Am Dienstag 26 Januar 2010 11:52:45 schrieb kane96@gmx.de:
Hello, I have: data Nat = Z | S Nat deriving (Eq,Ord,Show) and should write a function that works for Nats like foldr for list where: instance Enum Nat where toEnum i | i < 0 = error "foo"
| i == 0 = Z | otherwise = S (toEnum (i-1))
Can someone give me a hint how to do this?
I don't understand what "works for Nats like foldr for lists" means. A very nice explanation of how foldr works is foldr f z (a1 : (a2 : (a3 : (... : (an : []) ...)))) ~> (a1 `f` (a2 `f` (a3 `f` (... `f` (an `f` z) ...)))) So we replace the constructor [] with the base value z, and the constructor (:) with the function f. Maybe it's meant to be similar, foldNat f z should replace the constructor Z with the base value and the constructor S with the function.