@@ Linia 1: / Linia 1: @@
+====== Occurs ======
+{{tag>terms}}
+===== Description =====
+Occurs in
+**Source**:  The Art of Prolog
+===== Download =====
+Program source code: {{occurs.pl}}
+===== Listing =====
+<code prolog>
+/*
+   occurs_in(Sub,Term) :-
+	Sub is a subterm of the (possibly non-ground term) Term.
+*/
+% a:  Using ==
+   occurs_in(X,Term) :- subterm(Sub,Term), X == Sub.
+% b:  Using freeze    $$$$  freeze is not in Standard Prolog
+   occurs_in(X,_) :- freeze(X,Xf), freeze(_,Termf), subterm(Xf,Termf).
+/*
+   subterm(Sub,Term) :- Sub is a subterm of the ground term Term.
+*/
+     subterm(Term,Term).
+     subterm(Sub,Term) :-
+        compound(Term), functor(Term,_,N), subterm(N,Sub,Term).
+     subterm(N,Sub,Term) :-
+        arg(N,Term,Arg), subterm(Sub,Arg).
+     subterm(N,Sub,Term) :-
+        N > 1, N1 is N-1, subterm(N1,Sub,Term).
+%  Program
+.7    Occurs in
+</code>
+===== Comments =====

pl/prolog/pllib/occurs.txt · ostatnio zmienione: 2019/06/27 15:50 (edycja zewnętrzna)

AIwiki

Menu