On Dec 7, 2007 6:21 PM, Dan Weston
This is great! Two questions:
1) I want to make sure the function arity matches the list length (as a runtime check). I think I can do this with an arity function using Data.Typeable. I came up with:
arity f = a (typeOf f) where a tr | typeRepTyCon tr /= mkTyCon "->" = 0 | otherwise = 1 + (a . fromJust . funResultTy tr . head . typeRepArgs $ tr)
This looks awful. Is there a better way to get the function arity?
2) Once I have say arity (+) == 2 at runtime, how can I get it reified into Succ (Succ Zero)) at compile time to be able to use it as the first argument in your nary function? Can/should I use Template Haskell for this?
You can project the compile time numbers into runtime ones:
class ProjectN n where projectN :: n -> Int
instance ProjectN Zero where projectN _ = 0
instance (ProjectN n) => ProjectN (Succ n) where projectN _ = 1 + projectN (undefined :: n)
And then make sure the length matches the projected number of arguments. Other disagreements will be resolved at compile time. Luke
Dan
Victor Nazarov wrote:
{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-}
data Zero data Succ a
class Nary n x y | n x -> y where nary :: n -> x -> [String] -> y
instance Nary Zero x x where nary _ x [] = x
instance (Nary n y z, Read x) => Nary (Succ n) (x->y) z where nary _ f (x:xs) = nary (undefined::n) (f $ read x) xs
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