slightly too(?) complex list pattern matching

[Yoann Padioleau]

Hal Daume III <hdaume@ISI.EDU> writes:

> Hi all,
> 
> I don't think I can do this, but I figured I'd ask anyway.  Suppose I
> have:
> 
> > data X = X | Y | Z
> 
> and I want to match against a list of Xs.  But all I want is to ensure
> that there's a Y followed by a Z, but I don't care if there's anything
> else (or lots of other stuff) in between.  Namely, I want to match:
> 
>   .*Y.*Z.*

I think that http://www.dcs.gla.ac.uk/~meurig/regexp
 has some stuff for you


> 
> Now, I can match YZ.* easily with:
> 
> > foo (X:Y:_) = ...
> 
> furthermore, my matches never require any backtracking, so you won't see
> something like ".*X.*Y.*X.*Z" which could possibly make you backtrack (so,
> there are no duplicates in what I'm matching against).
> 
> Anway, the current way I do this is by having a function:
> 
> > match [] _ = True
> > match (m:ms) (x:xs)
> >     | m == x    = match ms xs
> >     | otherwise = match (m:ms) xs
> 
> (Actually, I do a bit more -- I return Maybe [Int] where Nothing signifies
> that the match failed and Just intList specifies the indices of the
> matched elements, but this is superfluous).
> 
> so this requires me to write things like:
> 
> > foo l
> >     | Just indices <- [X,Y] `match` l = 
> >       let [x,y] = map (l!!) indices
> >       in  ...
> 
> instead of the nice haskellesque syntax:
> 
> > foo [..,x@X,..,y@Y,..] = ...
> 
> anyway, I'm curious if anyone else has pounded against this wall at all
> and if there's any tool (drift, or something perhaps) that would allow me
> to write more succinct versions?
> 
> Thanks!
> 
>  - Hal
> 
> --
> Hal Daume III
> 
>  "Computer science is no more about computers    | hdaume@isi.edu
>   than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
> 
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-- 
          Yoann  Padioleau,  INSA de Rennes, France,
Opinions expressed here are only mine. Je n'écris qu'à titre personnel.
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