[Haskell-cafe] unsafeInterleaveIO respecting order of actions
I think I have a very similar problem to the currently discussed "WriterT [w] IO is not lazy in reading [w]". I want to defer IO actions, until they are needed, but they shall be executed in order. If I call unsafeInterleaveIO, they can be executed in any order. I understand that hGetContents does not defer the hGetChar operations but instead defers the call of the (:) constructors, thus preserving the order of the hGetChar calls. In the general case this is not so simple, since the result of a monadic block might not be a list and the result of particular actions might not be needed at all, e.g. (). If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On 2009 Jan 1, at 16:44, Henning Thielemann wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? I thought unsafeInterleaveIO and users of it (readFile, hGetContents) didn't guarantee the order of actions relative to independent IO actions (that is, those performed outside the unsafeInterleaveIO) and this was why it is generally disrecommended. For example the recurring situation where people try to readFile f >>= writeFile . someTransform and the writeFile fails with a "file locked" exception. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Thu, 1 Jan 2009, Brandon S. Allbery KF8NH wrote: On 2009 Jan 1, at 16:44, Henning Thielemann wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? I thought unsafeInterleaveIO and users of it (readFile, hGetContents) didn't guarantee the order of actions relative to independent IO actions (that is, those performed outside the unsafeInterleaveIO) and this was why it is generally disrecommended. For example the recurring situation where people try to readFile f >>= writeFile . someTransform and the writeFile fails with a "file locked" exception. Sure, it's dangerous. But for what I want to do, this situation cannot occur. I can come up with a simple example which might be generalized. It simulates what hGetContents does. liftLazy2 :: (a -> b -> c) -> IO a -> IO b -> IO c liftLazy2 f x y = fmap (\ ~(xr, ~(yr,())) -> f xr yr) $ unsafeInterleaveIO $ liftM2 (,) x $ unsafeInterleaveIO $ liftM2 (,) y $ return () test0, test1 :: IO String test0 = liftLazy2 (const) getLine getLine test1 = liftLazy2 (flip const) getLine getLine test0 only requests the first line, test1 expects two lines as user input. However, with liftLazy2 we have only Applicative functionality, not Monad, and it is not composable. For example: fmap (\((x,y),z) -> z) $ liftLazy2A (,) (liftLazy2A (,) getLine getLine) getLine This requests only one line, but should three ones. The reason is that the first two getLines are defered even until the last one. Thus, it is not enough that liftLazy2 returns (IO c). Instead it must return (IO (c,(a,(b,() and these pair emulated lists must somehow be combined in order to preserve the order of execution. This looks somehow like a writer monad transformer. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Sat, 3 Jan 2009, Henning Thielemann wrote: On Thu, 1 Jan 2009, Brandon S. Allbery KF8NH wrote: On 2009 Jan 1, at 16:44, Henning Thielemann wrote: If it is generally possible to use unsafeInterleaveIO such that it executes actions in the right order, wouldn't this allow the definition of a general lazy IO monad? I thought unsafeInterleaveIO and users of it (readFile, hGetContents) didn't guarantee the order of actions relative to independent IO actions (that is, those performed outside the unsafeInterleaveIO) and this was why it is generally disrecommended. For example the recurring situation where people try to readFile f >>= writeFile . someTransform and the writeFile fails with a "file locked" exception. Sure, it's dangerous. But for what I want to do, this situation cannot occur. I can come up with a simple example which might be generalized. It simulates what hGetContents does. liftLazy2 :: (a -> b -> c) -> IO a -> IO b -> IO c liftLazy2 f x y = fmap (\ ~(xr, ~(yr,())) -> f xr yr) $ unsafeInterleaveIO $ liftM2 (,) x $ unsafeInterleaveIO $ liftM2 (,) y $ return () I think I now have general Applicative functionality: apply :: (a -> b, ()) -> (a,()) -> (b,()) apply (f,fs) a = let (a0,as) = case fs of () -> a in (f a0, as) lazyIO :: IO a -> IO (a,()) lazyIO = unsafeInterleaveIO . fmap (\x -> (x,())) liftLazy2 :: (a -> b -> c) -> IO a -> IO b -> IO c liftLazy2 f x y = liftM2 (\xr yr -> fst $ (f,()) `apply` xr `apply` yr) (lazyIO x) (lazyIO y) The () is used to enforce the order of evaluation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeInterleaveIO respecting order of actions
On Sat, 3 Jan 2009, Henning Thielemann wrote:
I think I now have general Applicative functionality ...
I hope the following is a proper Monad implementation. In contrast to
Applicative a Writer for sequencing actions does no longer work, instead I
need a State monad.
newtype LazyIO a = LazyIO {runLazyIO :: StateT RunAll IO a}
data RunAll = RunAll
deriving Show
instance Monad LazyIO where
return x = LazyIO $ return x
x >>= f = LazyIO $
mapStateT unsafeInterleaveIO . runLazyIO . f =<<
mapStateT unsafeInterleaveIO (runLazyIO x)
instance MonadIO LazyIO where
liftIO m = LazyIO $ StateT $ \RunAll -> fmap (\x->(x,RunAll)) m
evalLazyIO :: LazyIO a -> IO a
evalLazyIO =
flip evalStateT RunAll . runLazyIO
I'll write some tests and upload it to Hackage.
Thank you for being a patient audience. ;-)
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