C Anthony R Hoare - Collected Quotes

"The most important property of a program is whether it accomplishes the intention of its user." (C Anthony R Hoare, Communications of the ACM, 1969)

"A deterministic process may be defined in terms of a mathematical function from its input channels to its output channels. Each channel is identified with the indefinitely extensible sequence of messages which pass along it. Such functions are defined in the usual way by recursion on the structure of the input sequences, except that the case of an empty input sequence is not considered." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"A process is defined by describing the whole range of its potential behaviour. Frequently, there will be a choice between several different actions [...]. On each such occasion, the choice of which event will actually occur can be controlled by the environment within which the process evolves. [...] . Fortunately, the environment of a process itself may be described as a process, with its behaviour defined by familiar notations. This permits investigation of the behaviour of a complete system composed from the process together with its environment, acting and interacting with each other as they evolve concurrently. The complete system should also be regarded as a process, whose range of behaviour is definable in terms of the behaviour of its component processes; and the system may in turn be placed within a yet wider environment. In fact, it is best to forget the distinction between processes, environments, and systems; they are all of them just processes whose behaviour may be prescribed, described, recorded and analysed in a simple and homogeneous fashion." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"In constructing a mathematical model of a physical system, it is a good strategy to define the basic concepts in terms of attributes that can be directly or indirectly observed or measured." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"In the design of a product, the designer has a responsibility to ensure that it will satisfy its specification; this responsibility may be discharged by the reasoning methods of the relevant branches of mathematics, for example, geometry or the differential and integral calculus." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"Recognition of the idea that a programming language should have a precise mathematical meaning or semantics dates from the early 1960s. The mathematics provides a secure, unambiguous, precise and stable specification of the language to serve as an agreed interface between its users and its implementors. Furthermore, it gives the only reliable grounds for a claim that different implementations are implementations of the same language. So mathematical semantics are as essential to the objective of language standardisation as measurement and counting are to the standardisation of nuts and bolts." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"Recursion permits the definition of a single process as the solution of a single equation. The technique is easily generalised to the solution of sets of simultaneous equations in more than one unknown. For this to work properly, all the right-hand sides must be guarded, and each unknown process must appear exactly once on the left-hand side of one of the equations." (C Anthony R Hoare, "Communicating Sequential Processes", 1985)

"There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies. It demands the same skill, devotion, insight, and even inspiration as the discovery of the simple physical laws which underlie the complex phenomena of nature." (C Anthony R Hoare, [lecture] 1987)

"The real value of tests is not that they detect bugs in the code, but that they detect inadequacies in the methods, concentration, and skills of those who design and produce the code." (C Anthony R Hoare, "How Did Software Get So Reliable Without Proof?", Lecture Notes in Computer Science Vol. 1051, 1996)

"Professional practice in a mature engineering discipline is based on relevant scientific theories, usually expressed in the language of mathematics. A mathematical theory of programming aims to provide a similar basis for specification, design and implementation of computer programs."  (C Anthony R Hoare, "Unified Theories of Programming", 1998)

"Programming languages on the whole are very much more complicated than they used to be: object orientation, inheritance, and other features are still not really being thought through from the point of view of a coherent and scientifically well-based discipline or a theory of correctness. My original postulate, which I have been pursuing as a scientist all my life, is that one uses the criteria of correctness as a means of converging on a decent programming language design - one which doesn’t set traps for its users, and ones in which the different components of the program correspond clearly to different components of its specification, so you can reason compositionally about it. [...] The tools, including the compiler, have to be based on some theory of what it means to write a correct program." (C Anthony R Hoare, [interview] 2002)