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local reference(+Name, ++Init)
Creates a named reference called Name with intial value Init.
- Name
- An atom.
- Init
- A ground term
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 the reference is Init, which must be a ground term.
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.
- (4) instantiation fault
- Init is not a ground term.
- (5) type error
- Name is not an atom.
Examples
[eclipse 1]: local reference(a,0).
yes.
[eclipse 2]: ( getval(a, Old), setval(a, 27), getval(a, New)
; getval(a, Then) ).
Old = 0
New = 27
Then = Then
Yes (0.00s cpu, solution 1, maybe more) ? ;
Old = Old
New = New
Then = 0
Yes (0.00s cpu, solution 2)
See Also
reference / 1, setval / 2, getval / 2, setarg / 3, arg / 3