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

[Ross Mellgren]

Wouldn't seq b only force (at minimum) the first character of the file?

-Ross

On Sep 15, 2009, at 4:08 PM, Cristiano Paris wrote:

> 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.
>
> Cristiano
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