CODEWITHSUNDEEP
haskellio
Tom Tetlaw
Tom Tetlaw

Reputation: 113

Couldn't match expected type while doing file IO

I'm trying to write a function that loads all files in a directory into a [ByteString], by calling loadFile on each element in the list that comes from getDirectoryContents

Right now I've got this:

import Data.ByteString
import System.Directory
import System.IO
import Control.Monad

loadFile :: String -> IO B.ByteString
loadFile fileName = withFile fileName ReadMode (\handle -> hGetContents handle)

loadFiles :: String -> IO [IO B.ByteString]
loadFiles x = return (map (loadFile) $ getDirectoryContents x)

And I'm getting this error: 

main.hs:28:60:
    Couldn't match type `[Char]' with `B.ByteString'
    Expected type: IO B.ByteString
      Actual type: IO String
    In the return type of a call of `hGetContents'
    In the expression: hGetContents handle
    In the third argument of `withFile', namely
      `(\ handle -> hGetContents handle)'

I am very new to haskell (I started learning yesterday) so this error message is confusing to me. I expected it to infer the type B.ByteString since that is part of the return value of loadFile. Also any comments on style/convention are welcome

Upvotes: 0

Views: 273

Answers (1)

kqr
kqr

Reputation: 15038

You are using hGetContents from System.IO, which returns an IO String. You probably want to be using B.hGetContents from Data.ByteString, which returns an IO ByteString.

The reason I know this is because the error message says it expects IO B.ByteString, but it has IO String, "In the return type of a call of hGetContents."

Upvotes: 1

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