Re: [Haskell-cafe] Custom monad using ST
In practice if you want to actually _use_ ST you'll find you'll need to let the world escape into your type. Otherwise you won't be able to create and pass around any STRefs or arrays and use them later. The universal quantification inside of MyST's definition will keep you from holding on to them. Okay, what you are saying is that two MyST action declared separately will not be compatible, right? I have another problem. One of my goal is to be able to alter an STRef when it is accessed. To do so, I use the following type: STRef s (ST s a). So the actual variable 'a' contained by my STRef is wrapped inside an ST action which goal is to modify the STRef and then return the value of type 'a'. My problem is that the STRef is not modfied, it always returns the same value. Example of this with IORefs, it is simpler to test: selfAlteringRef :: Int - IO (IORef (IO Int)) selfAlteringRef init = mfix $ \ref - newIORef $ do writeIORef ref (return 0) return init 2011/3/10 Edward Kmett ekm...@gmail.com On Wed, Mar 9, 2011 at 6:21 PM, Yves Parès limestr...@gmail.com wrote: Well, I want to hide the fact that I'm using ST, so if I can hide the existential type 's' it is better. In practice if you want to actually _use_ ST you'll find you'll need to let the world escape into your type. Otherwise you won't be able to create and pass around any STRefs or arrays and use them later. The universal quantification inside of MyST's definition will keep you from holding on to them. BTW, does someone know why the ST default implementation (the one exposed by Control.Monad.ST) is strict, whereas those of State et Writer are lazy? Mostly because of the principle of least surprise. It makes it act more like IO. -Edward ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Custom monad using ST
Hello,
I am trying to make a monad that uses ST internally.
But even when reducing this to the simplest case I'm still cramped by the
's' phantom type :
{-# LANGUAGE Rank2Types #-}
newtype MyST a = MyST (forall s. ST s a)
-- ^ I cannot use deriving (Monad) through GeneralizedNewtypeDeriving
runMyST (MyST m) = runST m
-- ^ works thanks to declaration of 's' at rank 2 in the definition of MyST
-- It refuses to compile if MyST is declared as such:
-- data MyST s a = MyST (ST s a)
instance Monad MyST where
return = MyST . return -- and this does not compile
(MyST m) = f = MyST $ do
x - m
case f x of
(MyST m) - m
If you try it, GHC will complain:
Simple.hs:13:20:
Couldn't match expected type `forall s. ST s a'
with actual type `ST s a'
Expected type: a - forall s1. ST s1 a
Actual type: a - ST s a
In the second argument of `(.)', namely
`(return :: a - (forall s. ST s a))'
In the expression: MyST . (return :: a - (forall s. ST s a))
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Re: [Haskell-cafe] Custom monad using ST
Try `return x = MyST (return x)`. It's (.) that throws it off. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Custom monad using ST
Thanks! It works this way. I often forget the dangers of point-free notation... 2011/3/9 Jake McArthur jake.mcart...@gmail.com Try `return x = MyST (return x)`. It's (.) that throws it off. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Custom monad using ST
On Wed, 9 Mar 2011, Yves Parès wrote:
Hello,
I am trying to make a monad that uses ST internally.
But even when reducing this to the simplest case I'm still cramped by the 's'
phantom
type :
{-# LANGUAGE Rank2Types #-}
newtype MyST a = MyST (forall s. ST s a)
-- ^ I cannot use deriving (Monad) through GeneralizedNewtypeDeriving
Would it make sense to make the 's' type explicit?
newtype MyST s a = MyST (ST s a)
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Re: [Haskell-cafe] Custom monad using ST
Well, I want to hide the fact that I'm using ST, so if I can hide the
existential type 's' it is better.
BTW, does someone know why the ST default implementation (the one exposed by
Control.Monad.ST) is strict, whereas those of State et Writer are lazy?
2011/3/9 Henning Thielemann lemm...@henning-thielemann.de
On Wed, 9 Mar 2011, Yves Parès wrote:
Hello,
I am trying to make a monad that uses ST internally.
But even when reducing this to the simplest case I'm still cramped by the
's' phantom
type :
{-# LANGUAGE Rank2Types #-}
newtype MyST a = MyST (forall s. ST s a)
-- ^ I cannot use deriving (Monad) through GeneralizedNewtypeDeriving
Would it make sense to make the 's' type explicit?
newtype MyST s a = MyST (ST s a)
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Re: [Haskell-cafe] Custom monad using ST
On Wed, Mar 9, 2011 at 6:21 PM, Yves Parès limestr...@gmail.com wrote: Well, I want to hide the fact that I'm using ST, so if I can hide the existential type 's' it is better. In practice if you want to actually _use_ ST you'll find you'll need to let the world escape into your type. Otherwise you won't be able to create and pass around any STRefs or arrays and use them later. The universal quantification inside of MyST's definition will keep you from holding on to them. BTW, does someone know why the ST default implementation (the one exposed by Control.Monad.ST) is strict, whereas those of State et Writer are lazy? Mostly because of the principle of least surprise. It makes it act more like IO. -Edward ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
