Adding space after prefix operator breaks code

[ursenzler]

Issue created from fantomas-online

Code

let inline (~%%) id = int id

let f a b = a + b

let foo () = f %%"17" %%"42"

Result

let inline (~%%) id = int id

let f a b = a + b

let foo () = f %% "17" %% "42"

Problem description

Fantomas adds a space between the prefix operator and the string. Note that this only happens when the string is specified after the operator. If a value is used, then formatting does not add a space.

This works correctly:

    let inline (~%%) id = int id
     
    let f a b = a + b

    let v = 17
    let w = 42
    let foo () = f %%v %%w

The current workaround is to add ( ) around the operator and value.
We use this in our tests to setup data; that's why we use strings there, not values containing strings.
It would also be more consistent never to add a space (as far as I understand it, a space is only added when the prefix operator is followed by a string definition)

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 v5.2 branch at 1/1/1990

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?}

[nojaf]

Hello, thank you for reporting this issue.
There seems to be a difference when you start your prefix operator with ~ versus !.

https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/operator-overloading#prefix-and-infix-operators

as far as I understand it, a space is only added when the prefix operator is followed by a string definition

That faintly rings a bell, I believe we at some point added this because of the operator we use in Fantomas.
let (!-) (s: string) = ... where !- "foo" was preferred over !-"foo".

Short term, I would accept a fix that takes the % character into account when dealing with a prefix operator. Long term, there should be guidance from the style guide for this matter.

Are you interested in submitting a PR?

[ursenzler]

I have no clue where and how to start :-(

[nojaf]

We have a fair amount of documentation to get started.

The Oak viewer in the online tool gives a pretty good starting point:

Look where ExprPrefixAppNode is constructed (ASTTransformer.fs) and consumed (CodePrinter.fs).