Allow any type constraints on SRTP

[WhiteBlackGoose]

Constrain any type instead of type parameter

I suggest adding the following feature to the language: being able to constrain any type in SRTP, not only the generic type parameter.

Let me show an example:

let inline addTwoThings< ^a, ^b, ^c when Tuple< ^a, ^b> : (member add : () -> ^c)> (a : ^a) (b : ^b) =
    (a, b).add()

So instead of constraining ^a, ^b I constrained Tuple< ^a, ^b> of these two arguments to have method add.

Same goes with any type computed of type arguments, it could be

when Tuple<^a list, ^b option> : (static member something : () -> ^a * ^b)

It integrates smooth with HKTs:

^a, ^b, ^h when ^h< ^a> : (static member bind : ^a -> ^h< ^b>)

Pros and Cons

Pros: useful for cases when we partially extend a generic type (that is, not covering its all type parameters), I mean the following example:

type Tuple<string, int> with
    member add () -> $"{Item1} {Item2}"

It also makes the C# interop more convenient. Here's sum via Linq:

let inline sum< ^a when seq< ^a> : (member Sum : unit -> ^a)> s = s.Sum()

Cons: complicates the syntax for SRTP.

Extra information

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

Related suggestions: HKT

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• [x] This is not a breaking change to the F# language design
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[dsyme]

It's an interesting proposal....

[kevmal]

Wouldn't add in the example need to be an extension method? So wouldn't this just be https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1043-extension-members-for-operators-and-srtp-constraints.md