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3.5 Comparison and Unification or Terms

3.5.1 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 < Atoms < Strings
  2. Old Variable < New Variable (7)
  3. Atoms are compared alphabetically.
  4. Strings are compared alphabetically.
  5. Numbers are compared by value. Integers and floats are treated identically.
  6. Compound terms are first checked on their arity, then on their functor-name (alphabetically) and finally recursively on their arguments, leftmost argument first.

If the feature (see feature/2) iso is defined, all floating point numbers precede all integers.

+Term1 == +Term2
Succeeds if Term1 is equivalent to Term2. A variable is only identical to a sharing variable.

+Term1 \== +Term2
Equivalent to \+Term1 == Term2.

+Term1 = +Term2
Unify Term1 with Term2. Succeeds if the unification succeeds.

+Term1 \= +Term2
Equivalent to \+Term1 = Term2.

+Term1 =@= +Term2
Succeeds 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

+Term1 \=@= +Term2
Equivalent to `\+Term1 =@= Term2'.

+Term1 @< +Term2
Succeeds if Term1 is before Term2 in the standard order of terms.

+Term1 @=< +Term2
Succeeds if both terms are equal (==/2) or Term1 is before Term2 in the standard order of terms.

+Term1 @> +Term2
Succeeds if Term1 is after Term2 in the standard order of terms.

+Term1 @>= +Term2
Succeeds 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.