method_to_function.fsx

// Link:	http://fssnip.net/7U0
// Posted:	5 years ago
// Author:	Evgeniy Andreev
// Tags:	#active patterns , #srtp

let inline (|HasLength|) x = 
  fun () -> (^a: (member Length: int) x)

// `this` is optional here, just for demonstration of `as` pattern
let inline length (HasLength f as this) = f()

// It works!
let foo = length [1; 2; 3]
let bar = length [|1; 2; 3|]

//vopo, alternative approach
let inline length' x = (^a : (member Length:int) x)