a sum-file program not working under Windows

[nanotitan@…]

This program just hangs under Windows, ghc-6-6.msi:

module Main where

main = print . sum . map read . lines =<< getContents

I compiled with:

ghc -O sumcol.hs -o sumcol_O.exe

and ran as:

sumcol_O.exe "sumcol-input.txt"

where sumcol-input.txt =  sumcol-input.txt :: Text_File (KB 6, Lines 1000)

Replacing 'getContents' with 'readFile "sumcol-input.txt" ' works.

Similar observed behavior under ghci:

Prelude> :set args "sumcol-input.txt"
Prelude> print . sum . map read . lines =<< getContents
_

and

Prelude> print . sum . map read . lines =<< readFile "sumcol-input.txt"
500
Prelude> _

It might also be relevant that when compiling and running this version of the sum-file program:

{-# OPTIONS -fbang-patterns #-}
module Main where

import Data.List
import qualified Data.ByteString.Lazy.Char8 as L

main = print . foldl' (+) 0 . unfoldr parse =<< L.getContents

parse !s | Just (n,t) <- L.readInt s = Just (n, L.tail t)
         | otherwise                 = Nothing

with

ghc -O -fglasgow-exts sumcol.hs -o sumcol_O.exe

and then

sumcol_O.exe "sumcol-input.txt"

I get

sumcol_O.exe: <stdin>: hGetBufNonBlocking: resource exhausted (Not enough space)

System: ghc 6.6 under WinXP Pro SP2 Intel Celeron 2.53GHz, 512 RAM

I'll be happy to provide any other necessary information:

Fernando Gonzalez, nanotitan at google's mail service

[duncan]

The confusion here is over what getContents does. It reads from the standard input 'stdin'. It has nothing to do with parsing command line arguments.

So that's why the program appears to hang, because it is waiting for input on its stdin, which by default is the console/terminal.

So to run the program you should say:

sumcol_O.exe < "sumcol-input.txt"

to direct he file "sumcol-input.txt" to the standard input of sumcol_O.exe.

If you want to use the command line arguments you should use the getArgs function and readFile as you noted.

On the other hand, the ByteString example is indeed broken because hGetBufNonBlocking is broken on Windows. I'm marking this bug as a duplicate of  the bug for that problem.