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local reference(+Name)

Creates a named reference called Name.
Name
An atom.

Description

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

Exceptions

(4) instantiation fault
Name is not instantiated.
(5) type error
Name is not an atom.

Examples

% comparison between references and nonlogical variables

      [eclipse 1]: local reference(a), variable(b).

      yes.
      [eclipse 2]: Term = p(X), setval(a, Term), getval(a, Y), Y == Term.
      X = X
      Y = p(X)
      Term = p(X)
      yes.
      [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

See Also

reference / 2, setval / 2, getval / 2, setarg / 3, arg / 3