maybe-monad.scm

; monads
;
; references:
;   http://onclojure.com/2009/03/05/a-monad-tutorial-for-clojure-programmers-part-1/
;   http://www.intensivesystems.net/tutorials/monads_101.html
;   http://en.wikipedia.org/wiki/Monad_(functional_programming)
;

(define-method return (monad)
  (make monad :value))

(define-method return (monad value)
  (make monad :value value))

;
; define the 'nothing' monad
;
(define-class <nothing> () ())

(define-method value-of ((monad <nothing>)) :nothing)

(define-method bind ((monad <nothing>) function) monad)


;
; define the 'maybe' monad
;
(define-class <maybe> ()
  ((value :init-keyword :value :accessor value-of)))

(define-method bind (monad function)
  (let ((result (function (value-of monad))))
    (if (is-a? result <nothing>)
        result
        (return <maybe> result))))

;
; define a few functions to use on the monadic values
;
(define (f v) (* v v))    ; square the value
(define (g v) (+ v v))    ; double the value

(define (h v)
  (if (> v 10)
      (return <nothing>)
      (- v 1)))


(value-of (bind (bind (bind (return <maybe> 4) f ) h) g))  ; -> :nothing   (g (h (f 4)))
(value-of (bind (bind (bind (return <maybe> 4) g ) h) f))  ; -> 49         (f (h (g 4)))

; notice that the functions f and g don't know anything about <nothing> and
; they still don't blow up when those functions are chained together