; Copyright 2005 Brian Alliet

(define (main args) (unsafe-perform-io (do io:
    (put-str-ln "Hello, World")
    (put-str-ln (show (e maybe: '(1 2 5 10) 12)))
    (put-str-ln (show (e list:  '(1 2 5 10) 12)))
    (put-str-ln (show (e maybe: '(2 5 10) 11)))
    (put-str-ln (show (e list:  '(2 5 10) 11)))
    (cs <- (read-file "monad.scm"))
    (write-file "dump.txt" cs)
    (io:return 0)
    )))

; This function is parameterized over any MonadPlus instance 
; It returns a combinaion of numbers in xs whose sum is t
; Running it under the Maybe monad yields 1 or 0 results
; Running it under the List monad yields all results
(define (e d: xs t)
    (match xs
        ((cons x xs) (cond
            ((= x t) (return d: (list x)))
            ((< x t) (msum d: (list
                ((lift-m d: (left-section cons x)) (e d: xs (- t x)))
                ((lift-m d: (left-section cons (- x))) (e d: xs (+ t x)))
                (e d: xs t))))
            (else (mzero d:))))
        (_ (mzero d:))))