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.* 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