Generalize runSTArray and runSTUArray

[David Feuer]

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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