This is the best I could come up with.
We let the compiler prove that "s" is irrelevant to the MArray
instance for a particular instance of STUArray, and package up that
knowledge using an existential type. We can then extract the instance
for any type; in particular, the instance for the current state
thread. I think this argues that the class context for MArray is too
constrained.
(I don't really use GADTs here, just pack up the class context, but I
like the GADT syntax for doing this)
-- ryan
{-# LANGUAGE GADTs, RankNTypes, FlexibleContexts, ScopedTypeVariables #-}
module StuTest where
import Control.Monad;
import Control.Monad.ST;
import Data.Array.ST;
import Data.Array.Unboxed;
import Data.Array.MArray;
import Data.Word;
data HasMArray s e where
HasMArray :: MArray (STUArray s) e (ST s) => HasMArray s e
newtype HasUnbox e = HasUnbox (forall s. HasMArray s e)
wombat :: forall s ix e. (IArray UArray e, Ix ix) => HasUnbox e ->
UArray ix e -> ST s (UArray ix e)
wombat (HasUnbox h) arr = case h of
(HasMArray :: HasMArray s e) ->
(unsafeThaw arr :: ST s (STUArray s ix e)) >>= unsafeFreeze
intHasUnbox :: HasUnbox Int
intHasUnbox = HasUnbox HasMArray
test :: (IArray UArray e, Ix ix) => HasUnbox e -> UArray ix e -> UArray ix e
test ctxt mem = runST (wombat ctxt mem)
simpleTest :: Ix ix => UArray ix Int -> UArray ix Int
simpleTest a = runST (wombat (HasUnbox HasMArray) a)
On Fri, Jun 19, 2009 at 6:43 PM, Scott Michel<scooter....@gmail.com> wrote:
> I'm trying to get my mind around how to thaw and then freeze a UArray.
> Theoretically, what I've written below should be a no-op, but I keep
> getting typing errors that I can't figure out. GHCI 6.10.3 says:
>
> Couldn't match expected type `UArray ix a'
> against inferred type `ST s (STUArray s ix1 e)'
> In the first argument of `(>>=)', namely
> `(unsafeThaw mem :: ST s (STUArray s ix e))'
> In the expression:
> (unsafeThaw mem :: ST s (STUArray s ix e))
> >>=
> (\ mmem -> unsafeFreeze mmem)
> In the definition of `wombat':
> wombat val idx mem
> = (unsafeThaw mem :: ST s (STUArray s ix e))
> >>=
> (\ mmem -> unsafeFreeze mmem)
>
> I'm figuring that usafeThaw with the type annotation should have given
> GHIC enough clue.
>
> Any suggestions?
>
>
> -scooter
> (WOMBAT = Waste Of Money Brains And Time)
>
> import Control.Monad;
> import Control.Monad.ST;
> import Data.Array.ST;
> import Data.Array.Unboxed;
> import Data.Array.MArray;
> import Data.Word;
>
> wombat :: (IArray UArray e, Ix ix, MArray (STUArray s) e (ST s)) => e
> -> ix -> UArray ix e -> UArray ix e
> wombat val idx mem = (unsafeThaw mem :: ST s (STUArray s ix e)) >>=
> (\mmem -> unsafeFreeze mmem)
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>
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