Matching Clauses

Matching clauses form a language for writing determinate Prolog programs. A matching clause takes the following form:

      H, G  => B

or alternatively

      H :- G : B

where H is an atomic formula, G and B are two sequences of atomic formulas. H is called the head, G the guard, and B the body of the clause. No call in G can bind variables in H and all calls in G must be in-line tests. In other words, the guard must be flat. The following types of predicates can occur in G:  

• Type checking
  • integer(X), real(X), float(X), number(X), var(X), nonvar(X), atom(X), atomic(X): X must be a variable that occurs before in either the head or some other call in the guard.

• Matching
  • X=Y: One of the arguments must be a non-variable term and the other must be a variable that occurs before. The non-variable term serves as a pattern and the variable refers to an object to be matched against the pattern. This call succeeds when the pattern and the object become identical after a substitution is applied to the pattern. For instance, the call f(X)=Y in a guard succeeds when Y is a structure whose functor is f/1.

• Term inspection
  • functor(T,F,N): T must be a variable that occurs before. The call succeeds if the T's functor is F/N. F can be either an atom or a variable. If F is not a first-occurrence variable, then the call is equivalent to functor(T,F1,N),F1==F. Similarly, N can be either an integer or a variable. If N is not a first-occurrence variable, then the call is equivalent to functor(T,F,N1),N1==N.
  • arg(N,T,A): T must be a variable that occurs before and N must be an integer that is in the range of 1 and the arity of T, inclusive. If A is a first-occurrence variable, the call succeeds and binds A to the Nth argument of T. If A is a variable that occurs before, the call is equivalent to arg(N,T,A1),A1==A. If A is a non-variable term, then the call is equivalent to arg(N,T,A1),A1=A where A is a pattern and A1 is an object to be matched against A.
  • T1 == T2: T1 and T2 are identical terms.
  • T1 \== T2: T1 and T2 are not identical terms.

• Arithmetic comparisons
  • E1 =:= E2,E1 =\= E2, E1 > E2, E1 >= E2, E1 < E2, E1 =< E2: E1 and E2 must be ground expressions.

For a call C, matching rather than unification is used to select a matching clause in its predicate. The matching clause 'H, G => B' is applicable to C if C matches H (i.e., C and H become identical after a substitution is applied to H) and G succeeds. When applying the matching clause to C, the system rewrites C into B.