[Haskell-cafe] unsafeInterleaveIO respecting order of actions
lemming at henning-thielemann.de
Sat Jan 3 09:58:10 EST 2009
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 =
(\xr yr -> fst $ (f,()) `apply` xr `apply` yr)
(lazyIO x) (lazyIO y)
The () is used to enforce the order of evaluation.
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