[Haskell-cafe] IO and lazyness.
schneegloeckchen at gmx.li
Tue Mar 6 15:27:20 EST 2007
On Tue, Mar 06, 2007 at 07:37:38PM +0100, D.V. wrote:
> I'm still learning Haskell and I am stuck on a probably simple problem.
> Assume I have a file where lines are of the form "key=value"
> I want to search a value in that file and came up with the following code.
> >rechf :: String -> IO (Maybe String)
> >rechf r = bracket (openFile "liste" ReadMode)
> (rechf2 r)
> >rechf2 :: String -> Handle -> IO (Maybe String)
> >rechf2 r h= do
> > f <- hGetContents h
> > --print f
> > return $ rech r $ lines f
> >rech :: String -> [ String ] -> Maybe String
> >rech r l = lookup r $ map span2 l
> >span2 :: String->(String,String)
> >span2 c = (a,b)
> > where a=takeWhile (/='=') c
> > b=drop 1 $ dropWhile (/='=') c
> Now the problem is this :
> 1) if I try rechf, it returns nothing even for a key that exists in the
> 2) if I uncomment the line where there is "print f", the key is found
> and the value returned.
I cannot help you with your question more than pointing you to
where Simon Marlow explains how to avoid "IO.bracket".
But heres a hint: you can use ParserCombinators instead of rolling your own parser,
especially if the complexity of your input formats will increase.
> I'm guessing print forces f to be evaluated, so the file is actually
> read, but I was wondering why it doesn't work without it and how to
> correct that.
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