Global variables have been introduced by various Prolog
implementations recently. The implementation of them in SWI-Prolog is
based on hProlog by Bart Demoen. In discussion with Bart it was decided
that the semantics of hProlog nb_setval/2,
which is equivalent to nb_linkval/2,
is not acceptable for normal Prolog users as the behaviour is influenced
by how built-in predicates that construct terms (read/1,
=../2, etc.) are implemented.
GNU-Prolog provides a rich set of global variables, including arrays.
Arrays can be implemented easily in SWI-Prolog using functor/3
and
setarg/3
due to the unrestricted arity of compound terms.
