Hash with Pipes

As cryptohash is designed to work well with foldl', it can work well with Pipes.Prelude.fold. For example, you can write a function to calculate a SHA1 digest of a specified file like this.

{-# LANGUAGE OverloadedStrings #-}

import qualified Control.Foldl as F
import Control.Monad
import qualified Crypto.Hash as H
import qualified Data.ByteString as B
import qualified Data.Vector as V
import qualified Pipes as P
import qualified Pipes.ByteString as PB
import qualified Pipes.Prelude as P
import System.Environment
import System.IO

main :: IO ()
main = liftM head getArgs >>= hashFile >>= print

hashFile :: FilePath -> IO (H.Digest H.SHA1)
hashFile path = withFile path ReadMode $ P.fold H.hashUpdate H.hashInit H.hashFinalize . PB.fromHandle

It’s a good idea to factor out a fold which can be used with any Foldables as well.

hash :: H.HashAlgorithm a => F.Fold B.ByteString (H.Digest a)
hash = F.Fold H.hashUpdate H.hashInit H.hashFinalize

For example, you can use this with a Vector.

hashVector :: V.Vector B.ByteString -> H.Digest H.SHA1
hashVector = F.fold hash

h = hashVector $ V.fromList ["a", "b"]

With this fold, it’s easy to write a function that reads a stream of ByteStrings from a producer and calculates a digest of it.

hashProducer :: (Monad m, H.HashAlgorithm a) => P.Producer B.ByteString m () -> m (H.Digest a)
hashProducer = F.purely P.fold hash

Here is a rewritten version of hashFile using hashProducer.

hashFile' :: FilePath -> IO (H.Digest H.SHA1)
hashFile' path = withFile path ReadMode $ hashProducer . PB.fromHandle