Bidoit, N. and Legay, P. WELL!: An evaluation procedure for all logic programs. In Proceedings of the Thirds Int. Conf. on Database Theory (1990) 335--348.  Home/Search   Document Not in Database   Summary   Related Articles   Check

.... direct extensions of SLDNF resolution have been proposed [26, 30] Besides the soundness and completeness of query evaluation, termination for function free programs is a major problem when there is recursion (possibly through negation) Various mechanisms for handling loops have been incorporated [4, 5, 8, 9, 24, 28, 38]. In addition, repeated evaluation of the same subgoal and evaluation of subgoals that are irrelevant to a query should be avoided so that the polynomial time data complexity of the well founded semantics [41] can be preserved by query processing procedures. The definition of stable models is not ....

.... win(a) move(a,X) win(X) win(b) win(b) move(b,Y) win(Y) win(a) win(c) win(a) fail Figure 2: SLDNF tree for win(a) A simple mechanism for negative loop detection is to associate with each call a negative context. This approach has been adopted in Well [4] and in XOLDTNF resolution [8] Consider a branch through negation in an SLDNF tree. The negative context of a call on the branch is the set of ground negative literals encountered along the path from the root to the call. In Figure 2, the initial call win(a) has an empty negative context. The ....

[Article contains additional citation context not shown here]

N. Bidoit and P. Legay. WELL!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....wait for potentially new answers for a call. As soon as a new answer is derived for the call, it is used to solve the repeated call in each node in the associated waiting list. OLDT resolution has been extended to handle negation and to compute the well founded semantics of normal logic programs [2, 3, 5]. We support two distinct modes of computation, namely Prolog execution and tabled evaluation. The latter is based upon SLG resolution [5] and has been implemented as a meta interpreter on top of Prolog. Three directives are available prolog, tabled and default for users to specify which ....

N. Bidoit and P. Legay. WELL!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....model of p p where p is false. In contrast, a negative loop, such as p p, is considered indeterminate since p is undefined in the well founded partial model of p p. Mechanisms for handling infinite recursion through negation have been studied in tabled evaluation of queries, including WELL [6] and XOLDTNF resolution [14] The key idea is to associate a set of ground negative literals, called the negative context , with each subgoal. The negative context for the initial subgoal is empty. When a subgoal A in a negative context N calls a ground negative literal B, B is replaced with an ....

....are selected before negative ones, and when only negative literals remain in a rule body, all the negative literals are selected and evaluated. The latter is required because the evaluation of one ground negative literal may go into an infinite loop while the evaluation of another may fail. WELL [6] and XOLDTNF resolution [14] represent a simple modification of SLS resolution with tabling to handle loops through negation. By maintaining a negative context with each subgoal, both can detect loops through negation, treat the ground negative literal involved in such a loop as undefined, and ....

N. Bidoit and P. Legay. WELL!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....To guarantee termination for non ground term bounded programs, we intend to introduce tabulation methods into SLX 23 . This will also decrease the computational complexity of the procedure. Such a modification differs significantly from existing procedures for WFS that use tabulation (e.g. [10, 12]) On the one hand, SLX is applicable to programs with explicit negation whilst others aren t; on the other hand, even for normal programs, SLX does not need a status distinct from successful and failed, as it is based on two types of proof, whilst others include a status unknown as well, because ....

N. Bidoit and P. Legay. Well!: An evaluation procedure for all logic programs. In Int. Conf. on Database Technology, pages 335--348, 1990.

....that may be assumed undefined, because they are encountered in a loop. In this way the tables must be constructed for all relevant negative contexts. This gives many redundant computations, and a rather complex result (the final construction is a forest of forests : Bidoit and Legay [23] proposed a similar system, computing the defined atoms and the potentially defined atoms separately. Recently, Bol and Degerstedt [25] proposed a simpler method that uses tabulation to detect both positive and negative loops. Only one table needs to be constructed, but their definition of failure ....

N. Bidoit and P. Legay. WELL!: An evaluation procedure for all logic programs. In Proceedings of the International Conference on Database Technology, pages 335--348, 1990.

....semantics by Przymusinski [16] and Ross [19] are extensions of SLDNF resolution with infinite failure. They are not suitable for effective computation of queries due to possible infinite loops even in programs that are function free. Effective top down computation with tabling is explored in Well [2] and XOLDTNF [8] for the well founded semantics. Two aspects of these approaches should be noted. First, a ground negative subgoal is solved by computing its positive counterpart up to a fixpoint as in Prolog. The fixpoint computation is a simple mechanism to guarantee that the positive ....

Bidoit, N. and Legay, P. Well!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....semantics by Przymusinski [16] and Ross [20] are extensions of SLDNF resolution with infinite failure. They are not suitable for effective computation of queries due to possible infinite loops even when programs are function free. Effective top down computation with tabling is explored in Well [2] and XOLDTNF [5] for the well founded semantics. Two aspects of these approaches should be noted. First, a ground negative subgoal is solved by computing its positive counterpart up to a fixpoint as in Prolog. The fixpoint computation is a simple mechanism to guarantee that the positive ....

Bidoit, N. and Legay, P. Well!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....termination are guaranteed for call consistent term bounded programs. To guarantee termination for nonground term bounded programs, we intend to introduce tabulation methods into SLX 15 . Such a modification would differ significantly from existing procedures for WFS that use tabulation (e.g. [10, 12]) On the one hand, SLX is applicable to programs with explicit negation whilst the others aren t; on the other hand, even for normal programs, SLX does not need a status distinct from successful and failed, and is based on two types of proof, whilst others include a status unknown as well and are ....

N. Bidoit and P. Legay. Well!: An evaluation procedure for all logic programs. In Int. Conf. on Database Technology, pages 335--348, 1990.

....to be completely evaluated once a fixpoint is reached. There is no analogue to this step in the technique of [8] Techniques for computing the well founded model: There are several query evaluation techniques in the literature that compute answers under the well founded model. For example, WELL [2] is based on global SLS resolution; XOLDTNF [4] is an extension of OLDT resolution; GUUS [9] is based on the alternating fixpoint semantics; and the technique of Kemp et al. 7] is based on alternating fixpoint semantics and magic sets. The class of programs handled by these techniques is larger ....

N. Bidoit and P. Legay. WELL! An evaluation procedure for all logic programs. In Proceedings of the International Conference on Database Theory, pages 335-- 348, Paris, France, December 1990.

....construction. For function free programs, it has a polynomial time data complexity [31] In addition to direct extensions of SLDNF resolution [21, 24] various mechanisms of positive and negative loop handling have been incorporated for effective query evaluation under the well founded semantics [2, 3, 6, 7, 23, 28]. However, not all of them can be extended directly for stable model computation. The definition of stable models requires guessing an interpretation and then verifying if it is a stable model. In fact, the problem of the existence of a stable model of a logic program is NPcomplete [17] There ....

....2 win(a) move(a,X) win(X) win(b) win(b) move(b,Y) win(Y) win(a) win(c) win(a) fail Figure 2: SLDNF tree for win(a) A simple mechanism for negative loop detection is to associate with each call a negative context. This approach has been adopted in Well [2] and in XOLDTNF resolution [6] Consider a branch through negation in an SLDNF tree. The negative context of a call on the branch is the set of ground negative literals encountered along the path from the root to the call. In Figure 2, the initial call win(a) has an empty negative context. The ....

[Article contains additional citation context not shown here]

N. Bidoit and P. Legay. Well!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....using only answers that have been computed or will be computed. The memoing technique not only detects positive loops, but also avoids redundant computation of identical subgoals. Mechanisms for detecting and handling negative loops in this framework have also been studied, including WELL [6] and XOLDTNF resolution [10] Imagine a negative looping branch in an SLDNF tree. The key idea to detect negative loops is to maintain for each subgoal the set of ground negative literals that have been encountered along a branch in a SLDNF tree, called the negative context in XOLDTNF resolution ....

....if a ground negative literal is not in the current negative context, it is solved as in Prolog by evaluating the corresponding positive subgoal up to a fixpoint. This allows a smooth integration of XOLDTNF and ordinary Prolog computation. A common problem with XOLDTNF resolution [10] or WELL [6]) though, is that the same subgoal may be solved in different negative contexts. XOLDTNF resolution provides an explicit representation of true and undefined answers, and thus reduces redundant computation by sharing definite answers across negative contexts. However, there may still be an ....

[Article contains additional citation context not shown here]

N. Bidoit and P. Legay. Well!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

....before using it, and thereby avoid unnecessary computation caused by treating them as undefined. As a special case of the above, for modularly stratified programs WF OS reduces to Ordered Search, and performs no irrelevant computation and repeats no computation. Our technique is better than WELL [7] and QSQR SLS resolution [22] since both perform repeated computation even for programs without negation. Unlike XOLDTNF [9] our technique is able to share answers to subgoals effectively; XOLDTNF repeats computation even for modularly stratified programs. The technique of [13] is not goal ....

N. Bidoit and P. Legay. WELL! An evaluation procedure for all logic programs. In Procs. of the International Conf. on Database Theory, pages 335--348, Dec. 1990.

....creation of false facts is limited to only those that contribute to derive new positive facts. Ross [23] adapts the magic sets rewriting technique for a subclass of logic programs called modularly stratified programs. There are at least two top down approaches to this problem. Bidoit and Legay [24] present a top down algorithm called Well . This approach applies only to the class of non floundering queries. Chen and Warren [25] present a goal oriented method for constructing the well founded model of general logic programs. It has the practical advantages of top down evaluation and ....

N. Bidoit and P. Legay. Well!: An evaluation procedure for all logic programs. In Proceedings of International Conference on Database Theory, pages 335--345. Lecture Notes in Computer Science, 470, Springer-Verlag, 1990.

....original top down derivation method for computing well founded models [37] was not effective even for DATALOG (i.e. function free) programs. Recently methods for top down computation of wellfounded models that are effective for arbitrary DATALOG programs with negation have been presented. WELL [12] extends the QSQR approach to computing the well founded model of DATALOG programs, while XOLDTNF [14] and SLG [15] extend the OLDT approach. In this paper, we presented a bottom up evaluation method for computing well founded models of arbitrary programs. Leone and Rullo [27] also defined a ....

....of Melbourne. Each of them was implemented using hand written code in Aditi RL a language which supports relational operations as well as constructs for iteration and procedure calls. We are unable to compare directly with the other methods applicable to modularly stratified programs ([12, 14, 15, 36, 38]) For the following program Morishita s method [30] is identical to the doubled program approach because the magic facts are independent of the working and has suspect part relations. Our example program is the (modularly stratified) program (modified from an example in [38] below. working(X) ....

[Article contains additional citation context not shown here]

Bidoit, N. and Legay, P. WELL!: An evaluation procedure for all logic programs. In Proceedings of the Thirds Int. Conf. on Database Theory (1990) 335--348.

....partial model of p p. In contrast, a negative loop, such as p p, is considered indeterminate since p is undefined in the well founded partial model of p p. Mechanisms for handling infinite recursion through negation have been studied in tabled evaluation of queries, including WELL [6] and XOLDTNF resolution [14] The key idea is to associate a set of ground negative literals, called negative context , with each subgoal. The negative context for the initial subgoal is empty. When a subgoal A in a negative context N calls a ground negative literal B, B is replaced with an ....

....are selected before negative ones, and when only negative literals remain in a rule body, all the negative literals are selected and evaluated. The latter is required because the evaluation of one ground negative literal may go into an infinite loop while the evaluation of another may fail. WELL [6] and XOLDTNF resolution [14] represent a simple extension of SLD resolution with tabling to handle loops through negation. By maintaining a negative context with each subgoal, both can detect loops through negation, treat the ground negative literal involved in such a loop as undefined, and avoid ....

N. Bidoit and P. Legay. WELL!: An evaluation procedure for all logic programs. In Intl. Conference on Database Theory, pages 335--348, 1990.

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Bidoit, N. and Legay, P. WELL!: An evaluation procedure for all logic programs. In Proceedings of the Thirds Int. Conf. on Database Theory (1990) 335--348.

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