toEnum problem

[Michael Ackerman]

Hello,

I tried this:

    data Nat = Zero | Succ Nat
               
    instance Enum Nat where
        fromEnum Zero = 0
        fromEnum (Succ n) = 1 + fromEnum n
        toEnum 0 = Zero
        toEnum n = if n > 0 then 1 + toEnum n
                   else error "bah"

and I get, from Hugs, the error message,

    Implementation of toEnum requires extra context
    Expected type: Enum Nat => Int -> Nat
    Missing context: (Num Nat, Num Nat)

I'm baffled (esp. by the last line). Any help will be appreciated.

                        Michael Ackerman