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;;;; License: CC0-1.0
(define-module (net ricketyspace sicp two thirtyeight)
#:export (fold-left
fold-right))
(define (accumulate op initial sequence)
(if (null? sequence)
initial
(op (car sequence)
(accumulate op initial (cdr sequence)))))
(define (fold-right op initial sequence)
(accumulate op initial sequence))
(define (fold-left op initial sequence)
(define (iter result rest)
(if (null? rest)
result
(iter (op result (car rest))
(cdr rest))))
(iter initial sequence))
;;; Guile REPL
;;;
;;; scheme@(guile-user)> ,use (net ricketyspace sicp two thirtyeight)
;;; scheme@(guile-user)> (fold-right / 1.0 (list 1 2 3))
;;; $2 = 1.5
;;; scheme@(guile-user)> (fold-left / 1.0 (list 1 2 3))
;;; $12 = 0.16666666666666666
;;; scheme@(guile-user)> (fold-right list '() (list 1 2 3))
;;; $13 = (1 (2 (3 ())))
;;; scheme@(guile-user)> (fold-left list '() (list 1 2 3))
;;; $14 = (((() 1) 2) 3)
;;; scheme@(guile-user)> (fold-right * 1 (list 1 2 3))
;;; $19 = 6
;;; scheme@(guile-user)> (fold-left * 1 (list 1 2 3))
;;; $20 = 6
;;;
;;;
;;; Conclusion: When the result produced by applying `op` on
;;; `sequence` is independent of the order in which `op` is applied to
;;; each element of the `sequence` then fold-right and fold-left will
;;; yield the same result.
;;;
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