[Haskell-cafe] Unboxed array of product type -> product type of unboxed arrays

[Daniel Fischer]

Am Mittwoch, 2. Juli 2008 18:48 schrieb Scott Dillard:
> Hi,
>
> I'm trying to extended the standard unboxed array types and array classes
> to my own product types, for now let's just say (,). So if the proper
> MArray and IArray instances exist for e and f, then I can make an instance
> for (e,f). The actual type of that array, something like (UArray i e,
> UArray i f), would be given by an associated type. This is how the uvector
> library does it, but that library defines its own array primitives and
> classes. I'd like to reuse the standard ones if possible.
>
> The problem I keep running into is the kind of the array, * -> * -> *, or
> 'a i e'. The crucial type there is e, which is used to dispatch the
> instance to the proper associated type, so if e = (a,b) then the array type
> would be (UArray i a, UArray I b), and if e is (a,b,c) then (UArray i a,
> UArray i b, UArray i c). If IArray was instead expecting the array type to
> be 'a e i' I could maybe do something like this:
>
> class UArrClass e where
>   data UArr e :: * -> *
> instance (IArray UArray e, IArray UArray f) => UArrClass (e,f) where
>   data UArr (e,f) i = UArrPair (UArray i e) (UArray i f)
>
> But as it stands, I can't do that. The 'i' type parameter has to be bound
> as a parameter of UArrClass. So instead I tried this.
>
> class UArrClass i e where
>   data UArr i e
>   unsafeAt_ :: UArr i e -> Int -> e
>   --mirror all IArray methods

Perhaps

class (Ix i) => UArrClass i e where ...

would work?

>
> instance
>     ( IArray UArray e
>     , IArray UArray f
>     , Ix i  --needed for unsafeAt
>     ) => UArrClass i (e,f)
>   where
>     newtype UArr i (e,f) = UArrPair (UArray i e) (UArray i f)
>     unsafeAt_ (UArrPair ea fa) i = (unsafeAt ea i , unsafeAt fa i)
>
> and then the instance for IArray could be defined as follows, just a
> mapping from the methods of that class onto my own:
>
> instance
>     ( IArray UArray e
>     , IArray UArray f
>     , UArrClass i (e,f)
>     ) => IArray UArr (e,f)
>   where
>     unsafeAt = unsafeAt_
>
> The problem I get now is from the 'Ix i' context of the IArray methods. The
> 'i' there is only mentioned in the context of the methods, not the class,
> so I have no 'handle' onto that 'i' that I can use to explicitly unify it
> with the 'i' mentioned in UArrClass. The compiler keeps complaining about
> rigid type variables. It would be great if I could leave that type variable
> unbound in my class, and only bind it in the methods, as IArray does, but
> as far as I can tell, I can't. I need to bind 'i' in my class because it's
> the first type-argument to the array type constructor, rather than the
> second. I don't care about the 'i', its the 'e' I'm after, but all
> applications of the associated type constructor need to be saturated.
>
> Can anyone see a way to do this? I understand there's about a million other
> ways to accomplish what I'm trying to do without IArray and MArray, but I'm
> just wondering if I should abandon those classes altogether, and use my own
> array classes, using something like uvector or unsafeIO/ForeignPtr. That
> seems to be trend.
>
> Thanks,
> Scott