% Simulated memory 
% Prolog variables are temporary local variables that can be lost
% during backtracking.
% The Prolog data base is global but does not associate a unique
% value with each variable.
% This file shows how to emulate a global random access memory.
% ram(x, v) is a clause when and only when x is a variable and v its
% current value.
:-dynamic(ram/2). % allow ram to change.
print_ram:-ram(X, VX), write('RAM['), write(X), write(']='),  write(VX), nl,fail.
print_ram:- write('-------------'), nl.

initial_ram:-abolish(ram,2 ), !, assert(ram(nothing, empty)).

% Now define semantics of assignment statements used with our RAM.
% Notice these do not evaluate the value being stored.
set(X,V):- remove_old(X), assert(ram(X, V)),!.

remove_old(X):- (ram(X,Old), retract(ram(X,Old)); true),!.

% The following two definitions will work with functions.plg to
% allow variables in RAM to be used in functional expressions
:-consult('functions.plg').
function(X,V):-ram(X,V),!.

let(X,E):-EV es E, remove_old(X), assert(ram(X,EV)),!.

:-write('intial_ram, print_ram, set(X,V), let(X,Expr) loaded'),nl,nl.