Question

When I try to run this code…

module Main where

import qualified Data.Text.Lazy.IO as LTIO
import qualified Data.Text.Lazy as LT
import System.IO (IOMode(..), withFile)

getFirstLine :: FilePath -> IO String
getFirstLine path =
        withFile path ReadMode (\f -> do
                contents <- LTIO.hGetContents f
                return ("-- "++(LT.unpack . head $ LT.lines contents)++" --"))

main::IO()
main = do
        firstLine <- getFirstLine "/tmp/foo.csv"
        print firstLine

I get

"-- *** Exception: Prelude.head: empty list

… where I would expect it to print the first line of “/tmp/foo.csv”. Could you please explain why? Ultimately, I’m trying to figure out how to create a lazy list of Texts from file input.

Answer 1

As Daniel Lyons mentions in a comment, this is due to IO and laziness interacting.

Imagine, if you will:

• withFile opens the file, to file handle f.
• Thunk using contents of f is returned.
• withFile closes the file.
• Thunk is evaluated. There are no contents in a closed file.

This trap is mentioned on the HaskellWiki / Maintaining laziness page.

To fix, you can either read the whole file contents within withFile (possibly by forcing it with seq) or lazily close the file instead of using withFile.