Using newtypes or any data with unboxed Vectors

[Ken Bateman]

I think I've managed to come up with an easy, useful, and sound way for
mere mortals to use unboxed vectors with arbitrary datatypes, without using
template Haskell.

I had a need for this capability, googled about it, and I didn't find a
satisfying way to do it.  This technique seemed useful and simple enough to
go into the vector library, but there's so much stuff going on in there I'm
not ready to submit a patch.

A user just needs to create an UnboxEquivalent instance for their type, and
then they can work with what looks and acts like a vector of their type,
but is in fact backed by a newtype-wrapped unboxed vector of the equivalent
type.

newtype SmallPos = Pos Int
    deriving (Eq, Ord, Show)

smallPos :: Int -> SmallPos
smallPos x | x > 0 && x < 100 = Pos x
           | otherwise = error "bad SmallPos"

instance UnboxEquivalent SmallPos where
    type UnboxEquiv SmallPos = Word8
    toUnbox (Pos x) = fromIntegral x
    fromUnbox x = (Pos (fromIntegral x))

test1 :: EVector SmallPos
test1 = Data.Vector.Generic.fromList $ map smallPos [5..15]

UnboxEquivalent.hs is at http://lpaste.net/136381
Some really simple test code is at http://lpaste.net/136382

I haven't performance tested it, or stuck {-# INLINE #-} annotations in,
but I think everything should be optimizing away to nothing, except of
course for the toUnbox and fromUnbox calls.

What do you think?

-Ken
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