[Haskell-cafe] Is it safe to use unsafePerformIO here?

Cristiano Paris frodo at theshire.org
Tue Sep 15 16:08:46 EDT 2009

On Tue, Sep 15, 2009 at 9:29 PM, Daniel Fischer
<daniel.is.fischer at web.de> wrote:
> Am Dienstag 15 September 2009 21:13:02 schrieb Daniel Fischer:
>> Still, the body should be read lazily.
>> I'm not sure, but the tracing message may be output because of its
>> position.
>> With
>> where
>>     readBody = withFile fn ReadMode $ \h -> do
>>         b <- hGetContents h
>>         seq b $ return (trace ("Read body from: " ++ fn) b)
>> there's no tracing output.
> Yes, tested with
> -rw-r--r-- 1 me users 243M 15. Sep 21:17 file1.txt
> -rw-r--r-- 1 me users 243M 15. Sep 21:18 file2.txt

Ok, Daniel, I got the point: the IO action gets performed but there's
no need to use unsafePerformIO as hGetContents is already lazy and the
IO action is "ineffective" anyway when the result is not needed. Yet,
I'm still confused as "seq b" should force the complete execution of
hGetContents. So I decided to run a different test:

I'm using this code:

module Main where

import System.IO
import System.IO.Unsafe
import Control.Applicative
import Data.List
import Data.Ord

import Debug.Trace

data Bit = Bit { index :: Integer, body :: String }

readBit fn =
  withFile fn ReadMode $ \h -> Bit <$> (hGetLine h >>= return . read)
<*> readBody
  where readBody = trace "In readBody"
                   $ withFile fn ReadMode
                     $ \h -> do b <- hGetContents h
                                seq b $ return $ trace ("Read body
from: " ++ fn) b

main = do bl <- mapM readBit ["file1.txt","file2.txt"]
          mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl
          putStrLn $ body $ head bl

(Hope this looks better than before).

I ran this on a 115KB-long file1.txt file and traced with dtruss (OSX
strace equivalent). You know what? Only the first 1024 bytes of
file1.txt are read and actually displayed.

So, it seems that "seq b" is completely ineffective and program is not correct.


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