[plt-scheme] Trying to use match.ss

plt-scheme at fast.cs.utah.edu writes:

|  You wrote the following on Wed, 28 May 2003 11:10:30 -0400 
|
||   Welcome to MzScheme version 204, Copyright (c) 1995-2003 PLT
||   | (require (lib \"match.ss\"))
|| 
||   | (match \'(2 2 2 3) ((a ... 3 ...) #t))
||   #t
||   | (match \'(2 2 2 3) ((a ... 3) #t))
||   match: no matching clause for (2 2 2 3):
||   (match (quote (2 2 2 3)) ((a ... 3) #t))
|
| This occurs because the match is \"greedy\" and not pre-emptive.  
| Since the pattern-variable _a_ will match anything, so _a ..._ alone matches the whole list.  

Aha, I did realize that the matching was greedy but I thought it
back-tracked on failure.  Thanks.

So I guess the way to match "a list with X anywhere in it" is

  ((not X) ... X _ ...) 

?  That could get a little painful when X gets complicated, since I
either have to duplicate it, or refactor it out of the match.  When I
get the sources I will try

  (list-no-order X _ ...)

although I don't know if that is an abuse of that construct or if it
is meant to be used that way.

Dan

-- 
http://www.dfan.org