7.10 Tabling and constraints

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.