[Haskell-cafe] trying to understand out of memory exceptions

[Clark Gaebel]

See the comment for hGetContents:

"This function reads chunks at a time, doubling the chunksize on each read.
The final buffer is then realloced to the appropriate size. For files >
half of available memory, this may lead to memory exhaustion. Consider
using readFile<http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Data-ByteString.html#v:readFile>
in
this case."

http://hackage.haskell.org/packages/archive/bytestring/0.9.2.1/doc/html/Data-ByteString-Char8.html#g:31

Maybe try lazy bytestrings?

  - Clark

On Tuesday, April 16, 2013, Anatoly Yakovenko wrote:

> -- So why does this code run out of memory?
>
> import Control.DeepSeq
> import System.IO
> import qualified Data.ByteString.Char8 as BS
>
> scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a]
> scanl' f q ls =  q : (case ls of
>                         []   -> []
>                         x:xs -> let q' = f q x
>                                 in q' `deepseq` scanl' f q' xs)
>
>
> main = do
>    file <- openBinaryFile "/dev/zero" ReadMode
>    chars <- BS.hGetContents file
>    let rv = drop 100000000000 $ scanl' (+) 0 $ map fromEnum $ BS.unpack
> chars
>    print (head rv)
>
> -- my scanl' implementation seems to do the right thing, because
>
> main = print $ last $ scanl' (+) (0::Int) [0..]
>
> -- runs without blowing up.  so am i creating a some thunk here?  or is
> hGetContents storing values?  any way to get the exception handler to print
> a trace of what caused the allocation?
>
>
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