Allow returning bool instead of unit option for partial active patterns

I propose we allow returning bool from partial active patterns that don't return a significant value apart from "matches" / "doesn't match".

The existing way of approaching this problem in F# is to return Some () if the value matches and None if it doesn't.

For example the active pattern for the following:

match key with
| CaseInsensitive "foo" -> ...
| CaseInsensitive "bar" -> ...

would currently be written as:

let (|CaseInsensitive|_|) (pattern: string) (value: string) =
    if String.Equals(value, pattern, StringComparison.OrdinalIgnoreCase) then
        Some ()
    else
        None

With this proposal, it could be written as:

let (|CaseInsensitive|_|) (pattern: string) (value: string) =
    String.Equals(value, pattern, StringComparison.OrdinalIgnoreCase)

Pros and Cons

The advantages of making this adjustment to F# are

• A simpler way to write active patterns that just check a condition without returning a value.
• Fewer memory allocations and probably better generated IL.

The disadvantages of making this adjustment to F# are

• Adding one more way to do the same thing.

Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions:

• Struct representation for active patterns [ RFC FS-1039 ] #612 / RFC FS-1039 would also remove the memory allocation, although I suspect that ValueSome () would still result in worse IL than true.

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
• This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

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