Generalize runSTArray and runSTUArray

[Jonathan S]

Even simpler, Freezer could just be a Monad denoting arbitrary "cleanup"
actions, as long as they don't involve writes.


-- | A computation that is guaranteed to run after all modifications are
complete.
newtype Freeze s a = {- private constructor and accessor -} Freeze {
getFreeze :: ST s a } deriving (Functor, Applicative, Monad)

freezeArray :: (Ix i, MArray a e (ST s), IArray b e) => a i e -> Freeze s
(b i e)
freezeArray arr = Freeze (unsafeFreeze arr)

-- Reads are fine too, just not writes
freezeRef :: STRef s a -> Freeze s a
freezeRef ref = Freeze (readSTRef ref)

runSTFreeze :: (forall s . ST s (Freeze s a)) -> a
runSTFreeze act = runST (act >>= getFreeze)



The proposed -Trav and -With functions can be implemented straightforwardly
from that if desired:
runSTArrayTrav m = runSTFreeze (m >>= traverse freezeArray)
runSTUArrayTrav m = runSTFreeze (m >>= traverse freezeArray)
runSTArrayWith tr m = runSTFreeze (m >>= tr freezeArray freezeArray)


On Wed, Aug 21, 2019 at 11:46 PM Henning Thielemann <
lemming at henning-thielemann.de> wrote:

>
> On Wed, 21 Aug 2019, Zemyla wrote:
>
> > The "runSTArray" and "runSTUArray" functions allow efficiently working
> > with Arrays in the ST monad before turning them immutable
>
> I think instead of some new functions we should have a Kleisli Arrow data
> type that hides unsafeFreeze for us.
>
> newtype Freezer s a b =
>     -- private constructor and accessor
>     Freezer {getFreezer :: a -> ST s b}
>
> instance Category Freezer where
> instance Arrow Freezer where
>
> quickFreeze :: Freezer s (STArray s i e) (Array i e)
> quickFreeze = Freezer unsafeFreeze
>
> runFrozen :: (forall s. Freezer s a b) -> (forall s. ST s a) -> b
> runFrozen freezer st = runST (getFreezer (freezer st))
>
>
> For moving two arrays and an additional value out, you would do
>
> runFrozen ((quickFreeze *** quickFreeze) *** id) st
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