Lambda in tuple in list on single line changes meaning of code

[nojaf]

Issue created from fantomas-online

Code

module A

let x =
    [
        1, fun () -> 1
        1, fun () -> 1
    ]

Result

module A

let x =
    [
        1,
        fun () ->
            1
            1, fun () -> 1
    ]

Problem description

Please describe here the Fantomas problem you encountered.
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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
                MultilineBracketStyle = aligned }

_{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.

Root Cause

I investigated this on the current main branch. The bug is confirmed and is a soundness issue — the formatted output changes the meaning of the code.

The current main produces:

let x = [ 1, fun () -> 1; 1, fun () -> 1 ]
```

This looks like a 2-element list, but F# actually parses it as a **1-element** list. The lambda `fun () -> 1` inside the first tuple captures the `;` and everything after it as a sequence expression:

```
[ 1, fun () -> (1; 1, fun () -> 1) ]   -- ONE element

Verified:

let x_orig = [ (* multiline *) 1, fun () -> 1; 1, fun () -> 1 (* newlines *) ]  // length = 2
let x_fmt  = [ 1, fun () -> 1; 1, fun () -> 1 ]  // length = 1 — DIFFERENT semantics!

Code Path

In genArrayOrList (CodePrinter.fs), the alwaysMultiline check forces multiline layout when:

1. Any element is if/then/else
2. All elements are lambdas or if/then/else

Case (2) prevents [fun () -> 1; fun () -> 2] from collapsing. But [1, fun () -> 1; ...] contains Expr.Tuple elements — not bare lambdas — so neither condition fires. The list falls through to the "fits on one line?" check, and since [ 1, fun () -> 1; 1, fun () -> 1 ] is short, it collapses.

Fix (draft PR)

I've created a draft PR that implements the "stay multiline" approach consistent with the vote on RFC #3279: when any non-last list element is a tuple whose last expression is a lambda or if/then/else, the list is forced to stay multiline. The ; newline-sensitive form prevents the capture.

Branch: repo-assist/fix-issue-3278-lambda-tuple-list-single-line

All 2738 tests pass with this fix.

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