[Haskell-beginners] haskell / prolog

[dan portin]

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 at 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.
>
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