[ Syntax Settings | Reference Manual | Alphabetic Index ]

op(+Precedence, +Associativity, ++Name)

local op(+Precedence, +Associativity, ++Name)

export op(+Precedence, +Associativity, ++Name)

Declare operator syntax.
Atom or List of atoms.


Declares Name as an operator of precedence Precedence and associativity Associativity. Operators are purely a syntactic convenience: they allow prefix-, infix- or postfix-syntax as an alternative to the canonical functor-syntax for structures with one or two arguments. If used carefully, they can contribute to making code more readable.

Precedence is an integer in the range 0 to 1200. An operator with a lower precedence number binds its arguments more strongly than an operator with higher precendence number.

The different classes of operators are distinguished through the Associativity argument:

    Operator class      Associativities
     prefix             fx, fy (unary) or fxx, fxy (binary)
     infix              xfx, xfy, yfx
     postfix            xf, yf
x represents an argument whose precedence must be lower than that of the operator. y represents an argument whose precedence must be lower or equal to that of the operator. y should be used if one wants to allow chaining of operators. The position of the y will determine the grouping within a chain of operators. Examples:
    Example declaration        will allow          to stand for
    :- op(500,xfx,in).         A in B              in(A,B)
    :- op(500,xfy,in).         A in B in C         in(A,in(B,C))
    :- op(500,yfx,in).         A in B in C         in(in(A,B),C)
    :- op(500,fx ,pre).        pre A               pre(A)
    :- op(500,fy ,pre).        pre pre A           pre(pre(A))
    :- op(500, xf,post).       A post              post(A)
    :- op(500, yf,post).       A post post         post(post(A))
    :- op(500,fxx,bin).        bin A B             bin(A,B)
    :- op(500,fxy,bin).        bin A bin B C       bin(A,bin(B,C))
Prefix, infix and postfix operators are independent of each other and may coexist. See the manual chapter on syntax about how ambiguities are resolved in this case.

Operators can be local or exported, depending on the definition. The default is local, i.e. the following declarations are equivalent:

    :- op(500, xfy, before).
    :- local op(500, xfy, before).
    :- export op(500, xfy, before).
will export the operator to all modules that import the module containing the declaration. There are also a number of predefined global operators, see the User Manual appendix for a complete list.

Name may be a single atom or a list of atoms, in which case each is given the specified precedence and associativity. A value of [] is interpreted as the empty list. To declare [] as an operator, use e.g. op(500,fx,[[]]).

A precedence of 0 has a special meaning. It erases a local operator definition and/or hides a global operator of the same name and associativity class.

Modes and Determinism


This predicate is sensitive to its module context (tool predicate, see @/2).


(4) instantiation fault
Any of the input arguments is uninstantiated.
(5) type error
Precedence is not an integer.
(5) type error
Name is not an atom or list of atoms.
(5) type error
Associativity is not an atom.
(6) out of range
Precedence is not in range 0 to 1200.
(6) out of range
Associativity is not one of xf, xfx, fy, etc.


      [eclipse]: module(a).
      [a]: current_op(X, Y, +).    % there are two global operators
      X = 500
      Y = yfx     More? (;)
      X = 500
      Y = fx     More? (;)
      no (more) solution.
      [a]: display(a+b*c).         % see how it is parsed
      +(a, *(b, c))

      [a]: op(100, xfy, +).        % define a local infix
      [a]: display(a+b*c).         % parsed differently now!
      *(+(a, b), c)

      [a]: op(0, xfy, +).          % just hide the global infix
      [a]: current_op(X, Y, +).
      X = 500
      Y = fx     More? (;)
      no (more) solution.

      op(X, fx, aaa).             (Error 4).
      op(a, fx, aaa).             (Error 5).
      op(100, xfx, 1).            (Error 5).
      op(100, abc, fred).         (Error 6).
      op(100, xfx, aaa),
          op(100,xf,aaa).         (Error 43).

See Also

current_op / 3, local / 1, export / 1