SRTP_extend_existing_types.fsx

// extend a type from another assembly with 'combine'
type List<'a> with
    static member combine (f, x: List<'a>) : List<'a> = x @ [f]

// extend another type from another assembly with 'combine'
type System.Int32 with
    static member combine (f, x: System.Int32) = x + f

// extend a type from this assembly with 'combine'
type GenTest<'a> = { value: 'a } with
    static member combine (f, x: GenTest<_>) = { value = f }

// the generic 'combine' function
let inline combine f (x: ^t) =
    (^t: (static member combine: _ * ^t -> ^t) (f,x))

///// usage:

// this is working:
List<_>.combine (2, [1;2;3])
System.Int32.combine (5, 10)
// not working ("The type '...' does not support the operator 'combine'"):
[1;2;3] |> combine 1
5 |> combine 10

// these are of course working:
{ value = 10 } |> combine 5
{ value = "yyy" } |> combine "zzz"