Optimized Fibonacci in F#

Vlad Chistyakov suggested to use [Memoze] macro attribute to speed up the Nemerle version of Fibonacci function. I don't think it's a very good idea to use macros in situations where plane functions work just well. Look at an optimized F# Fibonacci:

open FsCheck
open Prop
let rec slowFib x = if x < 2 then 1 else slowFib(x - 2) + slowFib(x - 1)
let fastFib x =
let rec fastFib' x n2 n1 =
match x with
|0 -> n2
|_ -> fastFib' (x - 1) n1 (n2 + n1)
if x < 2 then 1 else fastFib' x 1 1
[<Property>]
let ``Fast Fib matches the simple slow one``(x: int) =
(x >= 0 && x < 25) ==> lazy (slowFib x = fastFib x)
|> trivial (x < 2)
// Ok, passed 100 tests (30% trivial).
view raw gistfile1.fs hosted with ❤ by GitHub
The speed is fantastic, as expected:

[0..43] |> List.iter (fun x -> printfn "fib(%d) = %d" x (slowFib x))
// Real: 00:00:08.075, CPU: 00:00:07.987, GC gen0: 0, gen1: 0, gen2: 0
[0..43] |> List.iter (fun x -> printfn "fib(%d) = %d" x (fastFib x));;
// Real: 00:00:00.005, CPU: 00:00:00.000, GC gen0: 0, gen1: 0, gen2: 0
view raw gistfile1.fs hosted with ❤ by GitHub

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