Haskell lazy I/O and closing files

Question

I\'ve written a small Haskell program to print the MD5 checksums of all files in the current directory (searched recursively). Basically a Haskell version of md5deep

Answer 1

Lazy IO is very bug-prone.

As dons suggested, you should use strict IO.

You can use a tool such as Iteratee to help you structure strict IO code. My favorite tool for this job is monadic lists.

import Control.Monad.ListT (ListT) -- List
import Control.Monad.IO.Class (liftIO) -- transformers
import Data.Binary (encode) -- binary
import Data.Digest.Pure.MD5 -- pureMD5
import Data.List.Class (repeat, takeWhile, foldlL) -- List
import System.IO (IOMode(ReadMode), openFile, hClose)
import qualified Data.ByteString.Lazy as BS
import Prelude hiding (repeat, takeWhile)

hashFile :: FilePath -> IO BS.ByteString
hashFile =
    fmap (encode . md5Finalize) . foldlL md5Update md5InitialContext . strictReadFileChunks 1024

strictReadFileChunks :: Int -> FilePath -> ListT IO BS.ByteString
strictReadFileChunks chunkSize filename =
    takeWhile (not . BS.null) $ do
        handle <- liftIO $ openFile filename ReadMode
        repeat () -- this makes the lines below loop
        chunk <- liftIO $ BS.hGet handle chunkSize
        when (BS.null chunk) . liftIO $ hClose handle
        return chunk

I used the "pureMD5" package here because "Crypto" doesn't seem to offer a "streaming" md5 implementation.

Monadic lists/ListT come from the "List" package on hackage (transformers' and mtl's ListT are broken and also don't come with useful functions like takeWhile)