[Haskell-beginners] haskell / prolog

[dan portin]

I recently discovered that class and instance declarations in Haskell are
Horn clauses. So I encoded the arithmetic operations from the first chapter
of *Art of Prolog* into Haskell, in the standar

fac(0,s(0)).
fac(s(N),F) :- fac(N,X), mult(s(N),X,F).

class Fac x y | x -> y
instance Fac Z (S Z)
instance (Fac n x, Mult (S n) x f) => Fac (S n) f

I don't understand, however, to make Haskell instantiate values for
variables in a proof. I.e., I don't understand what the Haskell equivalent
of

?- fac(X,Y)

would be for some value of X.
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