How to use static extension methods with statically resolved type parameters?

Question

I'd like to use statically resolved type parameters with some extension methods I added to built-in types, like float32 and int32, so I tried the following:

module Foo =
    type System.Single with
        static member Bar x = x + 1.0f

    let inline bar (x : ^a) =
        (^a : (static member Bar : ^a -> ^a) x)

open Foo
[<EntryPoint>]
let main argv =
    System.Console.WriteLine (bar 1.0f) (* Compilation fails here *)
    0

The compiler complains that The type 'float32' doesn't support the operator 'Bar'. What am I doing wrong?

Answer 1

Statically resolved type parameters will not resolve against extension methods so this particular approach isn't possible.

I would advise thinking carefully about whether this is something you really need to do or whether you can express the problem you're trying to solve in a different way.

If you are sure that this is what you want and you're willing to brave the dragons that follow, there is a workaround which involves creating a helper type with some static methods:

type Ext = Ext
    with
        static member Bar (ext : Ext, flt : float) = 1.0 + flt
        static member Bar (ext : Ext, flt : float32) = 1.0f + flt

You can then define a new function using statically resolved type parameters like this:

let inline bar (x : ^a) =
    ((^b or ^a) : (static member Bar : ^b * ^a -> ^a) (Ext, x))

Then you can write:

bar 7.0f
val it : float32 = 8.0f

bar 5.0
val it : float = 6.0