SWI-Prolog -- nb

Documentation
- Reference manual
  - Built-in Predicates
    - Global variables
- Packages

Availability:built-in

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.