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From: maguire@sun.soe.clarkson.edu (Bill Maguire)
Newsgroups: comp.lang.scheme
Subject: Re: Scheme recursive procedures
Message-ID: <MAGUIRE.91Feb1104832@sun.clarkson.edu>
Date: 1 Feb 91 16:48:32 GMT
References: <11739@darkstar.ucsc.edu> <5834@idunno.Princeton.EDU>
Sender: @grape.ecs.clarkson.edu
Organization: Clarkson University, Potsdam NY
Lines: 52
In-Reply-To: markv@fourier.Princeton.EDU's message of 31 Jan 91 15:57:05 GMT


>In article <11739@darkstar.ucsc.edu> gforce@ucscb.UCSC.EDU (60766000) writes:
>>
>>Can one of you scheme hackers out there help me with a prog. problem ?
>>the version of scheme at my school doesn't have the function "reverse"
>>I am trying to write one but so far have only come up with one that is
>>recursive but needs to take in two arguments.. CAn someone show me a way
>>to do it with only one ?..
>>
>>Thanx

In article <5834@idunno.Princeton.EDU> markv@fourier.Princeton.EDU (Mark VandeWettering) writes:
>Sigh!  Someone should work harder to understand what is going on.  If you
>can't code reverse, then you can't code LOTS.
>But, in true spirit of a spoiler....
>Try any one of the following (done off the top of my head):
>
>  (define (reverse l) 
>    (if (null? l) '() 
>        (append (reverse (cdr l)) (car l))))
>
>  (define (reverse-tr l)
>    (define (rloop l r)
>      (if (null? l)
>	  r
>	  (rloop (cdr l) (cons (car l) r))))
>    (rloop l '()))
>
>By some strange coincidence, we also have gotten rid of the append problem.
>This will execute in all reasonable Scheme implementations with a constant
>amount of stack, since it is fully tail recursive.
>Mark VandeWettering
>markv@acm.princeton.edu

If you really want to hack though, why not:

(define reverse
  (lambda (l)
    (call-with-current-continuation
      (lambda (return)
        (let ((r nil)
              (loop 'any-value))
          (call-with-current-continuation
            (lambda (c) (set! loop c)))
          (if (null? l) (return r))
          (set! x (cons (car l) r))
          (set! l (cdr l))
          (loop 'any-value))))))

It also uses constant stack space...

                                 -Bill Maguire