%File: arithmetic.pro
%Implementation of arithmetic functions.

%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, _)).

%Crafted nondestructive arithmetic programs.

add(Variable1, Variable2, Answer) :-
    value_of(Variable1, Value1),
    value_of(Variable2, Value2),
    NewValue is Value1 + Value2,
    declare(Answer, NewValue).

sub(Variable1, Variable2, Answer) :-
    value_of(Variable1, Value1),
    value_of(Variable2, Value2),
    NewValue is Value1 - Value2,
    declare(Answer, NewValue).

mul(Variable1, Variable2, Answer) :-
    value_of(Variable1, Value1),
    value_of(Variable2, Value2),
    NewValue is Value1 * Value2,
    declare(Answer, NewValue).

div(Variable1, Variable2, Answer) :-
    value_of(Variable1, Value1),
    value_of(Variable2, Value2),
    NewValue is Value1 / Value2,
    declare(Answer, NewValue).

exp(Variable1, Variable2, Answer) :-
    value_of(Variable1, Value1),
    value_of(Variable2, Value2),
    NewValue is Value1 ^ Value2,
    declare(Answer, NewValue).