Hello, This is somewhat related to this thread: http://www.haskell.org/pipermail/glasgow-haskell-users/2007-May/012646.html In that email I asked about the performance of gaussian elimination on small matrices represented by unboxed arrays, because I noticed that unsafeRead was unexpectedly slower than readArray (the cause of this is still unknown.) Mirko Rahn replied with an indexless algorithm where unboxed arrays are replaced by lists-of-lists. http://www.haskell.org/pipermail/glasgow-haskell-users/2007-May/012648.html Besides being infinitely more elegant, I noticed this was considerably faster than my array-based functions, perhaps because of inlining/deforestation/list fusion or some other crazy GHC optimization. So I re-wrote my little affine geometry library representing vectors as lists and matrices as list-of-lists. One thing I wanted this library to do was to enforce dimensionality of the vectors at the type level, so you could not add a two-vector to a three-vector, even though both functions are just "zipWith (+)". After trying to tune my library I came across the SPECIALIZATION/RULES pagmas in the GHC manual, and thought that this phantom dimensionality type would be great for specializing the vector functions, and indeed it speeds things up quite a bit. For instance, if I define
zipWithV :: (Dim d) => (s->s->s) -> Vec d s -> Vec d s -> Vec d s zipWithV f (Vec u) (Vec v) = Vec( zipWith f u v )
then I can specialize
zipWithV3 :: (s->s->s) -> Vec Three s -> Vec Three s -> Vec Three s zipWithV3 f (Vec [x,y,z]) (Vec [i,j,k]) = Vec [ f x i, f y j, f k z ] {-# RULES "zipWith3" zipWithV = zipWithV3 #-}
and it makes a nice improvement to the performance of this function. But here comes my problem: Ideally, I should only have to specialize zipWith, map and foldr like this, because everything you could want to do with a vector can be implemented with these functions. (Well, everything I want to do, anyway.) But unfortunately, if I define, say...
instance (Num s) => Num (Vec d s ) where ... (-) = zipWithV (-) sqrNorm v = sumV ( mapV sqr v ) where sqr x = x*x
And I use this as
distance u v = sqrt ( sqrNorm (u-v) ) doSomethingWith (distance (u :: Vec Three Double) v)
The rules do not fire. They only seem to fire if the specialized function is called directly, such as
doSomethingWith ( zipWith (-) (u :: Vec Three Double) v )
I surely do not want to have to specialize all of my vector functions. Just the building blocks. Can anyone shed some light on my situation? Under what conditions do the rules fire? As I understand it, they fire whenever the types are the same. As far as I can tell, this is the case. How can I change things to make the rules fire more often? Thanks, Scott