Operators

Predefined operators

Predefined operators according to ISO/IEC 13211-1 and 13211-2:

Many systems provide further operators as an implementation specific extension:

Operator declaration

In Prolog, custom operators can be defined using op/3:

op(+Precedence, +Type, :Operator)

• Declares Operator to be an operator of a Type with a Precedence. Operator can also be a list of names in which case all elements of the list are declared to be identical operators.

• Precedence is an integer between 0 and 1200, where 0 removes the declaration.

• Type is one of: xf, yf, xfx, xfy, yfx, fy or fx where f indicates the position of the functor and x and y indicate the positions of the arguments. y denotes a term with a precedence lower or equal to the precedence of the functor, whereas x denotes a strictly lower precedence.

  • Prefix: fx , fy
  • Infix: xfx (not associative), xfy (right associative), yfx (left associative)
  • Postfix: xf , yf

Example usage:

:- op(900, xf, is_true).

X_0 is_true :-
  X_0.

Example query:

?- dif(X, a) is_true.
dif(X, a).

Term ordering

Two terms may be compared via the standard ordering:

variables @< numbers @< atoms @< strings @< structures @< lists

Notes:

• Structures compare alphabetically by functor first, then by arity and lastly by the comparison of each argument.

• Lists compare by length first, then by each element.

Example queries:

?- alpha @< beta.
true.

?- alpha(1) @< beta.
false.

?- alpha(X) @< alpha(1).
true.

?- alpha(X) @=< alpha(Y).
true.

?- alpha(X) @> alpha(Y).
false.

?- compound(z) @< compound(inner(a)).
true.

Term equality