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Date: Thu, 01 Mar 2012 00:07:21 -0600
From: Evan Driscoll <driscoll@cs.wisc.edu>
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To: Dan Stromberg <drsalists@gmail.com>
Subject: Re: Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp
References: <85fa5760-68c8-41a2-9116-2489165f7ca1@j5g2000yqm.googlegroups.com> <CAGGBd_ppMdmf4OaEPp_ZAkkzGt+Y23sh3U6oUMAMGnnqLeH1Kg@mail.gmail.com>
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Cc: python-list@python.org, Xah Lee <xahlee@gmail.com>
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On 2/29/2012 23:05, Dan Stromberg wrote:
>=20
> On Wed, Feb 29, 2012 at 8:07 PM, Xah Lee <xahlee@gmail.com
> <mailto:xahlee@gmail.com>> wrote:
>=20
>     This page tells you what's =93In-place algorithm=94, using {python,=
 perl,
>     emacs lisp} code to illustrate.
>=20
> 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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