Although unification is mostly done implicitly 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. It acts as if defined by the following fact:
=(Term, Term).

For behaviour on cyclic terms see the Prolog flag
occurs_check.
Calls to =/2 in a clause
body are compiled and may be (re)moved depending on the Prolog flag
optimise_unify.
See also section 2.18.3.

- [ISO]
`@Term1` **\=** `@Term2` - Equivalent to
`\+`

Term1 = Term2

.
This predicate is logically sound if its arguments are sufficiently
instantiated. In other cases, such as `?- X ``\=`

Y.

,
the predicate fails although there are solutions. This is due to the
incomplete nature of \+/1.

To make your programs work correctly also in situations where the
arguments are not yet sufficiently instantiated, use dif/2
instead.

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

`Variables` < `Numbers` < `Strings`
< `Atoms` < `Compound Terms`
- Variables are sorted by address.
`Numbers` are compared by value. Mixed rational/float are
compared using cmpr/2.^{66Up
to 9.1.4, comparison was done as float.} NaN is considered
smaller than all numbers, including `-inf`

. 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 rationals.
`Strings` are compared alphabetically.
`Atoms` are compared alphabetically.
`Compound` terms are first checked on their arity, then on
their functor name (alphabetically) and finally recursively on their
arguments, leftmost argument first.

Although variables are ordered, there are some unexpected properties
one should keep in mind when relying on variable ordering. This applies
to the predicates below as to predicate such as sort/2
as well as libraries that reply on ordering such as library `library(assoc)`

and library
`library(ordsets)`

. Obviously, an established relation `A` `@<`

`B`
no longer holds if `A` is unified with e.g., a number. Also
unifying `A` with `B` invalidates the relation because
they become equivalent (==/2) after unification.

As stated above, variables are sorted by address, which implies that
they are sorted by‘age', where‘older' variables are ordered
before‘newer' variables. If two variables are unified their‘shared'
age is the age of oldest variable. This implies we can examine a list of
sorted variables with‘newer' (fresh) variables without
invalidating the order. Attaching an *attribute*, see section
8.1, turns an‘old' variable into a‘new' one as
illustrated below. Note that the first always succeeds as the first
argument of a term is always the oldest. This only applies for the *first*
attribute, i.e., further manipulation of the attribute list does *not*
change the‘age'.

?- T = f(A,B), A @< B.
T = f(A, B).
?- T = f(A,B), put_attr(A, name, value), A @< B.
false.

The above implies you *can* use e.g., an assoc (from library
`library(assoc)`

, implemented as an AVL tree) to maintain
information about a set of variables. You must be careful about what you
do with the attributes though. In many cases it is more robust to use
attributes to register information about variables.

Note that the standard order is not well defined on
*rational trees*, also known as *cyclic terms*. This
issue
was identified by Mats Carlsson, quoted below. See also
issue#1162
on GitHub.

Consider the terms `A` and `B` defined by
the equations
[1] A = s(B,0).
[2] B = s(A,1).

- Clearly,
`A` and `B` are not identical, so either
`A @< B`

or `A @> B`

must hold.

- Assume
`A @< B`

. But then, `s(A,1) @> s(B,0)`

i.e.,
`B @< A`

. Contradicton.

- Assume
`A @> B`

. But then, `s(A,1) @< s(B,0)`

i.e.,
`B @< A`

. Contradicton.

- [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.
- [ISO]
**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.

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
predicate subsumes_term/2
defines‘one-sided unification' and is part of the ISO proposal
established in Edinburgh (2010). Finally, unifiable/3
is a‘what-if' version of unification that is often used as 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 queries below.
1 ?- A = f(A).
A = f(A).
2 ?- unify_with_occurs_check(A, f(A)).
false.

The first statement creates a *cyclic
term*, also called a
*rational tree*. 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.

The SWI-Prolog implementation of unify_with_occurs_check/2
is cycle-safe and only guards against *creating* cycles, not
against cycles that may already be present in one of the arguments. This
is illustrated in the following two queries:

?- X = f(X), Y = X, unify_with_occurs_check(X, Y).
X = Y, Y = f(Y).
?- X = f(X), Y = f(Y), unify_with_occurs_check(X, Y).
X = Y, Y = f(Y).

Some other Prolog systems interpret unify_with_occurs_check/2
as if defined by the clause below, causing failure on the above two
queries. Direct use of acyclic_term/1
is portable and more appropriate for such applications.

unify_with_occurs_check(X,X) :- acyclic_term(X).

`+Term1` **=@=** `+Term2`- True if
`Term1` is a *variant*
of (or *structurally equivalent* to) `Term2`. Testing
for a variant is weaker than equivalence (==/2),
but stronger than unification (=/2).
Two terms `A` and `B` are variants iff there exists a
renaming of the variables in `A` that makes `A`
equivalent (==) to `B` and vice versa.^{67Row 7
and 8 of this table may come as a surprise, but row 8 is satisfied
by (left-to-right) A ->, B -> and
(right-to-left) C ->, A ->. If the same
variable appears in different locations in the left and right term, the
variant relation can be broken by consistent binding of both terms.
E.g., after binding the first argument in row 8 to a value, both
terms are no longer variant.} Examples:
1 | `a =@= A` | false |

2 | `A =@= B` | true |

3 | `x(A,A) =@= x(B,C)` | false |

4 | `x(A,A) =@= x(B,B)` | true |

5 | `x(A,A) =@= x(A,B)` | false |

6 | `x(A,B) =@= x(C,D)` | true |

7 | `x(A,B) =@= x(B,A)` | true |

8 | `x(A,B) =@= x(C,A)` | true |

A term is always a variant of a copy of itself. Term copying takes
place in, e.g., copy_term/2, findall/3
or proving a clause added with
asserta/1.
In the pure Prolog world (i.e., without attributed variables), =@=/2
behaves as if defined below. With attributed variables, variant of the
attributes is tested rather than trying to satisfy the constraints.

A =@= B :-
copy_term(A, Ac),
copy_term(B, Bc),
numbervars(Ac, 0, N),
numbervars(Bc, 0, N),
Ac == Bc.

The SWI-Prolog implementation is cycle-safe and can deal with
variables that are shared between the left and right argument. Its
performance is comparable to ==/2,
both on success and (early) failure.
^{68The current implementation is
contributed by Kuniaki Mukai.}

This predicate is known by the name variant/2
in some other Prolog systems. Be aware of possible differences in
semantics if the arguments contain attributed variables or share
variables.^{69In many systems
variant is implemented using two calls to subsumes_term/2.}

`+Term1` **\=@=** `+Term2`- Equivalent to
`‘``\+`

Term1 =@= Term2'

. See =@=/2
for details.
- [ISO]
**subsumes_term**(`@Generic, @Specific`) - True if
`Generic` can be made equivalent to `Specific`
by only binding variables in `Generic`. The current
implementation performs the unification and ensures that the variable
set of `Specific` is not changed by the unification. On
success, the bindings are undone.^{70This
predicate is often named subsumes_chk/2
in older Prolog dialects. The current name was established in the ISO
WG17 meeting in Edinburgh (2010). The chk postfix was
considered to refer to determinism as in e.g., memberchk/2.}
This predicate respects constraints.
See section 5.6 for
defining clauses whose head is unified using
*single sided unification*.

**term_subsumer**(`+Special1,
+Special2, -General`)`General` is the most specific term that is a generalisation of
`Special1` and `Special2`. The implementation can
handle cyclic terms.
**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.^{71This
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 handled as normal variables. Associated hooks are *not*
executed.
**?=**(`@Term1, @Term2`)- Succeeds if the syntactic equality of
`Term1` and `Term2`
can be decided safely, i.e. if the result of `Term1 == Term2`

will not change due to further instantiation of either term. It behaves
as if defined by `?=(X,Y) :- \+ unifiable(X,Y,[_|_]).`