State monad in scheme.

state-monad.scm

(define (push element)
  (lambda (stack)
    (list '() (cons element stack))))

(define (pop)
  (lambda (stack)
    (let ((element (car stack))
          (new-stack (cdr stack)))
      (list element new-stack))))

(define (stack-of result)
  (cadr result))

(define (value-of result)
  (car result))

(define (>>= stack-action continuation)
  (lambda (stack)
    (let ((result (stack-action stack)))
      ((continuation (value-of result)) (stack-of result)))))

(define (return value)
  (lambda (stack)
    (list value stack)))

(define (run-stack computation stack)
  (computation stack))

(define (eval-stack computation stack)
  (value-of (computation stack)))

(define (exec-stack computation stack)
  (stack-of (computation stack)))

(define computation-1 (>>= (push 4) (lambda (_)
                      (>>= (push 5) (lambda (_)
                      (>>= (pop)    (lambda (a)
                      (>>= (pop)    (lambda (b)
                      (return (list a b)))))))))))

(define computation-2 (>>= (push 2) (lambda (_)
                      (>>= (push 3) (lambda (_)
                      (>>= (pop)    (lambda (a)
                      (>>= (pop)    (lambda (b)
                      (return (list a b)))))))))))

(define (main)
  (let ((initial-stack '())
        (composed (>>= computation-1 (lambda (a)
                  (>>= computation-2 (lambda (b)
                  (return (list a b))))))))
    (begin
      (display "Result: ")
      (display (eval-stack composed initial-stack)))))