Improved interface to UArray

[Zemyla]

We should probably use something similar, but I don't think a dependency on
primitive is a good idea.

On Tue, Mar 3, 2020, 14:28 Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> Hey Zemyla!
> one fuzzy question i have, and perhaps you have a perspective on this,
> would/could the PrimMonad machinery in primitive help simplify / clean up
> stuff here too?
>
> On Mon, Feb 24, 2020 at 9:46 AM Zemyla <zemyla at gmail.com> wrote:
>
>> It's currently hard to describe that a type should be usable with UArray
>> or IOUArray, and almost impossible to describe without QualifiedConstraints
>> that it should be usable with STUArray. And it's actually impossible to use
>> GeneralizedNewtypeDeriving to derive new instances of IArray or MArray. It
>> also means that it's difficult to keep the three sets of instances in sync.
>>
>> Therefore, there should be (taking inspiration from primitive) a class
>> that describes types which can be used in unboxed arrays. It'd be something
>> along the lines of:
>>
>> class Unboxed e where
>>   unsafeNewUArray :: Int# -> e -> State# s -> (# State# s,
>> MutableByteArray# s #)
>>   unsafeNewUArray_ :: Const (Int# -> State# s -> (# State# s,
>> MutableByteArray# s #)) e
>>   unsafeReadUArray :: MutableByteArray# s -> Int# -> State# s -> (#
>> State# s, e #)
>>   unsafeWriteUArray :: MutableByteArray# s -> Int# -> e -> State# s ->
>> State# s
>>   unsafeIndexUArray :: ByteArray# -> Int# -> e
>>
>> And then the instances for UArray, STUArray, and IOUArray would be:
>>
>> instance Unboxed e => IArray UArray e where ...
>> instance Unboxed e => MArray IOUArray e IO where ...
>> instance Unboxed e => MArray (STUArray s) e (ST s) where ...
>>
>> Incidentally, this also means it'd be less effort to add instances like
>> Unboxed CInt, Unboxed a => Unboxed (Complex a), and Unboxed Ordering.
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>
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