Re: [Haskell-beginners] Evaluate behaviour

...Oh, I just saw this: """ instead have; names <- evaluate (fmap (runGet $ readStrings numStrings) $ L.hGetContents h) """ My apologizes, I was tackling a different question. I believe the magic attributed to the (<-) operator results from the following: let FUN be "(fmap (runGet $ readStrings numStrings) $ L.hGetContents h)" then FUN :: IO [String] This: names <- FUN evaluate names bounds the result of the FUN computation to 'names'. Evaluate is run on names, therefore 'evaluate'ing the result of the FUN computation. On the other hand, this: names <- evaluate FUN bounds to 'names' the results of 'evaluate'ing FUN. The action is executed and the argument (FUN) is evaluated to WHNF. Let's see, evaluate :: a -> IO a evaluate FUN :: IO (IO [String]) names <- evaluate FUN ... and 'names' is bound to a [String] which has been evaluated to WHNF. You end up closing a handle before reading your input. If my reasoning is wrong, please (please!) let me know, you'd be helping me greatly. 2010/7/8, Elvio Rogelio Toccalino :

This is a common behaviour, Tom. I suggest you read about the "semi-closed" state of Handles in the documentation for System.IO.

It's kind of Zen :D

Basically, you are reading lazily from a bytestring (1 chunk at a time)... Although it may seem as your program read and processed the whole bytestring, it didn't (it's lazy!). If you close the handle before "using" the output (when you 'return' it from your main function), you're shutting down the incoming data channel before a single request for a chunk of the bytestring is made.

Check it out for yourself... don't hClose the handle, just leave it be. (It's messy, I know, but experiment with it.)

2010/7/7, 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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