% File: gv.pro
% Implementation of a global variable ADT.
% Essential functionality.

declare(Variable,Value) :-
    assert(binding(Variable, Value)).

display_bindings :-
    binding(Variable, Value),
    write(Variable), write(': '),
    write(Value), nl, fail.
display_bindings.

value_of(Variable, Value) :-
    binding(Variable, Value).

bind(Variable, Value) :-
    retract(binding(Variable, _)),
    assert(binding(Variable, Value)).

undeclare(Variable) :-
    retract(binding(Variable, _)).

inc(Variable) :-
	value_of(Variable, X),
	NewValue is X + 1,
	bind(Variable, NewValue).

dec(Variable) :-
    value_of(Variable, OldValue),
	NewValue is OldValue - 1,
	bind(Variable, NewValue).

add(Variable, Number) :-
	value_of(Variable, OldValue),
	NewValue is OldValue + Number,
	bind(Variable, NewValue).

sub(Variable, Number) :-
	value_of(Variable, OldValue),
	NewValue is OldValue - Number,
	bind(Variable, NewValue).