On Sat, Oct 15, 2011 at 11:00 AM, Bas van Dijk
Another problem is that the default implementation of popCount uses a '-':
popCount :: a -> Int popCount = go 0 where go !c 0 = c go c w = go (c+1) (w .&. w - 1)
I wonder what kind of bitwise algorithms will be hard to implement without using arithmetic. For example, the -1 trick is very common in bit twiddling: http://graphics.stanford.edu/~seander/bithacks.html I guess they can be implemented outside the type class, with an extra num constraint, but then we can't use specialized instructions where available, as in the case of popCount. Or perhaps we can if we use rules to rewrite e.g. popCount :: Word32 -> Int to popCount32# (which turns into a single assembly instruction). -- Johan