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Creates a named reference called Name.
- An atom.
This creates a named reference with the atomic name Name. A named
reference can be used to hold a reference to a term in the same way
as a logical variable. Unlike the non-logical variables, the value
of a reference is not a copy, but identical to the original term it
was set to. This implies that the value behaves logically, i.e.
it disappears on backtracking, bindings to the variables inside it
are undone on backtracking etc. A typical example of it use is global
state that a set of predicates wants to share without having to
pass an argument pair through all the predicate invocations.
Changing the value of a reference is similar to changing an argument
of a compound term using setarg/3.
The initial value of a reference is the integer 0. To have a different
initial value, use reference/2.
There are no arrays of references, but the same effect can be
achieved by storing a structure in a reference and using the
structure's arguments. The arguments can then be accessed and
modified using arg/3 and setarg/3 respectively.
Note: Declaring a reference twice is silently accepted, and the second
declaration is ignored.
Modes and Determinism
- (4) instantiation fault
- Name is not instantiated.
- (5) type error
- Name is not an atom.
% comparison between references and nonlogical variables
[eclipse 1]: local reference(a), variable(b).
[eclipse 2]: Term = p(X), setval(a, Term), getval(a, Y), Y == Term.
X = X
Y = p(X)
Term = p(X)
[eclipse 3]: Term = p(X), setval(b, Term), getval(b, Y), Y == Term.
no (more) solution.
% values of references are subject to backtracking:
[eclipse 4]: setval(a, 1), (setval(a, 2), getval(a, X); getval(a, Y)).
X = 2
Y = Y More? (;)
X = X
Y = 1
reference / 2, setval / 2, getval / 2, setarg / 3, arg / 3