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Monads/List monad: Difference between revisions
+Racket
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=={{header|Racket}}==
{{trans|JavaScript}}
Note that this also demonstrates how to use Racket's macro system to implement the do syntax.
<lang racket>#lang racket
(define (bind x f) (append-map f x))
(define return list)
(define ((lift f) x) (list (f x)))
(define listy-inc (lift add1))
(define listy-double (lift (λ (x) (* 2 x))))
(bind (bind '(3 4 5) listy-inc) listy-double)
;; => '(8 10 12)
(define (pythagorean-triples n)
(bind (range 1 n)
(λ (x)
(bind (range (add1 x) n)
(λ (y)
(bind (range (add1 y) n)
(λ (z)
(if (= (+ (* x x) (* y y)) (* z z))
(return (list x y z))
'()))))))))
(pythagorean-triples 25)
;; => '((3 4 5) (5 12 13) (6 8 10) (8 15 17) (9 12 15) (12 16 20))
(require syntax/parse/define)
(define-syntax-parser do-macro
[(_ [x {~datum <-} y] . the-rest) #'(bind y (λ (x) (do-macro . the-rest)))]
[(_ e) #'e])
(define (pythagorean-triples* n)
(do-macro
[x <- (range 1 n)]
[y <- (range (add1 x) n)]
[z <- (range (add1 y) n)]
(if (= (+ (* x x) (* y y)) (* z z))
(return (list x y z))
'())))
(pythagorean-triples* 25)
;; => '((3 4 5) (5 12 13) (6 8 10) (8 15 17) (9 12 15) (12 16 20))</lang>
=={{header|Ring}}==
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