diff options
| -rw-r--r-- | net/ricketyspace/sicp/two/thirtyeight.scm | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/net/ricketyspace/sicp/two/thirtyeight.scm b/net/ricketyspace/sicp/two/thirtyeight.scm new file mode 100644 index 0000000..40b4a13 --- /dev/null +++ b/net/ricketyspace/sicp/two/thirtyeight.scm @@ -0,0 +1,47 @@ +;;;; 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. +;;; |