open System.Numerics

​

let memoize<'a when 'a : comparison> (f : 'a -> 'b) : 'a -> 'b =

    let results = ref Map.empty

    (fun x ->

        match Map.tryFind x !results with

        | Some x' -> x'

        | None ->

            let result = f x

            results := Map.add x result !results

            result

    )

​

let fib =

    // remove memoize to get stack overflow error!

    let rec fib' (n : bigint) : bigint = 

        match n with

        | n when n = 0I -> 1I

        | n when n = 1I -> 1I

        | n -> fib'' (n - 1I) + fib'' (n - 2I)

    and fib'' = memoize fib'

    fib''

​

seq { 0I..100I }

|> Seq.map fib

|> Seq.iter (printfn "%O")

​