Attributed variables provide a technique for extending the Prolog unification algorithm Holzbaur, 1992 by hooking the binding of attributed variables. There is no consensus in the Prolog community on the exact definition and interface to attributed variables. The SWI-Prolog interface is identical to the one realised by Bart Demoen for hProlog Demoen, 2002. This interface is simple and available on all Prolog systems that can run the Leuven CHR system (see chapter 8 and the Leuven CHR page).
Binding an attributed variable schedules a goal to be executed at the first possible opportunity. In the current implementation the hooks are executed immediately after a successful unification of the clause-head or successful completion of a foreign language (built-in) predicate. Each attribute is associated to a module, and the hook (attr_unify_hook/2) is executed in this module. The example below realises a very simple and incomplete finite domain reasoner:
:- module(domain,
[ domain/2 % Var, ?Domain
]).
:- use_module(library(ordsets)).
domain(X, Dom) :-
var(Dom), !,
get_attr(X, domain, Dom).
domain(X, List) :-
list_to_ord_set(List, Domain),
put_attr(Y, domain, Domain),
X = Y.
% An attributed variable with attribute value Domain has been
% assigned the value Y
attr_unify_hook(Domain, Y) :-
( get_attr(Y, domain, Dom2)
-> ord_intersection(Domain, Dom2, NewDomain),
( NewDomain == []
-> fail
; NewDomain = [Value]
-> Y = Value
; put_attr(Y, domain, NewDomain)
)
; var(Y)
-> put_attr( Y, domain, Domain )
; ord_memberchk(Y, Domain)
).
% Translate attributes from this module to residual goals
attribute_goals(X) -->
{ get_attr(X, domain, List) },
[domain(X, List)].
Before explaining the code we give some example queries:
?- domain(X, [a,b]), X = cfail ?- domain(X, [a,b]), domain(X, [a,c]).X = a ?- domain(X, [a,b,c]), domain(X, [a,c]).domain(X, [a, c])
The predicate domain/2 fetches (first clause) or assigns (second clause) the variable a domain, a set of values the variable can be unified with. In the second clause, domain/2 first associates the domain with a fresh variable (Y) and then unifies X to this variable to deal with the possibility that X already has a domain. The predicate attr_unify_hook/2 (see below) is a hook called after a variable with a domain is assigned a value. In the simple case where the variable is bound to a concrete value, we simply check whether this value is in the domain. Otherwise we take the intersection of the domains and either fail if the intersection is empty (first example), assign the value if there is only one value in the intersection (second example), or assign the intersection as the new domain of the variable (third example). The nonterminal attribute_goals/3 is used to translate remaining attributes to user-readable goals that, when executed, reinstate these attributes.
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.
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. New infrastructure
dealing with communicating attribute values must be based on
copy_term/3
and its hook attribute_goals/3.//This building block is used by the top level to report pending attributes in a portable and understandable fashion. This predicate is the preferred way to reason about and communicate terms with constraints.
term_attvars(Term,[]) in an efficient test that Term
has
no attributes; scanning the term is aborted after the first
attributed variable is found.
Normal user code should deal with put_attr/3, get_attr/3 and del_attr/2. The routines in this section fetch or set the entire attribute list of a variable. Use of these predicates is anticipated to be restricted to printing and other special purpose operations.
att(Module, Value, MoreAttributes), where MoreAttributes
is
[] for the last attribute.