Wrap the predicate referenced by Head using Body. Subsequent calls
to Head call the given Body term. Body may call the original
definition through Wrapped. Wrapped is a term of the shape below,
where Closure is an opaque blob.
call(Closure(A1, ...))
Name names the wrapper for inspection using predicate_property/2 or
deletion using unwrap_predicate/2. If Head has a wrapper with Name
the Body of the existing wrapper is updated without changing the
order of the registered wrappers. The same predicate may be wrapped
multiple times. Multiple wrappers are executed starting with the
last registered (outermost).
The predicate referenced by Head does not need to be defined at the
moment the wrapper is installed. If Head is undefined, the predicate
is created instead of searched for using e.g., the auto loader.
Registered wrappers are not part of saved states (see
qsave_program/2) and thus need to be re-registered, for example
using initialization/1.
An example of using wrap_predicate/4 for computing GCD:
:- wrap_predicate(gcd(A,B,Gcd), gcd_wrap, W, gcd_wrap(W, A, B, Gcd)).
gcd(X, Y, Gcd), X < Y => gcd(X, Y-X, Gcd).
gcd(X, Y, Gcd), X > Y => gcd(Y, X-Y, Gcd).
gcd(X, _, Gcd) => Gcd = X.
gcd_wrap(call(Closure), X, Y, Gcd) :-
functor(Closure, ClosureBlob, 3),
X_eval is X,
Y_eval is Y,
call(ClosureBlob, X_eval, Y_eval, Gcd).
or (less efficient):
gcd_wrap(call(Closure), X, Y, Gcd) :-
functor(Closure, ClosureBlob, 3),
call(ClosureBlob, X_eval, Y_eval, G),
Gcd is G.
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