Technical Opinion: The Emperor with No Clothes
Examining the software problem from a scientific standpoint.
Henry F. Ledgard
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A sample set of Cobol programming standards is offered. These standards constrain code to be developed in a “structured” form for both data and control structures. They do not require syntax beyond the existing Cobol language and in fact utilize a typical limited subset of the 1974 ANS Cobol standard. These standards have proved extremely valuable in practice and have reduced the cost and time to produce and maintain large software systems that have been deployed in live multiple customer environments.
A genealogy of control structures
The issue of program control structures has had a history of heated controversy. To put this issue on a solid footing, this paper reviews numerous theoretical results on control structures and explores their practical implications.
The classic result of Böhm and Jacopini on the theoretical completeness of if-then-else and while-do is discussed. Several recent ideas on control structures are then explored. These include a review of various other control structures, results on time/space limitations, and theorems relating the relative power of control structures under several notions of equivalence.
In conclusion, the impact of theoretical results on the practicing programmer and the importance of one-in, one-out control structures as operational abstractions are discussed. It is argued further that there is insufficient evidence to warrant more than if-then-else, while-do, and their variants.
Production systems: or can we do better than BNF
Since the development of BNF, the definition of the syntax of programming languages has been almost universally associated with context-free requirements. Yet numerous interesting and difficult issues in syntax stem from the context-sensitive requirements, notably the compatibility between the declaration of an identifier and its uses, the correspondence between actual and formal parameters, and issues arising from block structure.
This paper explores the use of a formal notation called Production Systems in providing a readable and complete formal definition of syntax. As a practical illustration, a small but significant subset of PL/I is considered. A more detailed presentation, as well as the application to define abstract syntax and translations between languages, is given in a previous paper by the author.
A model for type checking: with an application to ALGOL 60
Most current programming languages treat computation over different classes of objects (e.g. numbers, strings, labels and functions). For correct compilation and execution, the following question then arises: is a program properly constructed so that its operations and operands are compatible? The activity of answering this question is usually called type checking.
This paper attempts to isolate the notion of type checking and presents a partial solution to the type checking problem based on the notions of abstraction and application of functions. In particular, a program is mapped into an expression within a decideable subset of the &lgr;-calculus, which characterizes the type relations within the program and eliminates all other information. The determination of the type-wise correctness or incorrectness of the program is resolved by reducing its corresponding &lgr;-calculus expression to one of two normal forms, the constant “correct” for a type-wise correct program or the constant “error.”
An application to type checking in Algol 60 is made, and the attendant problems faced for any notion of type checking are discussed.
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