/// https://stackoverflow.com/questions/40052256/how-does-continuation-monad-really-work/42062682#42062682

/// A continuation is a function that represents "the rest of the computation".
type Cont<'T, 'U> = ('T -> 'U)

/// An incomplete computation is a function which, when given a continuation,
/// will return a value.
type Inc<'T, 'U> = Cont<'T, 'U> -> 'U

/// Creates an incomplete computation that holds the given value.
let ret (t : 'T) : Inc<'T, _> =
    fun (cont : Cont<'T, _>) -> cont t

/// Composition of incomplete computations.
let bind (incT : Inc<'T, _>) (wrap : 'T -> Inc<'U, _>) : Inc<'U, _> =
    fun (contU : Cont<'U, _>) ->   // return an Inc, which is a function that takes a continuation as input
        incT (fun t ->             // force the given incomplete computation to cough up its wrapped value
            (wrap t) contU)        // re-wrap the raw value so it can be sent to the given continuation

/// Monad definition.
type ContinuationBuilder() =
    member __.Return(value) = ret value
    member __.Bind(inc, wrap) = bind inc wrap

/// Builder instance.
let continuation = ContinuationBuilder()

/// Continuation-ized version of Fibonacci function.
let rec fib n : Inc<int, _> =
    continuation {
        match n with
            | 0 -> return 0
            | 1 -> return 1
            | n ->
                let! x1 = fib (n - 1)
                let! x2 = fib (n - 2)
                return x1 + x2
    }

[<EntryPoint>]
let main argv =
    for i = 0 to 10 do
        fib i (fun x ->   // pass the continuation directly to the fib function
            printfn "%d: %d" i x)
    0