List expression items don't respect fsharp_blank_lines_around_nested_multiline_expressions

[cmeeren]

Issue created from fantomas-online

Code

[ verylonglinethatwillbreak arg1 arg2 arg3 arg4 arg5 arg6 arg7
  verylonglinethatwillbreak arg1 arg2 arg3 arg4 arg5 arg6 arg7
  verylonglinethatwillbreak arg1 arg2 arg3 arg4 arg5 arg6 arg7 ]

Result

[ verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7
  verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7
  verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7 ]

Expected result

[ verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7

  verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7

  verylonglinethatwillbreak
      arg1
      arg2
      arg3
      arg4
      arg5
      arg6
      arg7 ]

Problem description

List items (whether using implicit/explicit yield or yield!) do not seem to follow fsharp_blank_lines_around_nested_multiline_expressions, as shown above.

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.

Options

Fantomas main branch at 1/1/1990

    { config with
                MaxLineLength = 60 }

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[github-actions]

🤖 This is an automated response from Repo Assist.

Thanks for the detailed report! This is a good candidate for a fix.

Root cause: The fsharp_blank_lines_around_nested_multiline_expressions setting is applied to multiline expressions in certain known contexts (e.g. inside let bindings, match arms, etc.), but the code path for rendering list/array/seq items doesn't currently thread blank-line insertion between elements. Essentially, the per-item rendering in genCollectionExprItems doesn't consult the BlankLinesAroundNestedMultilineExpressions setting when emitting the separator between items.

Where to look: The relevant formatting lives in src/Fantomas.Core/CodePrinter.fs. The genExprList, genArrayOrList, and related functions emit items separated by newlines. The blank-line logic for MultilineExpressionsBetweenBrackets is implemented (see addNewlineIfMultiline), but the BlankLinesAroundNestedMultilineExpressions check for items in collections is either missing or not firing.

Suggested fix approach: Before emitting each item separator in list/array/seq expressions (when not the first item), check if both the preceding and current item are multiline — if so and the setting is enabled, emit an extra blank line. Similar logic already exists for module-level declarations.

This is labelled good first issue and the fix is self-contained. If you'd like to take a stab at it, the online tooling and the [YouTube series]((www.youtube.com/redacted) are good resources for getting started. The CONTRIBUTING.md also has useful pointers.

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[cmeeren]

check if both the preceding and current item are multiline

Should happen even if only one of them is multiline.

[nojaf]

/repo-assist The reporter is requesting an extension of fsharp_blank_lines_around_nested_multiline_expressions and presents their issue as fact. Currently, colWithNlnWhenMappedNodeIsMultiline applies to top-level bindings, nested module bindings, and type members. It could also be applied to lists and arrays, but a single reporter's request does not imply community consensus. I have added the needs-community-interest label. This issue requires upvotes before it can be taken seriously.

[github-actions]

🤖 This is an automated response from Repo Assist.

To clarify my earlier comment: I was too quick to treat this as a confirmed bug and suggest a fix. On reflection, this is actually a feature request — an extension of the existing fsharp_blank_lines_around_nested_multiline_expressions behaviour to cover list and array expressions, which is not currently in scope.

The colWithNlnWhenMappedNodeIsMultiline mechanism currently applies to top-level bindings, nested module bindings, and type members. Extending it to list and array items would be a deliberate design decision, not a bug fix.

The maintainer has added the needs-community-interest label to reflect that a single reporter's preference isn't sufficient basis for expanding the feature. If you'd like to see this implemented, the most helpful thing you can do is upvote the issue (👍 reaction) and encourage others who want this behaviour to do the same. Once there is broader community support, it becomes a stronger candidate for prioritisation.

Thank you for the detailed report — the reproduction and the clear expected/actual comparison are genuinely useful if this does move forward!

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[cmeeren]

@nojaf, I'd like to gently push back on bug vs. feature: The docs for fsharp_blank_lines_around_nested_multiline_expressions says "Surround nested multi-line expressions with blank lines." List expression items are nested and can be multiline. I see no reason why fsharp_blank_lines_around_nested_multiline_expressions should not be expected to apply to multiline list items. They are still nested multiline expressions.