IO monad in Scheme

monad.scm

#!/usr/bin/env guile
!#

(use-modules ((srfi srfi-1) #:select (fold)))

;;; An IO monad is a function which has no parameters.

(define (io-unit x) (lambda () x))

;;; The bind function.
(define (io->>= io . funcs)
  (fold (lambda (f io) (lambda () ((f (io))))) io funcs)
)

;;; The sequence function. When called with two arguments a and b, this is
;;; equivalent to (io->>= a (lambda (x) b)).
(define (io->> io . rest)
  (if (null? rest)
      io
      (lambda ()
	(io)
	((apply io->> rest))
      )
  )
)

(define (io-repeat io runs f init)
  (define (loop runs x)
    (if (< runs 1)
	x
	(loop (1- runs) (f x (io)))
    )
  )
  (lambda () (loop runs init))
)

(define io-done (io-unit #f))

(define (io-wrap proc) (lambda args (lambda () (apply proc args))))