[Haskell-beginners] Evaluate behaviour

[Tom Hobbs]

Hi guys,

Thanks to everyone who helped me, I hit a milestone this evening and  
finally got the (small) bit of functionality I was working on, working!  A  
huge "thank you" to everyone who has taken the time to use very small  
words to explain things to me!  (I just need to sort out some nicer error  
handling now...)

Just one more question... for today.

Here's my code;

import Network
import System.IO  
(hGetLine,hClose,hPutStrLn,hSetBuffering,BufferMode(..),Handle,stdout)
import Data.Bits
import Data.Binary
import Data.Binary.Put
import Data.Binary.Get
import qualified Data.ByteString.Lazy	as L
import qualified Data.ByteString.UTF8	as UTF
import Control.Monad
import Control.Exception (evaluate)

ping a t = do
	h <- connectTo a (PortNumber t)
	hSetBuffering h NoBuffering
	L.hPut h (encode (0xFAB10000 :: Word32))
	numStrings <- fmap (fromIntegral . runGet getWord64be) $ L.hGet h 8
	names <- (fmap (runGet $ readStrings numStrings) $ L.hGetContents h)
	evaluate names
	hClose h
	return names

readStrings :: Int -> Get [String]
readStrings n = replicateM n $ do
		len <- getWord32be
		name <- getByteString $ fromIntegral len
		return $ UTF.toString name

This works in GHCi exactly as I want it to.  Note the call to "evaluate"  
in the ping function which allows me to close the handle before returning  
the IO [String].  This behaviour prints out the list of Strings as read  
 from the handle in GHCi, exactly what I wanted it to do.  Here's the  
strange part;

If I remove the evalute line, and instead have;

names <- evaluate (fmap (runGet $ readStrings numStrings) $ L.hGetContents  
h)

GHCI claims that this is okay, but when I call ping I don't get any  
output.  I don't get any errors either, but I would expect to see the same  
list of Strings as before.

Can anyone explain to me why that happpens?  I'm assuming it's something  
to do with the "<-" magic, but I don't know what.

Thanks,

Tom


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