Apparently the answer is to `:set FlexibleContexts` in GHCi in addition to enabling the language extension in a source file. I was unaware it wasn't enabled by default on interpreting the source.

On Mon, Dec 20, 2010 at 5:54 PM, dan portin <danportin@gmail.com> wrote:
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.