_____ From: [email protected] [mailto:[email protected]] On Behalf Of Will Kurt Sent: 13 August 2010 03:53 To: [email protected] Subject: [racket] question about foldl implementation For a presentation I'm working on I've been implementing various higher order functions in javascript. When trying to work out the proper foldl implementation I came across something interesting in Racket > (foldl cons '() '(1 2 3 4)) (4 3 2 1) > (foldl - 0 '(1 2 3 4)) 2 However when I looked into various other implementations of foldl I came across very different behavior Haskell's foldl seems to be implemented like this: ;;haskell-like definition (define (foldl-haskell func accum lst) (if (null? lst) accum (foldl-haskell func (func accum (car lst)) (cdr lst)))) and yields these results: > (foldl-haskell cons '() '(1 2 3 4)) ((((() . 1) . 2) . 3) . 4) > (foldl-haskell - 0 '(1 2 3 4)) -10 > Unsure which was correct I broke out SICP which uses this definition: ;;sicp version (define (fold-left op initial sequence) (define (iter result rest) (if (null? rest) result (iter (op result (car rest)) or (op (car rest) result ) , which makes the difference. DrRacket does according to its docs. (cdr rest)))) (iter initial sequence)) with these results (same as the haskell-like version): > (fold-left cons '() '(1 2 3 4)) ((((() . 1) . 2) . 3) . 4) > (fold-left - 0 '(1 2 3 4)) -10 > So which of these is the canonical implementation of a left fold? Why the difference in Racket? For pure aesthetics I like the behavior of Racket in the case of cons, but for '-' the others seems to make more sense. Any insight into this is greatly appreciated! Thanks! --Will Kurt
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