Am Montag, 21. April 2008 01:35 schrieb Ben:
FWIW, installed bytestring-0.9.1.0, ran ghc-pkg hide bytestring-0.9.0.1, recompiled and reinstalled binary-0.4.1. then i played around with all that you suggested, and came to the conclusion that i don't understand seq!
import Control.Exception (bracket) import System.Directory import System.IO import Data.Binary import Data.ByteString.Lazy as B
strictDecodeFile :: Binary a => FilePath -> (a -> b) -> IO () strictDecodeFile path force = bracket (openFile path ReadMode) hClose $ \h -> do c <- B.hGetContents h force (decode c) `seq` return ()
This means that force (decode c) is reduced to head normal form, not fully evaluated. So if the result type of force were e.g. Either String Int, it would be evaluated just far enough to determine if force (decode c) is bottom, Left something or Right somethingelse; Left undefined and Right undefined won't cause an exception. In the case below, force (decode c) is an IO action, it will be evaluated as far as necessary to see it's (IO whatever) and not bottom, for that it need not be run, therefore you don't see "strict 1" printed out.
strictDecodeFile' :: Binary a => FilePath -> (a -> IO b) -> IO () strictDecodeFile' path force = bracket (openFile path ReadMode) hClose $ \h -> do c <- B.hGetContents h force (decode c) return ()
Here you say you want the IO action force (decode c) to be run, so it is run and "strict 2" is printed out. Hope that helps.