state monad

Program.fs

open FSharpPlus

type NewList<'a> =
  | Nil
  | Cons of 'a * NewList<'a>

type Monadic<'a, 'b> =
  | Monadic of ('a -> ('b * 'a))
  static member Return x = Monadic(fun c -> (x, c))
  static member (>>=) (Monadic f, g) =
    Monadic(fun cnt ->
      let result1, cnt1 = f cnt
      let (Monadic h) = g result1
      h cnt1)

let cons x xs = Monadic(fun cnt -> (Cons(x, xs), cnt + 1))
let run c (Monadic f) = f c

[<EntryPoint>]
let main _ =
  let m =
    monad { let! a = cons "a" Nil
            let! b = cons "b" a
            return b }
  let a = run 0 m
  printfn "%A" a
  0