% Operational semantics of the small language
% example of Chapter 13 expressed in Prolog
% as given in Section 13.2.5, pages 593-594 of
% Kenneth C. Louden, Programming Languages
% Principles and Practice 2nd Edition
% Copyright (C) Brooks-Cole/ITP, 2003

% sample small language program abstract syntax 
% from page 593:
% seq(assign(a,plus(2,3)),
%	seq(assign(b,times(a,4)),assign(a,sub(b,5))))

reduce(plus(V1,V2),R) :- 
	integer(V1), integer(V2), !, R is V1 + V2.

reduce(plus(E,E2),plus(E1,E2)) :- reduce(E,E1).

reduce(plus(V,E),plus(V,E1)) :-
	integer(V), !, reduce(E,E1).

reduce_all(V,V) :- integer(V), !.
reduce_all(E,E2) :- reduce(E,E1), reduce_all(E1,E2).

reduce(config(I,Env),config(V,Env)) :-
                    atom(I), !, lookup(Env, I, V).

reduce(config(assign(I,V),Env),Env1) :-
     integer(V), !, update(Env, value(I,V), Env1).