxProcessor<string>.Start(fun inbox ->
             do! sw.WriteLineAsync(msg) |> Async.AwaitTask
     })
 """
+
+[<Test>]
+let ``comment in app on lambda expr, 3128`` () =
+    formatSourceString
+        """
+f(
+    // some comment
+    fun x -> x)
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+f
+    (
+    // some comment
+    fun x -> x)
+"""

src/Fantomas.Core/CodePrinter.fs

@@ -2402,7 +2402,7 @@ let genAppWithLambda sep (node: ExprAppWithLambdaNode) =
         +> onlyIf (List.isNotEmpty node.Arguments) sepSpace
         +> (genSingleTextNode node.OpeningParen
             +> (match node.Lambda with
-                | Choice1Of2 lambdaNode -> genLambdaWithParen lambdaNode |> genNode lambdaNode
+                | Choice1Of2 lambdaNode -> genLambdaWithParen lambdaNode
                 | Choice2Of2 matchLambdaNode ->
                     genSingleTextNode matchLambdaNode.Function
                     +> indentSepNlnUnindent (genClauses matchLambdaNode.Clauses)
@@ -2418,7 +2418,7 @@ let genAppWithLambda sep (node: ExprAppWithLambdaNode) =
                     match node.Lambda with
                     | Choice1Of2 lambdaNode ->
                         genSingleTextNode node.OpeningParen
-                        +> (genLambdaWithParen lambdaNode |> genNode lambdaNode)
+                        +> genLambdaWithParen lambdaNode
                         +> onlyIf (not (isStroustrupStyleExpr ctx.Config lambdaNode.Expr)) sepNln
                         +> genSingleTextNode node.ClosingParen
                     | Choice2Of2 matchLambdaNode ->
@@ -2437,8 +2437,7 @@ let genAppWithLambda sep (node: ExprAppWithLambdaNode) =
                     +> (match node.Lambda with
                         | Choice1Of2 lambdaNode ->
                             leadingExpressionIsMultiline
-                                (genSingleTextNode node.OpeningParen
-                                 +> (genLambdaWithParen lambdaNode |> genNode lambdaNode))
+                                (genSingleTextNode node.OpeningParen +> genLambdaWithParen lambdaNode)
                                 (fun isMultiline ->
                                     onlyIf
                                         (isMultiline && not (isStroustrupStyleExpr ctx.Config lambdaNode.Expr))
@@ -2482,7 +2481,7 @@ let genAppWithLambda sep (node: ExprAppWithLambdaNode) =
                      +> col sepSpace node.Arguments genExpr
                      +> sep
                      +> genSingleTextNode node.OpeningParen
-                     +> (genLambdaWithParen lambdaNode |> genNode lambdaNode)
+                     +> genLambdaWithParen lambdaNode
                      +> sepNlnWhenWriteBeforeNewlineNotEmpty
                      +> addFixedSpaces startColumn
                      +> genSingleTextNode node.ClosingParen)
@@ -2496,7 +2495,7 @@ let genAppWithLambda sep (node: ExprAppWithLambdaNode) =
                         (col sepNln node.Arguments genExpr
                          +> onlyIfNot (List.isEmpty node.Arguments) sepNln
                          +> genSingleTextNode node.OpeningParen
-                         +> (genLambdaWithParen lambdaNode |> genNode lambdaNode)
+                         +> genLambdaWithParen lambdaNode
                          +> addFixedSpaces startColumn
                          +> genSingleTextNode node.ClosingParen)
                             ctx)