Extension method with F# function type

Question

The MSDN doc on Type Extensions states that "Before F# 3.1, the F# compiler didn't support the use of C#-style extension methods with a generic type variable, array type, tuple type, or an F# function type as the “this” parameter." (http://msdn.microsoft.com/en-us/library/dd233211.aspx) How can be a Type Extension used on F# function type? In what situations would such a feature be useful?

Answer 1

Here is how you can do it:

[<Extension>]
type FunctionExtension() =
    [<Extension>]
    static member inline Twice(f: 'a -> 'a, x: 'a) = f (f x)

// Example use
let increment x = x + 1
let y = increment.Twice 5  // val y : int = 7

Now for "In what situations would such a feature be useful?", I honestly don't know and I think it's probably a bad idea to ever do this. Calling methods on a function feels way too JavaScript-ey, not idiomatic at all in F#.

Answer 2

You may simulate the . notation for extension methods with F#'s |> operator. It's a little clumsier, given the need for brackets:

let extension f x =
    let a = f x
    a * 2

let f x = x*x

> f 2;;
val it : int = 4
> (f |> extension) 2;;
val it : int = 8
> let c = extension f 2;;  // Same as above
val c : int = 8