[Haskell-cafe] Space leak

[Arnoldo Muller]

Daniel,

Thank you so much for helping me out with this issue!

Thanks to all the other answers from haskel-cafe members too!

As a newbie, I am not able to understand why zip and map would make a
problem...

Is there any link I could read that could help me to understand why in this
case
zip and map created a leak? What are some function compositions that should
be
avoided when doing lazy I/O?

Regards,

Arnoldo


On Thu, Mar 11, 2010 at 11:46 PM, Daniel Fischer
<daniel.is.fischer at web.de>wrote:

> Am Donnerstag 11 März 2010 00:24:28 schrieb Daniel Fischer:
> > Hmm, offhand, I don't see why that isn't strict enough.
>
> Turns out, mapM_ was a red herring. The villain was (zip and map).
> I must confess, I don't know why it sort-of worked without the mapM_,
> though. "sort-of", because that also hung on to unnecessarily much memory,
> the space leak was just smaller than with the mapM_.
>
> A very small change that eliminates the space leak, is
>
> readFasta :: Int -> [Char] -> [Window]
> readFasta windowSize sequence =
>     -- get the header
>     let (header,rest) = span (/= '\n') sequence
>         chr = parseChromosome header
>        go i (w:ws) = Window w chr i : go (i+1) ws
>        go _ [] = []
>    in go 0 $ slideWindow windowSize $ filter (/= '\n') rest
>
> You can improve performance by eliminating slideWindow and the intermediate
> Window list (merging fastaExtractor and readFasta),
>
> {-# LANGUAGE BangPatterns #-}
>
> readFasta2 :: (String -> Bool) -> Int -> String
> readFasta2 test windowSize sequence =
>    let (header,rest) = span (/= '\n') sequence
>        chr = parseChromosome header
>        schr = show chr
>        go !i st@(_:tl)
>            | test w    = w ++ '\t' : schr ++ '\t' : show i ++ '\n' : go
> (i+1) tl
>            | otherwise = go (i+1) tl
>              where
>                w = take windowSize st
>        go _ [] = []
>    in go 0 (filter (/= '\n')) rest
>
>
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