Additional whitespace for LineCommentAfterSourceCode when last character is a ,

[stefan-schweiger]

Issue created from fantomas-online

Code

let x
    (
        a: string, // test
        b: string // test
    ) =
    print "hello"

Result

let x
    (
        a: string,  // test
        b: string // test
    ) =
    print "hello"

Problem description

When there is a , between source code and a comment the result prints an additional whitespace which doesn't get added if there is no ,.

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 2022-10-14T16:59:37Z - 35f093c

Default Fantomas configuration

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

[stefan-schweiger]

fsharp_space_after_comma always adds a space between commas, but this code already added a space before the comment:

fantomas/src/Fantomas.Core/Context.fs

Lines 1065 to 1067 in 35f093c

	let comment = sprintf "%s%s" (if addSpace then " " else String.empty) s
	writerEvent (WriteBeforeNewline comment)

Maybe there should be a different WriteEvent which tries to handle the spaces?

[nojaf]

The bigger problem here is that the comment is assigned to the string node and not the ,.
SynPat.Tuple does not contain any information about the commas. So we cannot construct a node to anchor it.

Theoretically speaking, we could allow the following to play out:
The comment is assigned to string, after visiting that node, we print it and force it*.
This would lead to

let x
    (
        a: string // comment

However, because of the comment, we can no longer put the comma on that same line

let x
    (
        a: string // comment, `<- Leads to invalid F# code`

There is a way to take this into account, but it would lead to the comment being on the next line:

let x
    (
        a: string // comment
        , b: string

I'm not exactly sure how it would play out, so it could also be more like

let x
    (
        a: string // comment
        , 
        b: string

Adding the commas to SynPat.Tuple isn't such a wild idea. SynExpr.Tuple has them for example. But that would require a PR to the F# compiler.

(*) The thing with comments after source code is that we don't write them directly but only once we move to the next line. This is to avoid exactly the invalid scenario as mentioned above.

[nojaf]

On a slightly more practical level, we could also check inside the update function of WriterModel if their is a trailing and leading space:

fantomas/src/Fantomas.Core/Context.fs

Line 87 in de8b5a7

let currentLine = String.Concat(List.head m.Lines, m.WriteBeforeNewline).TrimEnd()

Something like

let currentLine =
    let head = List.head m.Lines
    if head.EndsWith(" ") && m.WriteBeforeNewline.StartsWith(" ") then
        String.Concat(head, m.WriteBeforeNewline.TrimStart())
    else
        String.Concat(head, m.WriteBeforeNewline).TrimEnd()