8.1.2 Attributed variable hooks

Attribute names are linked to modules. This means that certain operations on attributed variables cause hooks to be called in the module whose name matches the attribute name.

attr_unify_hook(+AttValue, +VarValue)

A hook that must be defined in the module to which an attributed variable refers. It is called after the attributed variable has been unified with a non-var term, possibly another attributed variable. AttValue is the attribute that was associated to the variable in this module and VarValue is the new value of the variable. If this predicate fails, the unification fails. If VarValue is another attributed variable the hook often combines the two attributes and associates the combined attribute with VarValue using put_attr/3.

To be done

The way in which this hook currently works makes the implementation of important classes of constraint solvers impossible or at least extremely impractical. For increased generality and convenience, simultaneous unifications as in [X,Y]=[0,1] should be processed sequentially by the Prolog engine, or a more general hook should be provided in the future. See Triska, 2016 for more information.

attribute_goals(+Var)//

This nonterminal is the main mechanism in which residual constraints are obtained. It is called in every module where it is defined, and Var has an attribute. Its argument is that variable. In each module, attribute_goals//1 must describe a list of Prolog goals that are declaratively equivalent to the goals that caused the attributes of that module to be present and in their current state. It is always possible to do this (since these attributes stem from such goals), and it is the responsibility of constraint library authors to provide this mapping without exposing any library internals. Ideally and typically, remaining relevant attributes are mapped to pure and potentially simplified Prolog goals that satisfy both of the following:

• They are declaratively equivalent to the constraints that were originally posted.

• They use only predicates that are themselves exported and documented in the modules they stem from.

The latter property ensures that users can reason about residual goals, and see for themselves whether a constraint library behaves correctly. It is this property that makes it possible to thoroughly test constraint solvers by contrasting obtained residual goals with expected answers.

This nonterminal is used by copy_term/3, on which the Prolog top level relies to ensure the basic invariant of pure Prolog programs: The answer is declaratively equivalent to the query.

The copy_term/3 primitive uses attribute_goals//1 inside a findall/3 call. This implies that attribute_goals//1 can unify variables and modify attributes, for example, to tell other hooks that some attribute has already been taken care of. This nonterminal is also used by frozen/2 which does not create a copy. Ideally attribute_goals//1 should not modify anything to allow direct application in frozen/2. In the current implementation frozen/2 backtracks over attribute_goals//1 to tolerate the current behavior. This work-around harms the performance of frozen/2. New implementations of attribute_goals//1 should avoid relying on backtracking when feasible. Future versions of frozen/2 and copy_term/3 may require attribute_goals//1 not to modify any variables or attributes.

Note that instead of defaulty representations, a Prolog list is used to represent residual goals. This simplifies processing and reasoning about residual goals throughout all programs that need this functionality.

project_attributes(+QueryVars, +ResidualVars)

A hook that can be defined in each module to project constraints on newly introduced variables back to the query variables. QueryVars is the list of variables occurring in the query and ResidualVars is a list of variables that have attributes attached. There may be variables that occur in both lists. If possible, project_attributes/2 should change the attributes so that all constraints are expressed as residual goals that refer only to QueryVars, while other variables are existentially quantified.

[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.