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pl:prolog:pllib:occurs [2019/06/27 15:50] (aktualna)
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 +====== 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 ​
 +
 +0.7    Occurs in
 +
 +</​code>​
 +===== Comments =====
  
pl/prolog/pllib/occurs.txt · ostatnio zmienione: 2019/06/27 15:50 (edycja zewnętrzna)
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