Question

I want to take first five bytes from the fist file in zip archive. I use zip-archive package for decompression:

import qualified Data.ByteString.Lazy as L
import Data.Maybe
import System.Environment (getArgs)

import Codec.Archive.Zip

main = do
    f:_ <- getArgs
    print . L.take 5 . fromEntry . head . zEntries . toArchive =<< L.readFile f

This code works for small archives but I got heap overflow with big ones. For example:

./zip-arch test.zip +RTS -p -hy -M100M

for this archive gives this heap profile

Answer 1

I’ve read the explanation of the zip-archive author and decided to make recommended repairs. I’ve finished with a new library – zip-conduit. Its main feature is constant memory usage without lazy IO. To take first five bytes from the fist file in the zip archive you can write:

import           System.Environment
import           Data.Conduit
import qualified Data.Conduit.Binary as CB

import           Codec.Archive.Zip

main = do
    f:_ <- getArgs
    res <- withArchive f $ do
               name:_ <- fileNames
               source <- getSource name
               runResourceT $ source $$ CB.take 5
    print res