Joannes Lewi

The ELL(1) parser generator and the error recovery mechanism

SummaryA syntactic error recovery technique is presented that is simple and at the same time very powerful. It has the main property that it is phrase marker oriented, where phrase markers are considered symbols delimiting language constructions, e.g., begin and end for blocks, (and) for expressions, and [and] for indices. The basic idea of this error recovery technique originates from P. Branquart and has been worked out in the Algol 68 compiler project, see [8] and [9]. Here, we are especially concerned with the generation aspects of error recovery. In particular, it is investigated how error recovery can be mechanised in an ELL(1) and LALR(1) syntax directed translation scheme and which conditions the syntax must satisfy. Both the ELL(1) and LALR(1) generators are implemented and are part of the system LILA: a Language Implementation LAboratory [28, 29, 30]. Only the ELL(1) generator is described here.

Proving termination of (conditional) rewrite systems

In this paper an important problem in the domain of term rewriting, the termination of (conditional) rewrite systems, is dealt with. We show that in many applications, well-founded orderings on terms which only make use of syntactic information of a rewrite systemR, do not suffice for proving termination ofR. Indeed sometimes semantic information is needed to orient a rewrite rule. Therefore we integrate a semantic interpretation of rewrite systems and terms into a well-founded ordering on terms: the notion ofsemantic ordering is the first main contribution of this paper. The use and usefulness of the semantic ordering in proving termination is illustrated by means of some realistic examples.Furthermore the concept of semantic information induces a novel approach for proving termination inconditional rewrite systems. The idea is to employ not only semantic information contained in the terms that are to be compared, but also extra (semantic) information contained in the premiss of the conditional equation in which the terms appear. This leads to our second contribution in the termination problem area: the notion ofcontextual ordering andcontextual semantic ordering. Thecontextual approach allows to prove termination of conditional rewrite systems where all classical partial orderings would fail.

Proof by Consistency in Conditional Equational Theories

In this paper we deal with the problem of proving inductive theorems in conditional equational theories. We propose a proof by consistency method that can be employed when the theory is representable as a ground Church-Rosser conditional equational system. The method has a linear proof strategy and is shown to be sound and refutational complete, i.e. it refutes any conditional equation which is not an inductive theorem. Moreover it can handle rewrite rules as well as (unorientable) equations and therefore it will not fail when an unorientable equation comes up (as was the case in the earliest proof by consistency (inductionless induction) methods). The method extends the work on unconditional equational theories of [Bachmair 1988].

Formal verifications: an industrial case study

By means of the mu PABX example, the adequacy of temporal logic was demonstrated for the specification and formal verification of industrial reactive systems. The aim of the mu PABX system is to provide services that are issued by phone-users of the mu PABX. In contrast to a real PABX, the mu PABX offers only one type of service to its subscribers: two-party voice calls. Formal verification avoids the deficiencies of testing by proving mathematically that the system behaves according to the specification. For this to be possible, the system behavior must be defined with mathematical precision. The focus is on the formal verification of the mu PABX. It is demonstrated that, if temporal logic is used, the task of proof construction can be 100% automated. An implementation of the mu PABX system is discussed.<<ETX>>

SLS/1: A Translator Writing System

The translator writing system in its didactic version is an ideal tool upon which a course on compiler construction can be based. The student can treat non-trivial examples in a flexible and modular way within a reasonable limit of time [16]. This version is implemented in PL/I on the IBM 370 model 158 under OS.