[Haskell-cafe] [Pipes] Can pipes solve this problem? How?

Daniel Hlynskyi abcz2.uprola at gmail.com
Wed Aug 15 20:54:16 CEST 2012

Hello Cafe.
Consider code, that takes input from handle until special substring matched:

> matchInf a res s | a `isPrefixOf` s = reverse res
> matchInf a res (c:cs)                   = matchInf a (c:res) cs
> hTakeWhileNotFound str hdl = hGetContents hdl >>= return.matchInf str []

It is simple, but the handle is closed after running. That is not good,
because I want to reuse this function.
Code can be rewritten without hGetContent, but it is much less

hTakeWhileNotFound str hdl = fmap reverse$ findStr str hdl [0] []
   findStr str hdl indeces acc = do
     c <- hGetChar hdl
     let newIndeces = [ i+1 | i <- indeces, i < length str, str!!i == c]
     if length str `elem` newIndeces
       then return (c : acc)
       else findStr str hdl (0 : newIndeces) (c : acc)

So, the question is - can pipes (any package of them) be the Holy Grail in
this situation, to both keep simple code and better deal with handles (do
not close them specifically)? How?
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