SWI-Prolog -- attr_portray

Documentation
- Reference manual
  - Constraint Logic Programming
    - Attributed variables
      - Attributed variable hooks
        
        attr_unify_hook/2
        
        attribute_goals//1
        
        project_attributes/2
        
        attr_portray_hook/2
- Packages

[deprecated]attr_portray_hook(+AttValue, +Var)

Called by write_term/2 and friends for each attribute if the option attributes(portray) is in effect. If the hook succeeds the attribute is considered printed. Otherwise Module = ... is printed to indicate the existence of a variable. This predicate is deprecated because it cannot work with pure interface predicates like copy_term/3. Use attribute_goals//1 instead to map attributes to residual goals.