(define-module (data stack)
  #:export (pop peek push)
  #:use-module (control monad)
  #:use-module (control monad state))

;;; Simple stateful stack module for showing the state monad
;;; in action. These functions assume that they are in a
;;; (state list) monad. But dynamic types!

;;; TODO test these for empty stack

(define (pop)
  (do st <- (get)
      let top = (car st)
      (put (cdr st))
      (return-state top)))

(define (peek)
  (do st <- (get)
      (return-state (car st))))

(define (push v)
  (do st <- (get)
      (put (cons v st))))