Generate Unbox instances using TH

[Khudyakov]

It would be very nice and useful feature. Tuple-like data types are very frequently used.

data Point a = Point a a

It's not different from (a,a) and therefore could have Unboxed instance. However writing such instance is highly impractical. Obvious workaround is to make Point a newtype and to use GeneraliedNewtypeDeriving?:

newtype Point a = Point (a,a)

deriving instance (U.Unbox a) => U.Unbox             (Point a a)
deriving instance (U.Unbox a) => G.Vector  U.Vector  (Point a a)
deriving instance (U.Unbox a) => M.MVector U.MVector (Point a a)

This approach looks a little ugly to me. And if newtype couldn't be used for any reason it won't work.

Since to my knowledge TH doesn't offer any control on inlining it's not possible implement.

[rl]

Yes, this would be good! TH actually knows about INLINE pragmas (that's a relatively recent change) so it would be quite feasible, too. I'll gladly accept patches :-)

However, I suspect that with the generics support in GHC 7.2 we might not even need TH for this. I think it's best to wait and see if that's really the case.

[liyang]

Just noticed this ticket. I made a vector-th-unbox package that does exactly this. Would be quite happy for it to be absorbed into vector.

See https://github.com/liyang/vector-th-unbox and http://hackage.haskell.org/package/vector-th-unbox . The github repo is more up to date.

[Khudyakov]

Liyang, thank you for the work!

However I think it's better to keep deriving code in the separate package to avoid pulling TH dependency

[liyang]

In that case I think this can be closed.