[Haskell-beginners] Compile type Error

emmanuel.delaborde emmanuel.delaborde at cimex.com
Thu Jan 29 11:28:10 EST 2009

On 29 Jan 2009, at 16:10, beginners-request at haskell.org wrote:

> Content-Type: text/plain; charset="iso-8859-1"
> I didn't compile, but, by the looks of it, I would change
> import qualified Data.ByteString as BS (readFile)
> for
> import qualified Data.ByteString.Lazy as BS (readFile)
> as it seems sha1 needs a lazy bytestring

Thanks !
It compiles now but brings me to my real problem :)
If I uncomment this line

"md5"  -> md5sum

I get the following :


module Main where

import System.Environment (getArgs)
import Data.Digest.OpenSSL.MD5 (md5sum)
import Data.Digest.Pure.SHA (sha1, showDigest)
import qualified Data.ByteString.Lazy 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 now fails to compile with this error  :

     Couldn't match expected type `Data.ByteString.Internal.ByteString'
            against inferred type  
     In the expression: showDigest . sha1
     In a case alternative: "sha1" -> showDigest . sha1
     In the expression:
         case codec of {
           "md5" -> md5sum
           "sha1" -> showDigest . sha1
           _ -> error "Codec must be md5 or sha1 !" }

How can I both use md5sum and sha1 when one use a lazy ByteString end  
the other one does not ?



Emmanuel Delaborde
Web Technologist
53-55 Scrutton Street, London UK, EC2A 4PJ
T: +44 (0)20 7324 7780
F: +44 (0)20 7324 7781


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