[Haskell-beginners] Compile type Error

emmanuel.delaborde emmanuel.delaborde at cimex.com
Thu Jan 29 10:25:59 EST 2009


I have the following snippet :


module Main where

import System.Environment (getArgs)
import Data.Digest.OpenSSL.MD5 (md5sum)
import Data.Digest.Pure.SHA (sha1, showDigest)
import qualified Data.ByteString as BS (readFile)

-- sha1 :: ByteString -> Digest
-- readFile :: FilePath -> IO ByteString
-- md5sum :: ByteString -> String
-- showDigest :: Digest -> String

checkHash :: String -> String -> String -> IO ()
checkHash codec hash file =
   let f = case codec of
             --"md5"  -> md5sum
             "sha1" -> showDigest . sha1
             _      -> error "Codec must be md5 or sha1 !" in
   BS.readFile file >>= \fileBS ->
   let fileHash = f fileBS in
   print (hash == fileHash)

main =
   getArgs >>= \as ->
   case as of
     (codec:hash:file:_) -> checkHash codec hash file
     _                    -> usage

usage = print "checksum codec hash file"


which fails to compile, this is the error I get :

     Couldn't match expected type  
            against inferred type `Data.ByteString.Internal.ByteString'
     In the first argument of `f', namely `fileBS'
     In the expression: f fileBS
     In the definition of `fileHash': fileHash = f fileBS

it looks like  (showDigest . sha1) expects  
Data.ByteString.Lazy.Internal.ByteString but gets  

What can I do ?




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