cont-monad.fsx

type Cont<'T> = ('T -> unit) -> (exn -> unit) -> unit

let ret t = fun sc _ -> sc t
let (>>=) f g =
    fun sc ec ->
        let sc' (t : 'T) =
            match (try Choice1Of2 (g t) with e -> Choice2Of2 e) with
            | Choice1Of2 g -> g sc ec
            | Choice2Of2 e -> ec e

        f sc' ec

type ContBuilder() =
    member __.Zero() = ret ()
    member __.Return t = ret t
    member __.Bind(f : Cont<'T>, g : 'T -> Cont<'S>) : Cont<'S> = f >>= g
    member __.Delay(f : unit -> Cont<'T>) : Cont<'T> = ret () >>= f

let run (cont : Cont<'T>) =
    let result = ref Unchecked.defaultof<'T>
    cont (fun t -> result := t) raise
    !result

let cont = new ContBuilder()

/// test the implementation

let rec factorial n = cont {
    if n = 0 then return failwith "bug!"
    else
        let! pd = factorial (n-1)
        return n * pd
}


run (factorial 5)