PLOGHELP EQUAL                                Chris Mellish, August 1982
                                           Jonathan Laventhol, June 1983
                                Revised by Kathryn Seifert  October 1986

Operators which perform the "equals" operation, and related operations

    ?- X = Y.
    ?- X \= Y.
    ?- X =:= Y.
    ?- X =\= Y.
    ?- X == Y.
    ?- X \== Y.

Keywords: equality, unify, unification, identity, evaluation,
          arithmetic expressions


-- INTRODUCTION -------------------------------------------------------

There are three predicates which begin with the equals sign "=", and
have roughly the sense of the word equals.  (There is also =..  which is
described in PLOGHELP * UNIV) Notice that all these predicates can be
written in prefix form as well:

    ?- =(X, Y).

We normally use the infix form, and by default, all of these operators
have precedence 40 and fix "xfx". See PLOGHELP * OP and * OPERATORS.
Also note that there are negated forms as well.

-- DESCRIPTION OF THE DIFFERENT OPERATIONS ----------------------------

    ?- X = Y.
        X and Y are equal (can be matched).

    ?- X \= Y.
        X and Y are cannot be matched.

    ?- X =:= Y.
        Arithmetic expressions X and Y evaluate to the same number. X
        and Y can be simple integers or real numbers, or they can be
        constructed from the special functors recognised by 'is' (see
        PLOGHELP * IS).

    ?- X =\= Y.
        Arithmetic expression do not evaluate to the same number.

    ?- X == Y.
        X and Y are identical. Uninstantiated variables in corresponding
        positions must already be "sharing" for this to be true.

    ?- X \== Y.
        X and Y are not identical.

-- RELATED DOCUMENTATION ----------------------------------------------

PLOGHELP * IS
 Evaluating POP-11 and arithmetic expressions which return one result

PLOGHELP * SYMBOLS
 Summary and index of HELP files for symbols used in Prolog

PLOGHELP * OPERATORS
 Operator declarations made when the Prolog system is loaded

PLOGHELP * OP
 How to declare operators