[Haskell-cafe] Re: distinguish functions from non-functions in a class/instances

[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?

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
>