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An algorithm can be regarded as consisting of a logic component, which specifies the knowledge to be used in solving problems, and a control component, which determines the problem-solving strategies by means of which that knowledge is used. The logic component determines the meaning of the algorithm whereas the control component only affects its efficiency. The efficiency of an algorithm can often be improved by improving the control component without changing the logic of the algorithm. We argue that computer programs would be more often correct and more easily improved and modified if their logic and control aspects were identified and separated in the program text. [ABSTRACT FROM AUTHOR]

A system for proving that programs written in a high level language are correctly translated to a low level language is described. A primary use of the system is as a postoptimization step in code generation. The low level language programs need not be generated by a compiler and in fact could be hand coded Examples of the usefulness of such a system are given. Some interesting results are the ability to handle programs that implement recursion by bypassing the start of the program, and the detection and pinpointing of a wide class of errors in the low level language programs. The examples demonstrate that optimization of the genre of this paper can result in substantially faster operation and the saving of memory in terms of program and stack sizes. [ABSTRACT FROM AUTHOR]

Shallow binding is a scheme which allows the value of a variable to be accessed in a bounded amount of computation. An elegant model for shallow binding in Lisp 1.5 is presented in which context-switching is an environment tree transformation called rerooting. Rerooting is completely general and reversible, and is optional in the sense that a Lisp 1.5 interpreter will operate correctly whether or not rerooting is invoked on every context change. Since rerooting leaves assoc [ v, a] invariant, for all variables v and all environments a, the programmer can have access to a rerooting primitive, shallow[], which gives him dynamic control over whether accesses are shallow or deep, and which affects only the speed of execution of a program, not its semantics. In addition, multiple processes can be active in the same environment structure, so long as rerouting is an indivisible operation. Finally, the concept of rerooting is shown to combine the concept of shallow binding in Lisp with Dijkstra's display for Algol and hence is a general model for shallow binding. [ABSTRACT FROM AUTHOR]

The use of several levels of abstraction has proved to be very helpful in constructing and maintaining programs. When programs are designed with abstract data types such as sets and lists, programmer time can be saved by automating the process of fitting in low-level implementation details. In the past, programming systems have provided only a single general purpose implementation for an abstract type. Thus the programs produced using abstract types were often inefficient in space or time. In this paper a system for automatically choosing efficient implementations for abstract types from a library of implementations is discussed. This process is discussed in detail for an example program. General issues in data structure selection are reviewed. [ABSTRACT FROM AUTHOR]

Proofs of program correctness tend to be long and tedious, whereas testing, though useful in detecting errors, usually does not guarantee correctness. This paper introduces a technique whereby test data can be used in proving program correctness. In addition to simplifying the process of proving correctness, this method simplifies the process of providing accurate specification for a program. The applicability of this technique to procedures and recursive programs is demonstrated. [ABSTRACT FROM AUTHOR]

This paper introduces an alternative lo context-free grammars called regular right part (RRP) grammars, which resemble PASCAL syntax diagrams. Formally, RRP grammars have production right parts, which are nondeterministic finite state machines (FSMs), and, as a special case, regular expressions, since these can be converted to FSMs. RRP grammars describe the syntax of programming languages more concisely and more understandably than is possible with CF grammars. Also introduced is a class of parsers, RRP LR(m, k) parsers, which includes the CF LR(k) parsers and provides the same advantages. Informally, an RRP LR(m, k) parser can determine the right end of each handle by considering at most k symbols to the right of the handle and the left end, after the right end has been found, by considering at most m symbols to the left of the handle. A mechanism for determining the left end is required because there is no bound on the length of the handle. [ABSTRACT FROM AUTHOR]

In contrast to the predominant use of low-level intermediate text, high-level data flow analysis deals with programs essentially at source level and exploits the control flow information implicit in the parse tree. The need for high-level flow analysis arises from several aspects of recent work on advanced methods of program certification and optimization. This paper proposes a simple general method of high-level data flow analysis that allows free use of escape and jump statements, avoids large graphs when compiling large programs, facilitates updating of data flow information to reflect program changes, and derives new global information helpful in solving many familiar global flow analysis problems. An illustrative application to live variable analysis is presented. Many of the graphs involved are constructed and analyzed before any programs are compiled, thus avoiding certain costs that low-level methods incur repeatedly at compile time. [ABSTRACT FROM AUTHOR]

A common tool for proving the termination of programs is the well-founded set, a set ordered in such a way as to admit no infinite descending sequences. The basic approach is to find a termination function that maps the values of the program variables into some well-founded set, such that the value of the termination function is repeatedly reduced throughout the computation. All too often, the termination functions required are difficult to find and are of a complexity out of proportion to the program under consideration. Multisets (bags) over a given well-founded set S are sets that admit multiple occurrences of elements taken fróm S. The given ordering on S induces an ordering on the finite multisets over S. This multiset ordering is shown to be well-founded. The multiset ordering enables the use of relatively simple and intuitive termination functions in otherwise difficult termination proofs. In particular, the multiset ordering is used to prove the termination of production systems, programs defined in terms of sets of rewriting rules. [ABSTRACT FROM AUTHOR]

A data abstraction can be naturally specified using algebraic axioms. The virtue of these axioms is that they permit a representation-independent formal specification of a data type. An example is given which shows how to employ algebraic axioms at successive levels of implementation. The major thrust of the paper is twofold. First, it is shown how the use of algebraic axiomatizations can simplify the process of proving the correctness of an implementation of an abstract data type. Second, semi-automatic tools are described which can be used both to automate such proofs of correctness and to derive an immediate implementation from the axioms. This implementation allows for limited testing of programs at design time, before a conventional implementation is accomplished. [ABSTRACT FROM AUTHOR]

Hierarchical programming is being increasingly recognized as helpful in the construction of large programs. Users of hierarchical techniques claim or predict substantial increases in productivity and in the reliability of the programs produced. In this paper we describe a formal method for hierarchical program specification, implementation, and proof. We apply this method to a significant list processing problem and also discuss a number of extensions to current programming languages that ease hierarchical program design and proof. [ABSTRACT FROM AUTHOR]

In this paper we present a scheme for detecting and recovering from syntax errors in programs. The scheme, which is based on LR parsing. is driven by information which is directly and automatically obtainable from the information that is already present in an LR parser. The approach, which is patterned after that of Levy and Graham and Rhodes, appears to provide error recovery which is both simple and powerful. [ABSTRACT FROM AUTHOR]

This paper describes an approach for determining the minimum, maximum, and average times to parse sentences acceptable by a deterministic parser. These quantities are presented in the form of symbolic formulas, called time-formulas. The variables in these formulas represent not only the length of the input string but also the lime to perform elementary operations such as pushing, popping, subscripting, iterating, etc. By binding to the variables actual numerical values corresponding to a given compiler-machine configuration, one can determine the execution time for that configuration. Time-formulas are derived by examining the grammar rules and the program representing the algorithm one wishes to analyze. The approach is described by using a specific grammar that defines simple arithmetic expressions. Two deterministic parsers are analyzed: a top-down recursive descent LL(I) parser, and a bottom-up SLR(I) parser. The paper provides estimates for the relative efficiencies of the two parsers. The estimates applicable to a specific machine, the PDP-10, are presented and substantiated by benchmarks. Finally, the paper illustrates the proposed approach by applying it to the analyses of parsers for a simple programming language. [ABSTRACT FROM AUTHOR]

This paper describes an integrated procedure mechanism that permits procedures to be used as recursive functions or as coroutines. This integration is accomplished by treating procedures and their activation records (called environments) as data objects and by decomposing procedure invocation into three separate components at the source-language level. In addition, argument binding is under the control of the programmer, permitting the definition of various methods of argument transmission in the source language itself. The resulting procedure mechanism, which is part of the SL5 programming language, is well suited to goal-oriented problems and to other problems that are more readily programmed by using coroutines, Several examples are given. [ABSTRACT FROM AUTHOR]

The issues of how a programming language might aid in keeping tracking of physical units (feet, sec, etc.,) are discussed. A method is given for the introduction of relationships among units (a watt is volts-amps, a yard is three feet) and subsequent automatic conversion for syntax are considered. [ABSTRACT FROM AUTHOR]

Controlled sharing of information is needed and desirable for many applications and is supported in operating systems by access control mechanisms. This paper shows how to extend programming languages to provide controlled sharing. The extension permits expression of access constraints on shared data. Access constraints can apply both to simple objects. and to objects that are components of larger objects, such as bank account records in a bank's data base. The constraints are stated declaratively, and can be enforced by static checking similar to type checking. The approach can he used to extend any strongly-typed language, but is particularly suitable for extending languages that support the notion of abstract data types. [ABSTRACT FROM AUTHOR]

The basic ideas of nondeterministic programming are critically reconsidered to single out a proper attitude and programming style for languages allowing direct control of nondeterministic features. The proposed attitude aims at retaining the purity of the nondeterministic formulation of search processes on one level (the attempt level), deferring the coordination of problem solving efforts to another (the choice level). The feasibility of recognizing these two levels is discussed, stressing that the structure to be managed at the choice level is a tree of contexts. The leaves are computational environments, each holding an alternative under inspection, while the other nodes are associated with choice points. According to the proposed programming style, a generative function is associated with each choice point, which expresses the desired choice strategy. The main advantage of this approach is the localization of the search strategies: Each nonterminal node of the tree keeps track of the state of the computation as it was when the choice point was last interrogated, holding at the same time the strategy to coordinate the available alternatives. Examples are given in term of ND-Lisp, an extension of Lisp designed and implemented according to these guidelines. [ABSTRACT FROM AUTHOR]

A formal specification is given for a simple calendar program, and the derivation and proof of correctness of the program are sketched. The specification is easy to understand, and its correctness is manifest to humans. [ABSTRACT FROM AUTHOR]

A simple technique for reasoning about equalities that is fast and complete for ground formulas with function symbols and equality is presented. A proof of correctness is given as well. [ABSTRACT FROM AUTHOR]

Information on the conference titled History of Programming Languages is presented. The event was intended to initiate the preservation of a historical record of the development of thirteen programming languages. The conference preprints contained thirteen papers on the history of the various languages. The conference was attended by 350 people, most of whom had entered the computing field within the last ten years.

Problems involved in the production of reliable software are now attracting substantial attention. In many applications, reliability is at least as important as more traditional goals such as efficiency and portability. The programming community has started to examine the effects that its tools and techniques have on reliability. Reliability has not been an explicit goal in the design of most programming languages, and it is not surprising that each of today's dominant programming languages contains major sources of unreliability. Data about the effects of languages on the reliability of programs is difficult to obtain and easy to challenge. A new crop of languages promising improvements has appeared, but many issues are still vigorously debated by their designers. Users need guidelines for selecting "safe" subsets that avoid the worst problems of existing languages, for establishing suitable programming conventions, and for choosing between languages where that is an option.