%%%% FILE: gv2.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

  add(A, B, Sum) :-
    binding(A,ValA),
    binding(B,ValB),
    S is ValA + ValB,
    declare(Sum,S),!.

  sub(A, B, Sub) :-
    binding(A,ValA),
    binding(B,ValB),
    Subtr is ValA - ValB,
    declare(Sub,Subtr),!.
  
  mul(A, B, Prod) :-
    binding(A,ValA),
    binding(B,ValB),
    Product is ValA * ValB,
    declare(Prod,Product),!.

  div(A, B, Quote) :-
    binding(A,ValA),
    binding(B,ValB),
    Q is ValA / ValB,
    declare(Quote,Q),!.

  pow(A,B, Power) :-
    binding(A,ValA),
    binding(B,ValB),
    P is ValA ** ValB,
    declare(Power,P),!.