Learning Monads - Stack from the javascript example

stack_monad.fs

// Adapted from http://igstan.ro/posts/2011-05-02-understanding-monads-with-javascript.html

type StackResult = {value: int; stack: int list}
let push element stack =
  {value= element; stack = element :: stack}
let pop stack =
  {value= (List.head stack); stack = (List.tail stack)}

let bind operation continuation stack =
  let r = operation stack
  let newStack = r.stack
  continuation (r.value) r.stack
let result value stack = {value = value; stack = stack}

type StackBuilder() =
  member this.Bind(x, f) = bind x f
  member this.Delay(f) = f()
  member this.Return(x) = result x

let stackExpression = new StackBuilder()

let comp = bind (push 4) (fun r0 ->
  bind (push 5) (fun r1 ->
    bind (pop) (fun r2 ->
      bind (pop) (fun r3 ->
        result (r2 + r3)
      )
    )
  )
)
let cmp = stackExpression {
  let! _ = push 4
  let! _ = push 5
  let! a = pop
  let! b = pop
  return a + b
}

printfn "comp -- %d" (comp []).value

printfn "cmp -- %d" (cmp []).value