On Mon, Jan 07, 2008 at 09:27:17PM +0100, Peter Verswyvelen wrote:
If your compiler (pretty amazing job btw) does whole program optimization, can it remove the dictionary (aka v-table in C/C++ parlance?) overhead of type classes? Because if I understand it correctly, unless one uses existential types, a dictionary can be compiled away since the type in question is known at compile time?
Both jhc and ghc can completely remove the dictionary parameter whenever the type is known at compile time because an implicit SPECIALIZATION rule is generated that gets triggered whenever an overloaded function is called on a known type. Jhc can do a bit more though when the type is not fully known at compile time. it has a fairly interesting implementation of type classes, rather than turning each type class parameter into a dictionary of functions, it passes a reification of the type itself as a single argument whenever type class arguments are present (and used). This means that the generated code looks something like (+) :: (a::*) -> a -> a -> a (+) t x y = case t of Int -> primIntAdd x y Integer -> primIntegerAdd x y ... standard optimizations then can apply to eliminating and coalescing the type scrutinizations. John -- John Meacham - ⑆repetae.net⑆john⑈