Starting with version 9.3.24, SWI-Prolog offers some support for
tabled execution with constraints (attributed variables, see
section 8.1) All relevant
data structures support attributed variables, notably tries (see section
4.14.4). The basic attributed variable and tabling with attributed
variables tests from XSB have been ported and integrated in SWI-Prolog's
test suite.193Thanks to Theresa
Swift and David Warren. Some remarks:
- The variant is defined by the attributes and their values.
Note however that SWI-Prolog represents multiple attributes in a linked
list where the ordering depends on the order in which the attributes
were added while, ideally, the order is not relevant for the semantics
of attributes.
- Solving a goal with attributed variables may modify
attributes. As a result, enumeration of answers from the completed trie replaces
attributes rather than unifying with the attributes. The new set of
attributes is always a copy of the original set of attributes. For
example:
:- use_module(library(clpfd)).
:- table p/1.
p(X) :- X #>= 1, X #=< 6.
p(20).
?- X #> 0, p(X).
X in 1..6 ;
X = 20.
Note that this behaviour is unlike trie_gen/2.
If an attributed variable is inserted into a trie, trie_gen/2
unifies the stored attributed term with the second argument of the call.
