584 research outputs found
Projections for infinitary rewriting
Proof terms in term rewriting are a representation means for reduction
sequences, and more in general for contraction activity, allowing to
distinguish e.g simultaneous from sequential reduction. Proof terms for
finitary, first-order, left-linear term rewriting are described in the Terese
book, chapter 8. In a previous work, we defined an extension of the finitary
proof-term formalism, that allows to describe contractions in infinitary
first-order term rewriting, and gave a characterisation of permutation
equivalence.
In this work, we discuss how projections of possibly infinite rewrite
sequences can be modeled using proof terms. Again, the foundation is a
characterisation of projections for finitary rewriting described in Terese,
Section 8.7. We extend this characterisation to infinitary rewriting and also
refine it, by describing precisely the role that structural equivalence plays
in the development of the notion of projection. The characterisation we propose
yields a definite expression, i.e. a proof term, that describes the projection
of an infinitary reduction over another.
To illustrate the working of projections, we show how a common reduct of a
(possibly infinite) reduction and a single step that makes part of it can be
obtained via their respective projections. We show, by means of several
examples, that the proposed definition yields the expected behavior also in
cases beyond those covered by this result. Finally, we discuss how the notion
of limit is used in our definition of projection for infinite reduction
A new coinductive confluence proof for infinitary lambda calculus
We present a new and formal coinductive proof of confluence and normalisation
of B\"ohm reduction in infinitary lambda calculus. The proof is simpler than
previous proofs of this result. The technique of the proof is new, i.e., it is
not merely a coinductive reformulation of any earlier proofs. We formalised the
proof in the Coq proof assistant.Comment: arXiv admin note: text overlap with arXiv:1501.0435
Infinitary Combinatory Reduction Systems: Confluence
We study confluence in the setting of higher-order infinitary rewriting, in
particular for infinitary Combinatory Reduction Systems (iCRSs). We prove that
fully-extended, orthogonal iCRSs are confluent modulo identification of
hypercollapsing subterms. As a corollary, we obtain that fully-extended,
orthogonal iCRSs have the normal form property and the unique normal form
property (with respect to reduction). We also show that, unlike the case in
first-order infinitary rewriting, almost non-collapsing iCRSs are not
necessarily confluent
Infinitary -Calculi from a Linear Perspective (Long Version)
We introduce a linear infinitary -calculus, called
, in which two exponential modalities are available, the
first one being the usual, finitary one, the other being the only construct
interpreted coinductively. The obtained calculus embeds the infinitary
applicative -calculus and is universal for computations over infinite
strings. What is particularly interesting about , is that
the refinement induced by linear logic allows to restrict both modalities so as
to get calculi which are terminating inductively and productive coinductively.
We exemplify this idea by analysing a fragment of built around
the principles of and . Interestingly, it enjoys
confluence, contrarily to what happens in ordinary infinitary
-calculi
Automated Termination Proofs for Logic Programs by Term Rewriting
There are two kinds of approaches for termination analysis of logic programs:
"transformational" and "direct" ones. Direct approaches prove termination
directly on the basis of the logic program. Transformational approaches
transform a logic program into a term rewrite system (TRS) and then analyze
termination of the resulting TRS instead. Thus, transformational approaches
make all methods previously developed for TRSs available for logic programs as
well. However, the applicability of most existing transformations is quite
restricted, as they can only be used for certain subclasses of logic programs.
(Most of them are restricted to well-moded programs.) In this paper we improve
these transformations such that they become applicable for any definite logic
program. To simulate the behavior of logic programs by TRSs, we slightly modify
the notion of rewriting by permitting infinite terms. We show that our
transformation results in TRSs which are indeed suitable for automated
termination analysis. In contrast to most other methods for termination of
logic programs, our technique is also sound for logic programming without occur
check, which is typically used in practice. We implemented our approach in the
termination prover AProVE and successfully evaluated it on a large collection
of examples.Comment: 49 page
Infinitary Combinatory Reduction Systems: Normalising Reduction Strategies
We study normalising reduction strategies for infinitary Combinatory
Reduction Systems (iCRSs). We prove that all fair, outermost-fair, and
needed-fair strategies are normalising for orthogonal, fully-extended iCRSs.
These facts properly generalise a number of results on normalising strategies
in first-order infinitary rewriting and provide the first examples of
normalising strategies for infinitary lambda calculus
Modularity of Convergence and Strong Convergence in Infinitary Rewriting
Properties of Term Rewriting Systems are called modular iff they are
preserved under (and reflected by) disjoint union, i.e. when combining two Term
Rewriting Systems with disjoint signatures. Convergence is the property of
Infinitary Term Rewriting Systems that all reduction sequences converge to a
limit. Strong Convergence requires in addition that redex positions in a
reduction sequence move arbitrarily deep. In this paper it is shown that both
Convergence and Strong Convergence are modular properties of non-collapsing
Infinitary Term Rewriting Systems, provided (for convergence) that the term
metrics are granular. This generalises known modularity results beyond metric
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