On 7/13/06, Jared Warren wrote:

Haskell's type checking language is a logical programming language. The canonical logical language is Prolog. However, Idealised Prolog does not have data structures, and does Peano numbers like:

natural(zero). natural(x), succ(x,y) :- natural(y)

And I believe (but cannot confirm):

succ(zero,y). succ(x,y) :- succ(y,z)

That is not a valid encoding of peano numbers in prolog, so I think that's where your problems stem from. :-) % defining natural numbers natural(zero). natural(s(X)) :- natural(X). % translate to integers toInt(zero, 0). toInt(s(X), N) :- toInt(X, Y), N is Y + 1. In the same style of reasoning for Haskell, we would define

data Zero data Succ x

class Natural x instance Natural Zero instance (Natural x) => Natural (Succ x)

class Natural x => ToInt x where toInt :: x -> Int

instance ToInt Zero where toInt _ = 0

instance (ToInt x) => ToInt (Succ x) where toInt _ = toInt (undefined :: x) + 1

Or we could add the toInt method directly to the Natural class like you did above. So yes, we can mimic (some of) prolog in Haskell's type language. :-) /Niklas