On Wed, Aug 15, 2012 at 9:54 PM, Daniel Hlynskyi <abcz2.uprola@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?

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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