[Haskell-cafe] Automatically using boxed or unboxed vectors

Alp Mestanogullari alpmestan at gmail.com
Thu Oct 26 07:27:55 UTC 2017

Hello Jake,

Many libraries implement this "array of structures -> structure of arrays"
type of transformation, including for example the 'accelerate' library. I'm
however only aware of one that does it for good old vectors:

I don't think it implements any Generic-deriving mechanism for making this
work on user-specified types without any effort. This could be a pretty
nice addition though :)

On Wed, Oct 25, 2017 at 12:43 AM, Jake Waksbaum <jake.waksbaum at gmail.com>

> I am trying to create a data type that is an vector of values, but
> internally uses an unboxed vector when possible and a boxed array
> otherwise. If this already exists, let me know. For example, any easily
> unboxed type is stored in an unboxed array, but also any type that is a
> product type of only easily unboxed types like (Int, Int) is stored in
> multiple unboxed arrays. I don’t think there is a smart way of doing this
> for sum types, so those are stored in boxed arrays.
> I’m trying to create a typeclass
> class VectorElement a where
>   data Vector a
>   replicate :: Int -> a -> Vector a
>   length :: Vector a -> Int
> to represent things that can be stored in Vectors. I can then implement it
> for specific types that I know can be stored in unboxed vectors:
> instance VectorElement Int where
>   newtype Vector Int = VectorInt (U.Vector Int)
>                      deriving Show
>   replicate len x = VectorInt $ U.replicate len x
>   length (VectorInt v) = U.length v
> I also want to automatically derive instances of this class for other
> types using the Generic typeclass. Ideally these instances would be the
> most efficient possible, so that for example the instance for (Int, Int)
> used two unboxed arrays but the instance for Maybe Int uses a boxed array.
> To that end I created another typeclass and wrote instances for the Generic
> data constructors:
> class VectorElementG (r :: * -> *) where
>   data VectorG r
>   replicateG :: Int -> r a -> VectorG r
>   lengthG :: VectorG r -> Int
> instance VectorElementG V1 where
>   data VectorG V1
>   replicateG = undefined
>   lengthG = undefined
> instance VectorElementG U1 where
>   newtype VectorG U1 = VectorGUnit Int
>   replicateG i U1 = VectorGUnit i
>   lengthG (VectorGUnit i) = i
> instance VectorElement a => VectorElementG (K1 i a) where
>   newtype VectorG (K1 i a) = VectorGK (Vector a)
>   replicateG i (K1 x) = VectorGK $ replicate i x
>   lengthG (VectorGK v) = length v
> instance (VectorElementG r1, VectorElementG r2) => VectorElementG (r1 :*:
> r2) where
>   data VectorG (r1 :*: r2) = VectorGProd (VectorG r1) (VectorG r2)
>   replicateG i (a :*: b) = VectorGProd (replicateG i a) (replicateG i b)
>   lengthG (VectorGProd v _) = lengthG v
> instance VectorElement ((r1 :+: r2) p) where
>   newtype Vector ((r1 :+: r2) p) = VectorSum (V.Vector ((r1 :+: r2) p))
>   replicate i x = VectorSum $ V.replicate i x
>   length (VectorSum v) = V.length v
> instance VectorElementG f => VectorElementG (M1 i c f) where
>   newtype VectorG (M1 i c f) = VectorGMeta (VectorG f)
>   replicateG i (M1 f) = VectorGMeta $ replicateG i f
>   lengthG (VectorGMeta v) = lengthG v
> I’m not sure if these are correct, especially the one for :+:. I want
> basically base cases to be any type that already has an instance of
> VectorElement or a sum type which is automatically boxed, and the recursive
> case to basically just use parallel vectors for product types.
> I think this sort of worked insofar as it allowed me to write an instance
> for tuples:
> instance (VectorElement a, VectorElement b) => VectorElement (a,b) where
>   newtype Vector (a, b) = VectorTuple (VectorG (Rep (a, b)))
>   replicate i x = VectorTuple $ replicateG i (from x)
>   length (VectorTuple v) = lengthG v
> Ideally, however, the compiler would automatically derive this instance
> using the Generic instance. Is there a way to do that also?
> I would welcome any thoughts on this entire idea and approach.
> Thanks,
> Jake Waksbaum
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Alp Mestanogullari
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