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

[Mario Blažević]

On 12-08-15 02:54 PM, Daniel Hlynskyi 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 []
>
> 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?

It's more complex than Pipes, but SCC gives you what you need. If you 
cabal install it, you have the choice of using the shsh executable on 
the command line to accomplish your task:

$ shsh -c 'cat input-file.txt | select prefix (>! substring "search 
string")'

or using the equivalent library combinators from Haskell code:

 > import System.IO (Handle, stdin)
 > import Control.Monad.Coroutine (runCoroutine)
 > import Control.Concurrent.SCC.Sequential

 > pipeline :: String -> Handle -> Producer IO Char ()
 > pipeline str hdl = fromHandle hdl >-> select (prefix $ sNot $ 
substring str)

 > hTakeWhileNotFound :: String -> Handle -> IO String
 > hTakeWhileNotFound str hdl =
 >    fmap snd $ runCoroutine $ pipe (produce $ pipeline str hdl) 
(consume toList)

 > main = hTakeWhileNotFound "up to here" stdin >>= putStrLn