%%%% FILE: gv1.pro
%%%% LINE: very simple global variable ADT

%% Essential functionality

  declare(Var,Val) :- 
    retract(binding(Var,_)),
    assert(binding(Var,Val)),!.
  declare(Var,Val) :-
    assert(binding(Var,Val)),!.

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

  valueof(Variable,Value) :- 
    binding(Variable,Value),!.

  undeclare(Var,_) :-
    retract(binding(Var,_)),!.

%% Binding display functionality

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

%% Arithmatic operator functionality

  inc(Variable) :-
    retract(binding(Variable,Value)),
    NewValue is Value + 1,
    assert(binding(Variable,NewValue)),!.

  dec(Variable) :-
    retract(binding(Variable,Value)),
    NewValue is Value - 1,
    assert(binding(Variable,NewValue)),!.

  add(Variable, Number) :-
    retract(binding(Variable,Value)),
    NewValue is Value + Number,
    assert(binding(Variable,NewValue)),!.

  sub(Variable, Number) :-
    retract(binding(Variable,Value)),
    NewValue is Value + Number,
    assert(binding(Variable,NewValue)),!.

  mul(Variable, Number) :-
    retract(binding(Variable,Value)),
    NewValue is Value * Number,
    assert(binding(Variable,NewValue)),!.

  div(Variable, Number) :-
    retract(binding(Variable,Value)),
    NewValue is Value / Number,
    assert(binding(Variable,NewValue)),!.