Custom operator is applied to anonymous record after formatting

[nojaf]

Issue created from fantomas-online

Code

let a =
    fun x -> {| X = x |}
    <*| op

let b =
    fun x -> {|
               X = x |}
    <*| op

let c =
    fun x ->
        {|
            X = x
        |}
    <*| op

let d =
    fun x ->
    {|
        X = x
    |}
    <*| op
()

Result

let a = fun x -> {| X = x |} <*| op

let b = fun x -> {| X = x |} <*| op

let c = fun x -> {| X = x |} <*| op

let d = fun x -> {| X = x |} <*| op
()

Problem description

The meaning of the code changed here.
AST is most likely different after formatting.

Extra information

• The formatted result breaks my code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.
• I would like a release if this problem is solved.

Options

Fantomas main branch at 2025-09-08T06:57:30Z - 31029b1

    { config with
                MaxLineLength = 80 }

_{Did you know that you can ignore files when formatting by using a .fantomasignore file?
PS: It's unlikely that someone else will solve your specific issue, as it's something that you have a personal stake in.}

[github-actions]

🤖 This is an automated response from Repo Assist.

I've investigated this bug and submitted a fix: see the recently created PR.

Root cause: In genShortInfixExpr (the single-line layout for Expr.InfixApp), the left-hand side is formatted with genExpr node.LeftHandSide without any parentheses. When the LHS is a lambda expression, emitting fun x -> body (op) rhs on a single line is re-parsed by F# as fun x -> (body (op) rhs) — the lambda body absorbs the operator and its argument, which is semantically different from the original (fun x -> body) (op) rhs.

For newLineInfixOps (|>, >>, etc.), Fantomas already handles this by forcing multiline layout. But for other operators (<*|, <$>, >>=, etc.), the short form was used without parenthesising the lambda.

Fix: Add sepOpenT +> genExpr lhs +> sepCloseT when isLambdaOrIfThenElse node.LeftHandSide in genShortInfixExpr. All 2737 tests pass with no regressions.

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