191 research outputs found
Formalizing the Confluence of Orthogonal Rewriting Systems
Orthogonality is a discipline of programming that in a syntactic manner
guarantees determinism of functional specifications. Essentially, orthogonality
avoids, on the one side, the inherent ambiguity of non determinism, prohibiting
the existence of different rules that specify the same function and that may
apply simultaneously (non-ambiguity), and, on the other side, it eliminates the
possibility of occurrence of repetitions of variables in the left-hand side of
these rules (left linearity). In the theory of term rewriting systems (TRSs)
determinism is captured by the well-known property of confluence, that
basically states that whenever different computations or simplifications from a
term are possible, the computed answers should coincide. Although the proofs
are technically elaborated, confluence is well-known to be a consequence of
orthogonality. Thus, orthogonality is an important mathematical discipline
intrinsic to the specification of recursive functions that is naturally applied
in functional programming and specification. Starting from a formalization of
the theory of TRSs in the proof assistant PVS, this work describes how
confluence of orthogonal TRSs has been formalized, based on axiomatizations of
properties of rules, positions and substitutions involved in parallel steps of
reduction, in this proof assistant. Proofs for some similar but restricted
properties such as the property of confluence of non-ambiguous and (left and
right) linear TRSs have been fully formalized.Comment: In Proceedings LSFA 2012, arXiv:1303.713
Formalising Confluence in PVS
Confluence is a critical property of computational systems which is related
with determinism and non ambiguity and thus with other relevant computational
attributes of functional specifications and rewriting system as termination and
completion. Several criteria have been explored that guarantee confluence and
their formalisations provide further interesting information. This work
discusses topics and presents personal positions and views related with the
formalisation of confluence properties in the Prototype Verification System PVS
developed at our research group.Comment: In Proceedings DCM 2015, arXiv:1603.0053
Superdevelopments for Weak Reduction
We study superdevelopments in the weak lambda calculus of Cagman and Hindley,
a confluent variant of the standard weak lambda calculus in which reduction
below lambdas is forbidden. In contrast to developments, a superdevelopment
from a term M allows not only residuals of redexes in M to be reduced but also
some newly created ones. In the lambda calculus there are three ways new
redexes may be created; in the weak lambda calculus a new form of redex
creation is possible. We present labeled and simultaneous reduction
formulations of superdevelopments for the weak lambda calculus and prove them
equivalent
Certifying Confluence of Almost Orthogonal CTRSs via Exact Tree Automata Completion
Suzuki et al. showed that properly oriented, right-stable, orthogonal, and oriented conditional term rewrite systems with extra variables in right-hand sides are confluent. We present our Isabelle/HOL formalization of this result, including two generalizations. On the one hand, we relax proper orientedness and orthogonality to extended proper orientedness and almost orthogonality modulo infeasibility, as suggested by Suzuki et al. On the other hand, we further loosen the requirements of the latter, enabling more powerful methods for proving infeasibility of conditional critical pairs. Furthermore, we formalized a construction by Jacquemard that employs exact tree automata completion for non-reachability analysis and apply it to certify infeasibility of conditional critical pairs. Combining these two results and extending the conditional confluence checker ConCon accordingly, we are able to automatically prove and certify confluence of an important class of conditional term rewrite systems
Improving Automatic Confluence Analysis of Rewrite Systems by Redundant Rules
We describe how to utilize redundant rewrite rules, i.e., rules that can be simulated by other rules, when (dis)proving confluence of term rewrite systems. We demonstrate how automatic confluence provers benefit from the addition as well as the removal of redundant rules. Due to their simplicity, our transformations were easy to formalize in a proof assistant and are thus amenable to certification. Experimental results show the surprising gain in power
Improving automatic confluence analysis of rewrite systems by redundant rules
We describe how to utilize redundant rewrite rules, i.e., rules that can be simulated by other rules, when (dis)proving confluence of term rewrite systems. We demonstrate how automatic confluence provers benefit from the addition as well as the removal of redundant rules. Due to their simplicity, our transformations were easy to formalize in a proof assistant and are thus amenable to certification. Experimental results show the surprising gain in power
Confluence of Orthogonal Nominal Rewriting Systems Revisited
Nominal rewriting systems (Fernandez, Gabbay, Mackie, 2004;
Fernandez, Gabbay, 2007) have been introduced as a new framework
of higher-order rewriting systems based on the nominal approach
(Gabbay, Pitts, 2002; Pitts, 2003), which deals with variable
binding via permutations and freshness conditions on atoms.
Confluence of orthogonal nominal rewriting systems has been shown in
(Fernandez, Gabbay, 2007). However, their definition of
(non-trivial) critical pairs has a serious weakness so that the
orthogonality does not actually hold for most of standard nominal
rewriting systems in the presence of binders. To overcome this
weakness, we divide the notion of overlaps into the self-rooted and
proper ones, and introduce a notion of alpha-stability which
guarantees alpha-equivalence of peaks from the self-rooted
overlaps. Moreover, we give a sufficient criterion for uniformity and alpha-stability. The new definition of orthogonality and the
criterion offer a novel confluence condition effectively applicable to many standard nominal rewriting systems. We also report on an
implementation of a confluence prover for orthogonal nominal rewriting systems based on our framework
Certified Rule Labeling
The rule labeling heuristic aims to establish confluence of (left-)linear term rewrite systems via decreasing diagrams. We present a formalization of a confluence criterion based on the interplay of relative termination and the rule labeling in the theorem prover Isabelle. Moreover, we report on the integration of this result into the certifier CeTA, facilitating the checking of confluence certificates based on decreasing diagrams for the first time. The power of the method is illustrated by an experimental evaluation on a (standard) collection of confluence problems
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