Re: [Haskell-cafe] vector-simd: some code available, and some questions

On Sun, 2012-07-08 at 10:27 +0200, Gábor Lehel wrote:

On Sun, Jul 8, 2012 at 3:05 AM, Nicolas Trangez wrote:

...

I implemented the inductive alignment calculation over One and Twice (good idea, and easy to do), but I don't get the thing about superclasses. I've been trying several approaches (including definitions based on forall and other trickery I never used before), but didn't get things to work, at least: the compiler always said I'd need UndecidableInstances, and that sounds scary... Care to elaborate?

All I meant was

class (Alignment n, Alignment a) => AlignedToAtLeast n a

but I got a bit ahead of myself, because that rules out the instance on tuples.

Maybe that's one of the things I ran into, can't remember OTOH.

(I suppose you *could* write some kind of Alignment instance for them, taking their minimum or something, but that's getting a bit too subversive for me).

Heh, it already feels evil-but-in-a-good-and-powerful-way to me right now ;-) Thanks for all your input!

The alternative, if you want both Alignment as a superclass and the ability to constrain multiple types at once, is to have the above, remove the instance on tuples, and instead something like:

class (AlignedToAtLeast n a, AlignedToAtLeast n b) => AlignedToAtLeast2 n a b instance (AlignedToAtLeast n a, AlignedToAtLeast n b) => AlignedToAtLeast2 n a b class (AlignedToAtLeast n a, AlignedToAtLeast n b, AlignedToAtLeast n c) => AlignedToAtLeast3 n a b c instance (AlignedToAtLeast n a, AlignedToAtLeast n b, AlignedToAtLeast n c) => AlignedToAtLeast3 n a b c (feel free to think of better names!)

unsafeXorSSE42 :: (Storable a, SV.AlignedToAtLeast3 SV.A16 o1 o2 o3) => SV.Vector o1 a -> SV.Vector o2 a -> SV.Vector o3 a

Implemented in [1]. Code says unsafeXorSSE42 :: (Storable a, SV.AlignedToAtLeast3 SV.A16 o1 o2 o3) => SV.Vector o1 a -> SV.Vector o2 a -> SV.Vector o3 a ghci says unsafeXorSSE42 :: (AlignedToAtLeast (Data.Vector.SIMD.Mutable.Twice A8) o2, AlignedToAtLeast (Data.Vector.SIMD.Mutable.Twice A8) o1, AlignedToAtLeast (Data.Vector.SIMD.Mutable.Twice A8) o3, Foreign.Storable.Storable a) => Vector o1 a -> Vector o2 a -> Vector o3 a which is the same thing, so should do.

That will require UndecidableInstances, but all that means is that GHC can't prove to itself that instance checking will terminate. So you could end up getting the compiler into an infinite loop (or in practice, to exceed its recursion limit). But it doesn't allow anything unsafe to happen at runtime, and there's plenty of perfectly good instances which terminate even if GHC can't prove it.

I see. I had some rather unsafe/undecidable-by-a-developer things in mind, but I guess I should read up on some of the language extensions GHC provides. Thanks, Nicolas [1] https://github.com/NicolasT/vector-simd/commit/8f934891c9630a96ce009fafa7f6b...