On Wed, Aug 15, 2012 at 9:54 PM, Daniel Hlynskyi <abcz2.upr...@gmail.com>wrote:
> 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 > comprehensible: > > hTakeWhileNotFound str hdl = fmap reverse$ findStr str hdl [0] [] > where > 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? > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > This is essentially what we do in wai-extra for multipart body parsing[1]. This code uses `conduit`. The tricky part is that you have to remember that the substring you're looking for might be spread across multiple chunks, so you need to take that into account. A simple approach would be: * If the search string is a substring of the current chunk, success. * If the end of the current chunk is a prefix of the search string, grab the next chunk, append the two, and repeat. (Note: there are more efficient approaches than appending.) * Otherwise, skip to the next chunk. * If no more chunks available, the substring was not found. Michael [1] https://github.com/yesodweb/wai/blob/master/wai-extra/Network/Wai/Parse.hs#L270
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