On 2/29/2012 23:05, Dan Stromberg wrote: > > On Wed, Feb 29, 2012 at 8:07 PM, Xah Lee <xah...@gmail.com > <mailto:xah...@gmail.com>> wrote: > > This page tells you what's “In-place algorithm”, using {python, perl, > emacs lisp} code to illustrate. > > Aren't in-place reversals rather non-functional?
There is one place where they're reasonably idiomatic in Lispy languages, at least by my understanding. That occurs when you are writing a function that returns a list and there is a natural recursive way to build up the answer -- but backwards. The idiom then is to build up a temporary list up backwards, then call an in-place reversal function. (NREVERSE in Common Lisp. I thought there was a reverse! in Scheme, but apparently not.) This doesn't break the external view of a pure function because the list that's being reversed is a fresh, temporary list, which is why this idiom would even fit in pretty well in Scheme. Evan
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