741 research outputs found
Fifty years of Hoare's Logic
We present a history of Hoare's logic.Comment: 79 pages. To appear in Formal Aspects of Computin
An observationally complete program logic for imperative higher-order functions
We establish a strong completeness property called observational completeness of the program logic for imperative, higher-order functions introduced in [1]. Observational completeness states that valid assertions characterise program behaviour up to observational congruence, giving a precise correspondence between operational and axiomatic semantics. The proof layout for the observational completeness which uses a restricted syntactic structure called finite canonical forms originally introduced in game-based semantics, and characteristic formulae originally introduced in the process calculi, is generally applicable for a precise axiomatic characterisation of more complex program behaviour, such as aliasing and local state
ConSUS: A light-weight program conditioner
Program conditioning consists of identifying and removing a set of statements which cannot be executed when a condition of interest holds at some point in a program. It has been applied to problems in maintenance, testing, re-use and re-engineering. All current approaches to program conditioning rely upon both symbolic execution and reasoning about symbolic predicates. The reasoning can be performed by a ‘heavy duty’ theorem prover but this may impose unrealistic performance constraints.
This paper reports on a lightweight approach to theorem proving using the FermaT Simplify decision procedure. This is used as a component to ConSUS, a program conditioning system for the Wide Spectrum Language WSL. The paper describes the symbolic execution algorithm used by ConSUS, which prunes as it conditions.
The paper also provides empirical evidence that conditioning produces a significant reduction in program size and, although exponential in the worst case, the conditioning system has low degree polynomial behaviour in many cases, thereby making it scalable to unit level applications of program conditioning
A comparative study of structured and un-structured remote data access in distributed computing systems
Recently, the use of distributed computing systems has been growing rapidly due to the result of cheap and advanced microelectronic technology. In addition to the decrease in hardware costs, the tremendous development in machine to machine communication interfaces, especially in local area networking, also favours the use of distributed systems. Distributed systems often require remote access to data stored at different sites. Generally, two models of access to remote data storage exist: the un structured and structured models. In the former, data is simply stored as row of bytes, whereas in the latter, data is stored along with the associated access codes. The objective of this thesis is to compare these two models and hence determines the tradeoffs of each model. First of all, an extended review of the field of distributed data access is provided which addressing key issues such as the basic design principles of distributed computing systems, the notions of abstract data types, data inheritance, data type system and data persistence. Secondly, a distributed system is implemented using the persistent programming language PS-algol and the high level language C in conjunction with the remote procedure call facilities available in Unix(^1) 4.2 BSD operating system. This distributed system makes extensive use of Unix's software tools and hence it is called DCSUNIX for Distributed Computing System on UNIX. Thirdly, two specific applications which employ the implemented system will be given so that a comparison can be made between the two remote data access models mentioned above. Finally, the implemented system is compared with the criteria established earlier in the thesis. keywords: abstract data types, class, database management, data persistence, information hiding, inheritance, object oriented programming, programming languages, remote procedure calls, transparency, and type checking
The pragmatic formalization of computing systems relative to a given high-level language
Imperial Users onl
Trust, but Verify: Two-Phase Typing for Dynamic Languages
A key challenge when statically typing so-called dynamic languages is the
ubiquity of value-based overloading, where a given function can dynamically
reflect upon and behave according to the types of its arguments. Thus, to
establish basic types, the analysis must reason precisely about values, but in
the presence of higher-order functions and polymorphism, this reasoning itself
can require basic types. In this paper we address this chicken-and-egg problem
by introducing the framework of two-phased typing. The first "trust" phase
performs classical, i.e. flow-, path- and value-insensitive type checking to
assign basic types to various program expressions. When the check inevitably
runs into "errors" due to value-insensitivity, it wraps problematic expressions
with DEAD-casts, which explicate the trust obligations that must be discharged
by the second phase. The second phase uses refinement typing, a flow- and
path-sensitive analysis, that decorates the first phase's types with logical
predicates to track value relationships and thereby verify the casts and
establish other correctness properties for dynamically typed languages
Topics in Programming Languages, a Philosophical Analysis through the case of Prolog
[EN]Programming languages seldom find proper anchorage in philosophy of logic, language and science. is more, philosophy of language seems to be restricted to natural languages and linguistics, and even philosophy of logic is rarely framed into programming languages topics. The logic programming paradigm and Prolog are, thus, the most adequate paradigm and programming language to work on this subject, combining natural language processing and linguistics, logic programming and constriction methodology on both algorithms and procedures, on an overall philosophizing declarative status. Not only this, but the dimension of the Fifth Generation Computer system related to strong Al wherein Prolog took a major role. and its historical frame in the very crucial dialectic between procedural and declarative paradigms, structuralist and empiricist biases, serves, in exemplar form, to treat straight ahead philosophy of logic, language and science in the contemporaneous age as well.
In recounting Prolog's philosophical, mechanical and algorithmic harbingers, the opportunity is open to various routes. We herein shall exemplify some:
- the mechanical-computational background explored by Pascal, Leibniz, Boole, Jacquard, Babbage, Konrad Zuse, until reaching to the ACE (Alan Turing) and EDVAC (von Neumann), offering the backbone in computer architecture, and the work of Turing, Church, Gödel, Kleene, von Neumann, Shannon, and others on computability, in parallel lines, throughly studied in detail, permit us to interpret ahead the evolving realm of programming languages. The proper line from lambda-calculus, to the Algol-family, the declarative and procedural split with the C language and Prolog, and the ensuing branching and programming languages explosion and further delimitation, are thereupon inspected as to relate them with the proper syntax, semantics and philosophical élan of logic programming and Prolog
Decentralised Control Flow: A Computational Model for Distributed Systems
PhD ThesisThis thesis presents two sets of principles for the organisation of
distributed computing systems. Details of models of computation based on
these principles are together given, with proposals for programming
languages based on each model of computation.
The recursive control flow principles are based on the concept of
recursive control flow computing system structuring. A recursive
comprises a group of subordinate computing systems connected together by
Each subordinate computing system may either be a communications medium.
which a a computing system consists of a processing unit, memory
some is itself a recursive component, and input/output devices, or
computing components control flow system. The memory of all the
computing systems within a recursive control flow computing subordinate
system are arranged in a hierarchy. Using suitable addresses, any part
of the hierarchy is accessible to any sequence of instructions which may
be executed by the processing unit of a subordinate computing system.
This rise to serious difficulties in the global accessibility gives
understanding of programs written the meaning of in a programming
language recursive control flow on the model of computation. based
Reasoning about a particular program in isolation is difficult because of
the potential interference between the execution different programs
cannot be ignored .
alternative principles, decentralised control flow, restrict the The
accessibility of subordinate global the memory components of the
computing The basis of the concept of objects forms the systems.
principles. Information channels may flow along unnamed between
instances of these objects, this being the only way in which one instance
of an object may communicate with some other instance of an object.
Reasoning particular program written in a programming language about a
based on the decentralised control flow model of computation is easier
since it is that there will be no interference between the guaranteed
execution of different programs.Science and Engineering Research Council of Great Britain, International Computers Limite
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