Toy implementation of imperative variables and manual scoping, Prolog

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%commands to the interpreter are submitted from stdin input ('show input' box below)
%'halt.' will be automatically appended to stdin input.
%swi-prolog 7.6.4

%(basic) emulation of imperative local variables (with manual scoping)
% In the following "scope" means a scope manually created with new_scope, or the global scope.
% set(Symbol,Value) binds the value Value to the name Symbol in the current scope, and returns true.
%                   e.g. set(abc,12) binds the value 12 to the name abc
%                        set(f(X,a),5) binds the value 5 to all terms in the form f(_,a)
% set_global(Symbol,Value) binds the value Value to the name Symbol in the global scope.
% fetch(Symbol,Value) checks if Symbol is bound to Value, in the topmost scope in which it is bound.
%                     If Symbol isn't bound in any scope visible from the curret scope, ir returns false.
% new_scope/0 creates a new scope. Always returns true.
% end_scope/0 ends the topmost scope. Returns false iff it is called in the global scope.

:- dynamic get_var/3.
set_var(Scope,Name,Value) :- asserta(
    (get_var(Scope,Name1,Value1) :- Name1=Name, !,
                                    Value1=Value) ).

:- dynamic next_scope_id/1.
:- dynamic active_scope/1.
next_scope_id(0) :- !.
active_scope([]) :- !.
new_scope :- next_scope_id(Id),
             NextId is Id+1,
             asserta(next_scope_id(NextId) :- !),
             active_scope(L),
             asserta(active_scope([Id|L]) :- !), !.

end_scope :- active_scope(S), S=[_|T],
             asserta(active_scope(T) :- !), !.

set(Var,Value) :- active_scope(S),
                  set_var(S,Var,Value).
set_global(Var,Value) :- set_var([],Var,Value).

fetch_in_scope(S,Var,Value) :- (get_var(S,Var,Value), !);
                               S=[_|T],
                               fetch_in_scope(T,Var,Value).
fetch(Var,Value) :- active_scope(S),
                    fetch_in_scope(S,Var,Value).

%% EXAMPLE %%

main :- set(x,"in outer scope"),
        new_scope,
          set(x,"in inner scope"),
          fetch(x,Y),
          write(Y), nl, % ==> prints "in inner scope"
        end_scope,
        fetch(x,X),
        write(X), nl. % ==> prints "in outer scope"

:- main.

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