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4.6 Comparison and Unification of Terms

Although unification is mostly done implicitely while matching the head of a predicate, it is also provided by the predicate =/2.

[ISO]+Term1 = +Term2
Unify Term1 with Term2. True if the unification succeeds. For behaviour on cyclic terms see the Prolog flag occurs_check. It acts as if defined by the following rule. 
=(Term, Term).
[ISO]+Term1 \= +Term2
Equivalent to \+Term1 = Term2. See also dif/2.

4.6.1 Standard Order of Terms

Comparison and unification of arbitrary terms. Terms are ordered in the so called ``standard order''. This order is defined as follows:

  1. Variables < Numbers < Atoms < Strings < Compound Terms26Strings might be considered atoms in future versions. See also section 4.23
  2. Variables are sorted by address. Attaching attributes (see section 6.1) does not affect the ordering.
  3. Atoms are compared alphabetically.
  4. Strings are compared alphabetically.
  5. Numbers are compared by value. Mixed integer/float are compared as floats. If the comparison is equal, the float is considered the smaller value. If the Prolog flag iso is defined, all floating point numbers precede all integers.
  6. Compound terms are first checked on their arity, then on their functor-name (alphabetically) and finally recursively on their arguments, leftmost argument first.
[ISO]+Term1 == +Term2
True if Term1 is equivalent to Term2. A variable is only identical to a sharing variable.
[ISO]+Term1 \== +Term2
Equivalent to \+Term1 == Term2.
[ISO]+Term1 @< +Term2
True if Term1 is before Term2 in the standard order of terms.
[ISO]+Term1 @=< +Term2
True if both terms are equal (==/2) or Term1 is before Term2 in the standard order of terms.
[ISO]+Term1 @> +Term2
True if Term1 is after Term2 in the standard order of terms.
[ISO]+Term1 @>= +Term2
True if both terms are equal (==/2) or Term1 is after Term2 in the standard order of terms.
compare(?Order, +Term1, +Term2)
Determine or test the Order between two terms in the standard order of terms. Order is one of <, > or =, with the obvious meaning.

4.6.2 Special unification and comparison predicates

This section describes special purpose variations on Prolog unification. The predicate unify_with_occurs_check/2 provides sound unification and is part of the ISO standard. The predicates subsumes/2 and subsumes_chk/2 define `one-sided-unification' and are found in many Prolog systems. Finally, unifiable/3 is a `what-if' version of unification that is often qused a building block in constraint reasoners.

[ISO]unify_with_occurs_check(+Term1, +Term2)
As =/2, but using sound-unification. That is, a variable only unifies to a term if this term does not contain the variable itself. To illustrate this, consider the two goals below: 
1 ?- A = f(A).

A = f(f(f(f(f(f(f(f(f(f(...))))))))))
2 ?- unify_with_occurs_check(A, f(A)).

No

I.e. the first creates a cyclic-term, which is printed as an infinitely nested f/1 term (see the max_depth option of write_term/2). The second executes logically sound unification and thus fails. Note that the behaviour of unification through =/2 as well as implicit unification in the head can be changed using the Prolog flag occurs_check.

+Term1 =@= +Term2
True if Term1 is `structurally equal' to Term2. Structural equivalence is weaker than equivalence (==/2), but stronger than unification (=/2). Two terms are structurally equal if their tree representation is identical and they have the same `pattern' of variables. Examples:
a=@=Afalse
A=@=Btrue
x(A,A)=@=x(B,C)false
x(A,A)=@=x(B,B)true
x(A,B)=@=x(C,D)true

The predicates =@=/2 and \=@=/2 are cycle-safe. Attributed variables are considered structurally equal iff their attributes are structurally equal. This predicate is known by the name variant/2 in some other Prolog systems.

+Term1 \=@= +Term2
Equivalent to `\+Term1 =@= Term2'.
subsumes(+Generic, @Specific)
A term is told to subsume another term if instantiation in the generic term produces the specific term. The subsumption relation is also called one sided unification or semi-unification. It behaves as if defined by27This implementation relies on the fact that term_variables/2 orders its variables based on depth-first left-to-right traversal of the term. 
subsumes(General, Specific) :-
        term_variables(Specific, SVars),
        General = Specific,
        term_variables(SVars, SVars2),
        SVars == SVars2.
subsumes_chk(+Generic, @Specific)
Equivalent to \+ \+ subsumes(Generic, Specific).
unifiable(@X, @Y, -Unifier)
If X and Y can unify, unify Unifier with a list of Var = Value, representing the bindings required to make X and Y equivalent.28This predicate was introduced for the implementation of dif/2 and when/2 after discussion with Tom Schrijvers and Bart Demoen. None of us is really happy with the name and therefore suggestions for a new name are welcome. This predicate can handle cyclic terms. Attributed variables are handles as normal variables. Associated hooks are not executed.
?=(@Term1, @Term2)
Decide whether the equality of Term1 and Term2 can be compared safely, i.e. whether the result of Term1 == Term2 can change due to further instantiation of either term. It is defined as by ?=(A,B) :- (A==B ; A \= B), !. See also dif/2.