FWIW, I like to assert that the familiar "FOR A = 1 TO 10" is actually
not often needed in idiomatic Scheme. More often, you're processing a
sequence, or you're doing functional programming such that you need to
recurse anyway to avoid mutation, or you need premature exits
sometimes. One possible exception that comes to mind is if you're
writing a matrix math library, but in that case you might make your own
procedures or syntax for iterating/folding over matrices in different ways.
Anyway, for the benefit of anyone new to syntax extensions, here is a
syntax definition that supports the "for-loop" example (warning: it
doesn't error-check as much as it should, because that would clutter the
example). You can paste this into DrRacket and use the Macro Stepper to
watch how it expands.
#lang scheme/base
(define-syntax for-loop
(syntax-rules ()
((_ (VAR START END) BODY0 BODY1 ...)
(let ((end-val END))
(let loop ((VAR START))
(if (> VAR end-val)
(void)
(begin BODY0 BODY1 ...
(loop (add1 VAR)))))))))
(for-loop (i 1 10) (print i))
Robby Findler wrote at 06/27/2010 09:34 PM:
Please see 'for' in the docs. Here's the relevant section of the Guide:
http://docs.racket-lang.org/guide/for.html
Robby
On Sun, Jun 27, 2010 at 8:32 PM, Brad Long <b...@longbrothers.net> wrote:
Dear racketeers,
What is the reason for not offering a looping construct in racket? For
example, something like:
(loop (i 1 10) (print i))
Just for the masses, it seems simpler to use.
Any comments?
--
http://www.neilvandyke.org/
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