The acts of the mind, wherein it exerts its power over simple ideas, are chiefly these three: 1. Combining several simple ideas into one compound one, and thus all complex ideas are made. 2. The second is bringing two ideas, whether simple or complex, together, and setting them by one another so as to take a view of them at once, without uniting them into one, by which it gets all its ideas of relations. 3. The third is separating them from all other ideas that accompany them in their real existence: this is called abstraction, and thus all its general ideas are made.John LockeAn Essay Concerning Human Understanding 1690
We are about to study the idea of a computational process. Computational processes are abstract beings that inhabit computers. As they evolve, processes manipulate other abstract things called data. The evolution of a process is directed by a pattern of rules called a program. People create programs to direct processes. In effect, we conjure the spirits of the computer with our spells.
A computational process is indeed much like a sorcerer's idea of a spirit. It cannot be seen or touched. It is not composed of matter at all. However, it is very real. It can perform intellectual work. It can answer questions. It can affect the world by disbursing money at a bank or by controlling a robot arm in a factory. The programs we use to conjure processes are like a sorcerer's spells. They are carefully composed from symbolic expressions in arcane and esoteric programming languages that prescribe the tasks we want our processes to perform.
A computational process, in a correctly working computer, executes programs precisely and accurately. Thus, like the sorcerer's apprentice, novice programmers must learn to understand and to anticipate the consequences of their conjuring. Even small errors (usually called bugs or glitches) (usually called bugs) in programs can have complex and unanticipated consequences.
Fortunately, learning to program is considerably less dangerous than learning sorcery, because the spirits we deal with are conveniently contained in a secure way. Real-world programming, however, requires care, expertise, and wisdom. A small bug in a computer-aided design program, for example, can lead to the catastrophic collapse of an airplane or a dam or the self-destruction of an industrial robot.
Master software engineers have the ability to organize programs so that they can be reasonably sure that the resulting processes will perform the tasks intended. They can visualize the behavior of their systems in advance. They know how to structure programs so that unanticipated problems do not lead to catastrophic consequences, and when problems do arise, they can debug their programs. Well-designed computational systems, like well-designed automobiles or nuclear reactors, are designed in a modular manner, so that the parts can be constructed, replaced, and debugged separately.
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Despite its inception as a mathematical formalism, Lisp is a practical programming language. A Lisp interpreter is a machine that carries out processes described in the Lisp language. The first Lisp interpreter was implemented by McCarthy with the help of colleagues and students in the Artificial Intelligence Group of the MIT Research Laboratory of Electronics and in the MIT Computation Center.[1] Lisp, whose name is an acronym for LISt Processing, was designed to provide symbol-manipulating capabilities for attacking programming problems such as the symbolic differentiation and integration of algebraic expressions. It included for this purpose new data objects known as atoms and lists, which most strikingly set it apart from all other languages of the period. |
Despite its inception as a language for scripting the web, JavaScript is a general-purpose programming language. A JavaScript interpreter is a machine that carries out processes described in the JavaScript language. The first JavaScript interpreter was implemented by Eich at Netscape Communications Corporation for the Netscape Navigator web browser. JavaScript inherited its core features from the Scheme and Self programming languages. Scheme is a dialect of Lisp, and was used as the programming language for the original version of this book. From Scheme, JavaScript inherited its most fundamental design principles, such as lexically scoped first-class functions and dynamic typing. |
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Lisp was not the product of a concerted design effort. Instead, it
evolved informally in an experimental manner in response to users'
needs and to pragmatic implementation considerations. Lisp's
informal evolution has continued through the years, and the community of
Lisp users has traditionally resisted attempts to promulgate any
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JavaScript bears only superficial resemblance to the language Java,
after which it was
(eventually) named; both Java and JavaScript use the block structure of
the language C. In contrast with Java and C, which usually
employ compilation to lower-level
languages, JavaScript programs were initially
interpreted
by web browsers.
After Netscape Navigator, other web browsers provided interpreters
for the language, including Microsoft's Internet Explorer, whose
JavaScript version is called
JScript. The popularity of JavaScript for controlling web
browsers gave rise to a standardization effort, culminating in
ECMAScript. The
first edition of the ECMAScript standard was led by Guy Lewis
Steele Jr. and completed in June 1997
( |
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Because of its experimental character and its emphasis on symbol manipulation, Lisp was at first very inefficient for numerical computations, at least in comparison with Fortran. Over the years, however, Lisp compilers have been developed that translate programs into machine code that can perform numerical computations reasonably efficiently. And for special applications, Lisp has been used with great effectiveness.[3] Although Lisp has not yet overcome its old reputation as hopelessly inefficient, Lisp is now used in many applications where efficiency is not the central concern. For example, Lisp has become a language of choice for operating-system shell languages and for extension languages for editors and computer-aided design systems. |
The practice of embedding JavaScript programs in web pages encouraged the developers of web browsers to implement JavaScript interpreters. As these programs became more complex, the interpreters became more efficient in executing them, eventually using sophisticated implementation techniques such as Just-In-Time (JIT) compilation. The majority of JavaScript programs as of this writing (2021) are embedded in web pages and interpreted by browsers, but JavaScript is increasingly used as a general-purpose programming language, using systems such as Node.js. |