[Haskell-beginners] Help for type matching

6 Nov 2009

      I have the following code:

$$$$$$$$
module Main where

import System.Environment (getArgs)
import Text.CSV.ByteString
import qualified Data.ByteString.Lazy.Char8 as L

main = do
       [args] <- getArgs
       file <- L.readFile args
       let result = parseCSV file
       case result of
            Nothing       -> putStrLn "Error when parsing!"
            Just contents -> do
                               putStrLn "parsing OK!"
                               -- map_header_records contents
$$$$$$$$

which yielded the error as follows:

    Couldn't match expected type `Data.ByteString.Internal.ByteString'
           against inferred type `L.ByteString'
    In the first argument of `parseCSV', namely `file'
    In the expression: parseCSV file
    In the definition of `result': result = parseCSV file

readFile in the Data.ByteString.Lazy.Char8 module returns ByteString type.
Since the module is qualified as L, L.readFile returns L.ByteString. But
parseCSV expects a ByteString. Sometimes this is annoying, because it makes
type matching difficult (at least for me, a beginner). I really wish Haskell
can intelligently treat L.ByteString, M.ByteString, and whatever
X.ByteString the same as ByteString, and match them.

Can someone tell me how to solve the above problem?

Thanks,

Hong

[Haskell-beginners] Help for type matching

Hong Yang