[Haskell-cafe] problem with IO, strictness, and "let"
mvanier at cs.caltech.edu
Fri Jul 13 00:54:08 EDT 2007
That makes sense. Thanks!
Stefan O'Rear wrote:
> On Thu, Jul 12, 2007 at 09:22:09PM -0700, Michael Vanier wrote:
>> I stumbled across a problem with IO and strictness that I could fix, but I
>> can't understand why the fix works. I've compressed it down into a program
>> which simply computes the number of lines in a file. Here is a version
>> that doesn't work:
>> module Main where
>> import System.IO
>> import System.Environment
>> process_file :: FilePath -> IO ()
>> process_file filename =
>> do h <- openFile filename ReadMode
>> c <- hGetContents h
>> let cs = unlines $ lines c
>> hClose h
>> putStrLn $ show $ length cs
> The problem is that you're closing the file twice. When you call any
> function of the getContents family, you assign to that function the
> responsibility to close the file, no sooner than it is no longer needed.
> Don't call hClose yourself, Bad Things will happen.
> If you get rid of hClose, laziness will not hurt you - infact it will
> help you, by allowing your program to run in constant space.
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