Generalize runSTArray and runSTUArray

[David Feuer]

Here's a link to that old PR:

https://github.com/haskell/primitive/pull/109

On Thu, Aug 22, 2019, 1:27 AM David Feuer <david.feuer at gmail.com> wrote:

> I did some work on this sort of thing for primitive, which didn't want it.
> But maybe array does. If I don't link to it in the next day, please ping me.
>
> On Thu, Aug 22, 2019, 1:25 AM Zemyla <zemyla at gmail.com> wrote:
>
>> The "runSTArray" and "runSTUArray" functions allow efficiently working
>> with Arrays in the ST monad before turning them immutable; however, they
>> don't allow any way to return supplemental or alternative information with
>> the array. There are many times when I've wanted to get an (Array i e, w)
>> or a Maybe (UArray i e), but I couldn't, and had to use the
>> far-more-inefficient freezeArray and hope it inlined properly.
>>
>> What I want are functions that generalize the return types given:
>>
>> runSTArrayTrav :: Traversable t => (forall s. ST s (t (STArray s i e)))
>> -> t (Array i e)
>> runSTArrayTrav m = runST $ m >>= traverse unsafeFreezeSTArray
>>
>> runSTUArrayTrav :: Traversable t => (forall s. ST s (t (STUArray s i e)))
>> -> t (UArray i e)
>> runSTUArrayTrav m = runST $ m >>= traverse unsafeFreezeSTUArray
>>
>> And then an even more generalized version, which takes a sort of
>> Lens-like iterator, and allows returning multiple arrays of different
>> kinds, types, and indices:
>>
>> runSTArrayWith :: (forall f s. Applicative f => (forall i e. STArray s i
>> e -> f (Array i e)) -> (forall i e. STUArray s i e -> f (UArray i e)) -> u
>> s -> f v) -> (forall s. ST s (u s)) -> v
>> runSTArrayWith tr m = runST $ m >>= tr unsafeFreezeSTArray
>> unsafeFreezeSTUArray
>>
>> The advantage of the runSTArrayTrav/runSTUArrayTrav functions, if they're
>> subsets of the runSTArrayWith function, is that it works with standard
>> things like (,) and Either, and doesn't require wrapping it in a newtype so
>> that the s is at the end.
>>
>> The names of the functions are up for debate, and I know there will be
>> one, because naming things is one of the two hard problems in computer
>> science, along with cache invalidation and off-by-one errors.
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>
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