A comment before an anonymous function gets swallowed up

[naartjie]

Issue created from fantomas-online

Code

(   
    (* comment before gets swallowed *)
    fun x -> x * 42
)

(
    fun x -> x * 42
    (* comment after is OK *)
)

(   (* comment on first line is OK too *)
    fun x -> x * 42
)

Result

(fun x -> x * 42)

(fun x ->
    x * 42 (* comment after is OK *)
    )

( (* comment on first line is OK too *) fun x -> x * 42)

Problem description

A comment above/before an anonymous function is getting dropped from the formatted output. This also happens if you use // for a line comment instead of the (* *) block comment syntax.

Extra information

• The formatted result breaks by code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas Master at 10/02/2020 17:06:53 - 31305c2

Default Fantomas configuration

Possibly related issues

#1179 #1172 #932

[nojaf]

Hello @naartjie, I'm in the process of writing some more 'getting started' content. See #1195.

As for this problem, it is a trivia bug.
See online tool.

Fantomas assigns the block comment to a SynExpr.Lambda node. And in CodePrinter.fs we need to call genTrivia or variant in order that the comment gets restored.

In CodePrinter I believe it is not printed around here.
Call it a gut feeling.

[naartjie]

Thank you for fixing this 🚀 🚀