Unwanted space before DU cases when pattern matching

[drhumlen]

Issue created from fantomas-online

Code

type Type =
    | TyLam of Type * Type
    | TyVar of string
    | TyCon of string * Type list
    override this.ToString() =
        match this with
        | TyLam(t1, t2) -> sprintf "(%O -> %O)" t1 t2
        | TyVar a -> a
        | TyCon(s, ts) -> s

Result

type Type =
    | TyLam of Type * Type
    | TyVar of string
    | TyCon of string * Type list

    override this.ToString() =
        match this with
        | TyLam (t1, t2) -> sprintf "(%O -> %O)" t1 t2
        | TyVar a -> a
        | TyCon (s, ts) -> s

Problem description

Fantomas adds a space before the parenthesis for the Discriminated Union cases when pattern matching. According to the F# style guide, there should not be such a space: https://docs.microsoft.com/en-us/dotnet/fsharp/style-guide/formatting#formatting-pattern-matching

It also makes sense to not have it since you contruct your object like let myShape = Rect(4.0, 6.0), so the destructuring should look similar match myShape with Rect(w, h) -> .....

Also worth noting here is that the output from Fantomas does not match the example given in the README because of this behavior (see example 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 master branch at 2022-07-15T14:30:45Z - 87cd212

Default Fantomas configuration

[nojaf]

Hello, thank you for raising this issue. It might be me, and I haven't finished my first coffee on a Monday morning, but where does the guide mention this explicitly of the space?

It makes sense to change this but this can be a bit of an impactful change for many users, so I really would like to have crystal clear guidance to be able to justify this change. A code sample in the F# style guide without any explicit text isn't really reliable.

Currently, the setting fsharp_space_before_parameter is being used to determine if there should be a space or not. I believe it might make more sense to use the same rules as for function invocation.
There, depending on the capitalisation of the function name, fsharp_space_before_lowercase_invocation or fsharp_space_before_uppercase_invocation is used.

[drhumlen]

Hello, thank you for raising this issue. It might be me, and I haven't finished my first coffee on a Monday morning, but where does the guide mention this explicitly of the space?

Hmm on reading I can't find anything explicit actually, but all the examples are without the space. I also think of it as exactly the same the construction of something – ie let shape = Rect(2.0, 6.0), and not let shape = Rect (2.0, 6.0), so I think of it as covered "implicitly" by that rule. But some official statement on this would be nice.

Currently, the setting fsharp_space_before_parameter is being used to determine if there should be a space or not. I believe it might make more sense to use the same rules as for function invocation.
There, depending on the capitalisation of the function name, fsharp_space_before_lowercase_invocation or fsharp_space_before_uppercase_invocation is used.

Yea I think that makes sense. It should always follow the same spacing rule as the construction of a DU – which is very similar to an uppercase invocation.

@nojaf Is there anyone at Microsoft/F# committee we can confer with? Philip Carter?

[nojaf]

Please raise this over at https://github.com/fsharp/fslang-design#style-guide.

[auduchinok]

The adding of space is very inconsistent to how expressions are handled. I'd expect Fantomas to use SpaceBeforeUppercaseInvocation setting here.

[auduchinok]

Created an issue: fsharp/fslang-design#712.

[nojaf]

This is available in the 5.1 series.