Hi. In
class NatToInt n where natToInt :: n -> Int
the class parameter is of kind "Nat", but a function argument has to be of kind "*". However, you only want the "n" argument in order to "guide" the instance resolution mechanism. For this, you can use a "Proxy". A Proxy is a datatype that is parameterized by an arbitrary argument (of arbitrary kind), but has only one value, also called "Proxy", so it's perfect for an argument that has no computational meaning and is just there to make the type checker happy:
{-# LANGUAGE DataKinds, KindSignatures, GADTs, PolyKinds, ScopedTypeVariables #-}
import Data.Proxy
data Nat = Z | S Nat
class NatToInt n where natToInt :: Proxy n -> Int
instance NatToInt Z where natToInt _ = 0
instance NatToInt n => NatToInt (S n) where natToInt _ = 1 + natToInt (Proxy :: Proxy n)
Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com Registered in England & Wales, OC335890 250 Ice Wharf, 17 New Wharf Road, London N1 9RF, England