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

[Daniel Fischer]

Am Dienstag 15 September 2009 22:25:31 schrieb Cristiano Paris:
> On Tue, Sep 15, 2009 at 10:11 PM, Cristiano Paris <frodo at theshire.org> wrote:
> > On Tue, Sep 15, 2009 at 10:08 PM, Cristiano Paris <frodo at theshire.org> wrote:
> >> ...
> >> So, it seems that "seq b" is completely ineffective and program is not
> >> correct.
> >
> > Correction: removing "seq b" results in nothing being displayed :)
> >
> > So, it's not "completely" effective. I suspect this is related to the
> > fact that a String in Haskell is just a list of Char so we should use
> > seq on every element of b. Let me try...
>
> Now it works as expected:
>
> ---
> 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
>                                 let b' = foldr (\e a -> seq e (a ++ [e])) [] b

Aaawww. 
let b' = length b
or 
let b' = foldl' seq () b
or
let b' = b `using` rnf

if you want to force the whole file to be read. But then you should definitely be using 
ByteStrings.

> 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
> ----
>
> Two points:
>
> 1 - I had to cut off file1.txt to be just above 1024 bytes otherwise
> the program becomes extremely slow even on a 100KB file with a line
> being output every 5 seconds and with my CPU being completely busy
> (I'm using a modern MacBook Pro).
>
> 2 - Omitting the last line in my program actually causes the body to
> be completely read even if it's not used: this is consistent with my
> hypotesis on seq which now works properly.
>
> :)
>
> Cristiano