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Re: Left- and right-ness of folds

This page is part of the web mail archives of SRFI 44 from before July 7th, 2015. The new archives for SRFI 44 contain all messages, not just those from before July 7th, 2015.

On Fri, Oct 24, 2003 at 01:43:45PM -0700, oleg@xxxxxxxxx wrote:
> I would like to remark that 'left' and 'right' in the traditional
> fold-left and fold-right do *not* refer to the order in which the
> elements of a collection are fetched: from the left or from the
> right. Rather, these labels refer to associativity. Let us consider an
> ordered collection, e.g., a list of three elements (e1 e2 e3). Then
> fold-left  opl seed lst ===  (((seed `opl` e1) `opl` e2) `opl` e3)
> fold-right opr seed lst ===  (e1 `opr` (e2 `opr` (e3 `opr` seed)))


Ah.  That is most enlightening as always.  Bradd, I believe this means 
you'll have to add

(define collection-fold collection-fold-left) 

to your code. :)


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