Skip to content

Instantly share code, notes, and snippets.

What would you like to do?
State Monad in Scheme
(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))))))))
(display "Result: ")
(display (eval-stack composed initial-stack)))))
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment