## 4.33 Global variables

Global variables are associations between names (atoms) and terms. They differ in various ways from storing information using assert/1 or recorda/3.

- The value lives on the Prolog (global) stack. This implies that lookup time is independent of the size of the term. This is particularly interesting for large data structures such as parsed XML documents or the CHR global constraint store.

- They support both global assignment using nb_setval/2 and backtrackable assignment using b_setval/2.

- Only one value (which can be an arbitrary complex Prolog term) can be associated to a variable at a time.

- Their value cannot be shared among threads. Each thread has its own namespace and values for global variables.

- Currently global variables are scoped globally. We may consider module scoping in future versions.

Both b_setval/2 and nb_setval/2 implicitly create a variable if the referenced name does not already refer to a variable.

Global variables may be initialised from directives to make them available during the program lifetime, but some considerations are necessary for saved states and threads. Saved states do not store global variables, which implies they have to be declared with initialization/1 to recreate them after loading the saved state. Each thread has its own set of global variables, starting with an empty set. Using thread_initialization/1 to define a global variable it will be defined, restored after reloading a saved state and created in all threads that are created after the registration. Finally, global variables can be initialised using the exception hook exception/3. The latter technique is used by CHR (see chapter 8).

b_setval(+Name, +Value)
  Associate the term Value with the atom Name or replace the currently associated value with Value. If Name does not refer to an existing global variable, a variable with initial value `[]` is created (the empty list). On backtracking the assignment is reversed.
b_getval(+Name, -Value)
  Get the value associated with the global variable Name and unify it with Value. Note that this unification may further instantiate the value of the global variable. If this is undesirable the normal precautions (double negation or copy_term/2) must be taken. The b_getval/2 predicate generates errors if Name is not an atom or the requested variable does not exist.

nb_setval(+Name, +Value)
  Associates a copy of Value created with duplicate_term/2 with the atom Name. Note that this can be used to set an initial value other than `[]` prior to backtrackable assignment.
nb_getval(+Name, -Value)
  The nb_getval/2 predicate is a synonym for b_getval/2, introduced for compatibility and symmetry. As most scenarios will use a particular global variable using either non-backtrackable or backtrackable assignment, using nb_getval/2 can be used to document that the variable is non-backtrackable. Raises `existence_error(variable, Name)` if the variable does not exist. Alternatively, nb_current/2 can used to query a global variable. This version fails if the variable does not exist rather than raising an exception.
nb_linkval(+Name, +Value)
  Associates the term Value with the atom Name without copying it. This is a fast special-purpose variation of nb_setval/2 intended for expert users only because the semantics on backtracking to a point before creating the link are poorly defined for compound terms. The principal term is always left untouched, but backtracking behaviour on arguments is undone if the original assignment was trailed and left alone otherwise, which implies that the history that created the term affects the behaviour on backtracking. Consider the following example:

      demo_nb_linkval :-
              T = nice(N),
              (   N = world,
                  nb_linkval(myvar, T),
                  fail
              ;   nb_getval(myvar, V),
                  writeln(V)
              ).

nb_current(?Name, ?Value)
  Enumerate all defined variables with their value. The order of enumeration is undefined. Note that nb_current/2 can be used as an alternative for nb_getval/2 to request the value of a variable and fail silently if the variable does not exists.
nb_delete(+Name)
  Delete the named global variable. Succeeds also if the named variable does not exist.

### 4.33.1 Compatibility of SWI-Prolog Global Variables

Global variables have been introduced by various Prolog implementations recently. The implementation of them in SWI-Prolog is based on hProlog by Bart Demoen. In discussion with Bart it was decided that the semantics of hProlog nb_setval/2, which is equivalent to nb_linkval/2, is not acceptable for normal Prolog users as the behaviour is influenced by how built-in predicates that construct terms (read/1, =../2, etc.) are implemented.

GNU-Prolog provides a rich set of global variables, including arrays. Arrays can be implemented easily in SWI-Prolog using functor/3 and setarg/3 due to the unrestricted arity of compound terms.