sig.fs

open System

module MyMod1 =

    type Fib = bigint -> bigint //function signatures!
    let fib:Fib = fun n ->
        let rec loop acc1 acc2 = function
            | n when n = 0I -> acc1
            | n -> loop acc2 (acc1 + acc2) (n - 1I)
        loop 0I 1I n


Console.WriteLine(MyMod1.fib 6I)