Programming language: Wikis

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Programming language
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A programming language is an artificial language designed to express computations that can be performed by a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine, to express algorithms precisely, or as a mode of human communication.

Many programming languages have some form of written specification of their syntax (form) and semantics (meaning). Some languages are defined by a specification document. For example, the C programming language is specified by an ISO Standard. Other languages, such as Perl, have a dominant implementation that is used as a reference.

The earliest programming languages predate the invention of the computer, and were used to direct the behavior of machines such as Jacquard looms and player pianos. Thousands of different programming languages have been created, mainly in the computer field, with many more being created every year. Most programming languages describe computation in an imperative style, i.e., as a sequence of commands, although some languages, such as those that support functional programming or logic programming, use alternative forms of description.

Contents

Definitions

A programming language is a notation for writing programs, which are specifications of a computation or algorithm.[1] Some, but not all, authors restrict the term "programming language" to those languages that can express all possible algorithms.[1][2] Traits often considered important for what constitutes a programming language include:

  • Function and target: A computer programming language is a language[3] used to write computer programs, which involve a computer performing some kind of computation[4] or algorithm and possibly control external devices such as printers, disk drives, robots,[5] and so on. For example PostScript programs are frequently created by another program to control a computer printer or display. More generally, a programming language may describe computation on some, possibly abstract, machine. It is generally accepted that a complete specification for a programming language includes a description, possibly idealized, of a machine or processor for that language.[6] In most practical contexts, a programming language involves a computer; consequently programming languages are usually defined and studied this way.[7] Programming languages differ from natural languages in that natural languages are only used for interaction between people, while programming languages also allow humans to communicate instructions to machines.
  • Abstractions: Programming languages usually contain abstractions for defining and manipulating data structures or controlling the flow of execution. The practical necessity that a programming language support adequate abstractions is expressed by the abstraction principle;[8] this principle is sometimes formulated as recommendation to the programmer to make proper use of such abstractions.[9]

Markup languages like XML, HTML or troff, which define structured data, are not generally considered programming languages.[12][13][14] Programming languages may, however, share the syntax with markup languages if a computational semantics is defined. XSLT, for example, is a Turing complete XML dialect.[15][16][17] Moreover, LaTeX, which is mostly used for structuring documents, also contains a Turing complete subset.[18][19]

The term computer language is sometimes used interchangeably with programming language.[20] However, the usage of both terms varies among authors, including the exact scope of each. One usage describes programming languages as a subset of computer languages.[21] In this vein, languages used in computing that have a different goal than expressing computer programs are generically designated computer languages. For instance, markup languages are sometimes referred to as computer languages to emphasize that they are not meant to be used for programming.[22] Another usage regards programming languages as theoretical constructs for programming abstract machines, and computer languages as the subset thereof that runs on physical computers, which have finite hardware resources.[23] John C. Reynolds emphasizes that formal specification languages are just as much programming languages as are the languages intended for execution. He also argues that textual and even graphical input formats that affect the behavior of a computer are programming languages, despite the fact they are commonly not Turing-complete, and remarks that ignorance of programming language concepts is the reason for many flaws in input formats.[24]

Elements

All programming languages have some primitive building blocks for the description of data and the processes or transformations applied to them (like the addition of two numbers or the selection of an item from a collection). These primitives are defined by syntactic and semantic rules which describe their structure and meaning respectively.

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Syntax

Parse tree of Python code with inset tokenization
Syntax highlighting is often used to aid programmers in recognizing elements of source code. The language above is Python.

A programming language's surface form is known as its syntax. Most programming languages are purely textual; they use sequences of text including words, numbers, and punctuation, much like written natural languages. On the other hand, there are some programming languages which are more graphical in nature, using visual relationships between symbols to specify a program.

The syntax of a language describes the possible combinations of symbols that form a syntactically correct program. The meaning given to a combination of symbols is handled by semantics (either formal or hard-coded in a reference implementation). Since most languages are textual, this article discusses textual syntax.

Programming language syntax is usually defined using a combination of regular expressions (for lexical structure) and Backus–Naur Form (for grammatical structure). Below is a simple grammar, based on Lisp:

expression ::= atom   | list
atom       ::= number | symbol
number     ::= [+-]?['0'-'9']+
symbol     ::= ['A'-'Z''a'-'z'].*
list       ::= '(' expression* ')'

This grammar specifies the following:

  • an expression is either an atom or a list;
  • an atom is either a number or a symbol;
  • a number is an unbroken sequence of one or more decimal digits, optionally preceded by a plus or minus sign;
  • a symbol is a letter followed by zero or more of any characters (excluding whitespace); and
  • a list is a matched pair of parentheses, with zero or more expressions inside it.

The following are examples of well-formed token sequences in this grammar: '12345', '()', '(a b c232 (1))'

Not all syntactically correct programs are semantically correct. Many syntactically correct programs are nonetheless ill-formed, per the language's rules; and may (depending on the language specification and the soundness of the implementation) result in an error on translation or execution. In some cases, such programs may exhibit undefined behavior. Even when a program is well-defined within a language, it may still have a meaning that is not intended by the person who wrote it.

Using natural language as an example, it may not be possible to assign a meaning to a grammatically correct sentence or the sentence may be false:

  • "Colorless green ideas sleep furiously." is grammatically well-formed but has no generally accepted meaning.
  • "John is a married bachelor." is grammatically well-formed but expresses a meaning that cannot be true.

The following C language fragment is syntactically correct, but performs an operation that is not semantically defined (because p is a null pointer, the operations p->real and p->im have no meaning):

complex *p = NULL;
complex abs_p = sqrt (p->real * p->real + p->im * p->im);

If the type declaration on the first line were omitted, the program would trigger an error on compilation, as the variable "p" would not be defined. But the program would still be syntactically correct, since type declarations provide only semantic information.

The grammar needed to specify a programming language can be classified by its position in the Chomsky hierarchy. The syntax of most programming languages can be specified using a Type-2 grammar, i.e., they are context-free grammars.[25] Some languages, including Perl and Lisp, contain constructs that allow execution during the parsing phase. Languages that have constructs that allow the programmer to alter the behavior of the parser make syntax analysis an undecidable problem, and generally blur the distinction between parsing and execution.[26] In contrast to Lisp's macro system and Perl's BEGIN blocks, which may contain general computations, C macros are merely string replacements, and do not require code execution.[27]

Static semantics

The static semantics defines restrictions on the structure of valid texts that are hard or impossible to express in standard syntactic formalisms.[1] For compiled languages, static semantics essentially include those semantic rules that can be checked at compile time. Examples include checking that every identifier is declared before it is used (in languages that require such declarations) or that the labels on the arms of a case statement are distinct.[28] Many important restrictions of this type, like checking that identifiers are used in the appropriate context (e.g. not adding a integer to a function name), or that subroutine calls have the appropriate number and type of arguments can be enforced by defining them as rules in a logic called a type system. Other forms of static analyses like data flow analysis may also be part of static semantics. Newer programming languages like Java and C# have definite assignment analysis, a form of data flow analysis, as part of their static semantics.

Type system

A type system defines how a programming language classifies values and expressions into types, how it can manipulate those types and how they interact. The goal of a type system is to verify and usually enforce a certain level of correctness in programs written in that language by detecting certain incorrect operations. Any decidable type system involves a trade-off: while it rejects many incorrect programs, it can also prohibit some correct, albeit unusual programs. In order to bypass this downside, a number of languages have type loopholes, usually unchecked casts that may be used by the programmer to explicitly allow a normally disallowed operation between different types. In most typed languages, the type system is used only to type check programs, but a number of languages, usually functional ones, perform type inference, which relieves the programmer from writing type annotations. The formal design and study of type systems is known as type theory.

Typed versus untyped languages

A language is typed if the specification of every operation defines types of data to which the operation is applicable, with the implication that it is not applicable to other types.[29] For example, "this text between the quotes" is a string. In most programming languages, dividing a number by a string has no meaning. Most modern programming languages will therefore reject any program attempting to perform such an operation. In some languages, the meaningless operation will be detected when the program is compiled ("static" type checking), and rejected by the compiler, while in others, it will be detected when the program is run ("dynamic" type checking), resulting in a runtime exception.

A special case of typed languages are the single-type languages. These are often scripting or markup languages, such as REXX or SGML, and have only one data type—most commonly character strings which are used for both symbolic and numeric data.

In contrast, an untyped language, such as most assembly languages, allows any operation to be performed on any data, which are generally considered to be sequences of bits of various lengths.[29] High-level languages which are untyped include BCPL and some varieties of Forth.

In practice, while few languages are considered typed from the point of view of type theory (verifying or rejecting all operations), most modern languages offer a degree of typing.[29] Many production languages provide means to bypass or subvert the type system.

Static versus dynamic typing

In static typing all expressions have their types determined prior to the program being run (typically at compile-time). For example, 1 and (2+2) are integer expressions; they cannot be passed to a function that expects a string, or stored in a variable that is defined to hold dates.[29]

Statically typed languages can be either manifestly typed or type-inferred. In the first case, the programmer must explicitly write types at certain textual positions (for example, at variable declarations). In the second case, the compiler infers the types of expressions and declarations based on context. Most mainstream statically typed languages, such as C++, C# and Java, are manifestly typed. Complete type inference has traditionally been associated with less mainstream languages, such as Haskell and ML. However, many manifestly typed languages support partial type inference; for example, Java and C# both infer types in certain limited cases.[30]

Dynamic typing, also called latent typing, determines the type-safety of operations at runtime; in other words, types are associated with runtime values rather than textual expressions.[29] As with type-inferred languages, dynamically typed languages do not require the programmer to write explicit type annotations on expressions. Among other things, this may permit a single variable to refer to values of different types at different points in the program execution. However, type errors cannot be automatically detected until a piece of code is actually executed, making debugging more difficult. Ruby, Lisp, JavaScript, and Python are dynamically typed.

Weak and strong typing

Weak typing allows a value of one type to be treated as another, for example treating a string as a number.[29] This can occasionally be useful, but it can also allow some kinds of program faults to go undetected at compile time and even at runtime.

Strong typing prevents the above. An attempt to perform an operation on the wrong type of value raises an error.[29] Strongly typed languages are often termed type-safe or safe.

An alternative definition for "weakly typed" refers to languages, such as Perl and JavaScript, which permit a large number of implicit type conversions. In JavaScript, for example, the expression 2 * x implicitly converts x to a number, and this conversion succeeds even if x is null, undefined, an Array, or a string of letters. Such implicit conversions are often useful, but they can mask programming errors.

Strong and static are now generally considered orthogonal concepts, but usage in the literature differs. Some use the term strongly typed to mean strongly, statically typed, or, even more confusingly, to mean simply statically typed. Thus C has been called both strongly typed and weakly, statically typed.[31][32]

Execution semantics

Once data has been specified, the machine must be instructed to perform operations on the data. For example, the semantics may define the strategy by which expressions are evaluated to values, or the manner in which control structures conditionally execute statements. The execution semantics (also known as dynamic semantics) of a language defines how and when the various constructs of a language should produce a program behavior. There are many ways of defining execution semantics. Natural language is often used to specify the execution semantics of languages commonly used in practice. A significant amount of academic research went into formal semantics of programming languages, which allow execution semantics to be specified in a formal manner. Results from this field of research have seen limited application to programming language design and implementation outside academia.

Core library

Most programming languages have an associated core library (sometimes known as the 'standard library', especially if it is included as part of the published language standard), which is conventionally made available by all implementations of the language. Core libraries typically include definitions for commonly used algorithms, data structures, and mechanisms for input and output.

A language's core library is often treated as part of the language by its users, although the designers may have treated it as a separate entity. Many language specifications define a core that must be made available in all implementations, and in the case of standardized languages this core library may be required. The line between a language and its core library therefore differs from language to language. Indeed, some languages are designed so that the meanings of certain syntactic constructs cannot even be described without referring to the core library. For example, in Java, a string literal is defined as an instance of the java.lang.String class; similarly, in Smalltalk, an anonymous function expression (a "block") constructs an instance of the library's BlockContext class. Conversely, Scheme contains multiple coherent subsets that suffice to construct the rest of the language as library macros, and so the language designers do not even bother to say which portions of the language must be implemented as language constructs, and which must be implemented as parts of a library.

Design and implementation

Programming languages share properties with natural languages related to their purpose as vehicles for communication, having a syntactic form separate from its semantics, and showing language families of related languages branching one from another.[3] But as artificial constructs, they also differ in fundamental ways from languages that have evolved through usage. A significant difference is that a programming language can be fully described and studied in its entirety, since it has a precise and finite definition.[33] By contrast, natural languages have changing meanings given by their users in different communities. While constructed languages are also artificial languages designed from the ground up with a specific purpose, they lack the precise and complete semantic definition that a programming language has.

Many languages have been designed from scratch, altered to meet new needs, combined with other languages, and eventually fallen into disuse. Although there have been attempts to design one "universal" programming language that serves all purposes, all of them have failed to be generally accepted as filling this role.[34] The need for diverse programming languages arises from the diversity of contexts in which languages are used:

  • Programs range from tiny scripts written by individual hobbyists to huge systems written by hundreds of programmers.
  • Programmers range in expertise from novices who need simplicity above all else, to experts who may be comfortable with considerable complexity.
  • Programs must balance speed, size, and simplicity on systems ranging from microcontrollers to supercomputers.
  • Programs may be written once and not change for generations, or they may undergo continual modification.
  • Finally, programmers may simply differ in their tastes: they may be accustomed to discussing problems and expressing them in a particular language.

One common trend in the development of programming languages has been to add more ability to solve problems using a higher level of abstraction. The earliest programming languages were tied very closely to the underlying hardware of the computer. As new programming languages have developed, features have been added that let programmers express ideas that are more remote from simple translation into underlying hardware instructions. Because programmers are less tied to the complexity of the computer, their programs can do more computing with less effort from the programmer. This lets them write more functionality per time unit.[35]

Natural language processors have been proposed as a way to eliminate the need for a specialized language for programming. However, this goal remains distant and its benefits are open to debate. Edsger W. Dijkstra took the position that the use of a formal language is essential to prevent the introduction of meaningless constructs, and dismissed natural language programming as "foolish".[36] Alan Perlis was similarly dismissive of the idea.[37]

A language's designers and users must construct a number of artifacts that govern and enable the practice of programming. The most important of these artifacts are the language specification and implementation.

Specification

The specification of a programming language is intended to provide a definition that the language users and the implementors can use to determine whether the behavior of a program is correct, given its source code.

A programming language specification can take several forms, including the following:

Implementation

An implementation of a programming language provides a way to execute that program on one or more configurations of hardware and software. There are, broadly, two approaches to programming language implementation: compilation and interpretation. It is generally possible to implement a language using either technique.

The output of a compiler may be executed by hardware or a program called an interpreter. In some implementations that make use of the interpreter approach there is no distinct boundary between compiling and interpreting. For instance, some implementations of BASIC compile and then execute the source a line at a time.

Programs that are executed directly on the hardware usually run several orders of magnitude faster than those that are interpreted in software.[citation needed]

One technique for improving the performance of interpreted programs is just-in-time compilation. Here the virtual machine, just before execution, translates the blocks of bytecode which are going to be used to machine code, for direct execution on the hardware.

Usage

Thousands of different programming languages have been created, mainly in the computing field.[41] Programming languages differ from most other forms of human expression in that they require a greater degree of precision and completeness. When using a natural language to communicate with other people, human authors and speakers can be ambiguous and make small errors, and still expect their intent to be understood. However, figuratively speaking, computers "do exactly what they are told to do", and cannot "understand" what code the programmer intended to write. The combination of the language definition, a program, and the program's inputs must fully specify the external behavior that occurs when the program is executed, within the domain of control of that program.

A programming language provides a structured mechanism for defining pieces of data, and the operations or transformations that may be carried out automatically on that data. A programmer uses the abstractions present in the language to represent the concepts involved in a computation. These concepts are represented as a collection of the simplest elements available (called primitives).[42] Programming is the process by which programmers combine these primitives to compose new programs, or adapt existing ones to new uses or a changing environment.

Programs for a computer might be executed in a batch process without human interaction, or a user might type commands in an interactive session of an interpreter. In this case the "commands" are simply programs, whose execution is chained together. When a language is used to give commands to a software application (such as a shell) it is called a scripting language.[citation needed]

Measuring language usage

It is difficult to determine which programming languages are most widely used, and what usage means varies by context. One language may occupy the greater number of programmer hours, a different one have more lines of code, and a third utilize the most CPU time. Some languages are very popular for particular kinds of applications. For example, COBOL is still strong in the corporate data center, often on large mainframes; FORTRAN in engineering applications; C in embedded applications and operating systems; and other languages are regularly used to write many different kinds of applications.

Various methods of measuring language popularity, each subject to a different bias over what is measured, have been proposed:

  • counting the number of job advertisements that mention the language[43]
  • the number of books sold that teach or describe the language[44]
  • estimates of the number of existing lines of code written in the language—which may underestimate languages not often found in public searches[45]
  • counts of language references (i.e., to the name of the language) found using a web search engine.

Combining and averaging information from various internet sites, langpop.com claims that [46] in 2008 the 10 most cited programming languages are (in alphabetical order): C, C++, C#, Java, JavaScript, Perl, PHP, Python, Ruby, and SQL.

Taxonomies

There is no overarching classification scheme for programming languages. A given programming language does not usually have a single ancestor language. Languages commonly arise by combining the elements of several predecessor languages with new ideas in circulation at the time. Ideas that originate in one language will diffuse throughout a family of related languages, and then leap suddenly across familial gaps to appear in an entirely different family.

The task is further complicated by the fact that languages can be classified along multiple axes. For example, Java is both an object-oriented language (because it encourages object-oriented organization) and a concurrent language (because it contains built-in constructs for running multiple threads in parallel). Python is an object-oriented scripting language.

In broad strokes, programming languages divide into programming paradigms and a classification by intended domain of use. Traditionally, programming languages have been regarded as describing computation in terms of imperative sentences, i.e. issuing commands. These are generally called imperative programming languages. A great deal of research in programming languages has been aimed at blurring the distinction between a program as a set of instructions and a program as an assertion about the desired answer, which is the main feature of declarative programming.[47] More refined paradigms include procedural programming, object-oriented programming, functional programming, and logic programming; some languages are hybrids of paradigms or multi-paradigmatic. An assembly language is not so much a paradigm as a direct model of an underlying machine architecture. By purpose, programming languages might be considered general purpose, system programming languages, scripting languages, domain-specific languages, or concurrent/distributed languages (or a combination of these).[48] Some general purpose languages were designed largely with educational goals.[49]

A programming language may also be classified by factors unrelated to programming paradigm. For instance, most programming languages use English language keywords, while a minority do not. Other languages may be classified as being esoteric or not.

History

A selection of textbooks that teach programming, in languages both popular and obscure. These are only a few of the thousands of programming languages and dialects that have been designed in history.

Early developments

The first programming languages predate the modern computer. The 19th century had "programmable" looms and player piano scrolls which implemented what are today recognized as examples of domain-specific languages. By the beginning of the twentieth century, punch cards encoded data and directed mechanical processing. In the 1930s and 1940s, the formalisms of Alonzo Church's lambda calculus and Alan Turing's Turing machines provided mathematical abstractions for expressing algorithms; the lambda calculus remains influential in language design.[50]

In the 1940s, the first electrically powered digital computers were created. The first high-level programming language to be designed for a computer was Plankalkül, developed for the German Z3 by Konrad Zuse between 1943 and 1945. However, it was not implemented until 1998 and 2000.[51]

Programmers of early 1950s computers, notably UNIVAC I and IBM 701, used machine language programs, that is, the first generation language (1GL). 1GL programming was quickly superseded by similarly machine-specific, but mnemonic, second generation languages (2GL) known as assembly languages or "assembler". Later in the 1950s, assembly language programming, which had evolved to include the use of macro instructions, was followed by the development of "third generation" programming languages (3GL), such as FORTRAN, LISP, and COBOL.[52] 3GLs are more abstract and are "portable", or at least implemented similarly on computers that do not support the same native machine code. Updated versions of all of these 3GLs are still in general use, and each has strongly influenced the development of later languages.[53] At the end of the 1950s, the language formalized as ALGOL 60 was introduced, and most later programming languages are, in many respects, descendants of Algol.[53] The format and use of the early programming languages was heavily influenced by the constraints of the interface.[54]

Refinement

The period from the 1960s to the late 1970s brought the development of the major language paradigms now in use, though many aspects were refinements of ideas in the very first Third-generation programming languages:

Each of these languages spawned an entire family of descendants, and most modern languages count at least one of them in their ancestry.

The 1960s and 1970s also saw considerable debate over the merits of structured programming, and whether programming languages should be designed to support it.[57] Edsger Dijkstra, in a famous 1968 letter published in the Communications of the ACM, argued that GOTO statements should be eliminated from all "higher level" programming languages.[58]

The 1960s and 1970s also saw expansion of techniques that reduced the footprint of a program as well as improved productivity of the programmer and user. The card deck for an early 4GL was a lot smaller for the same functionality expressed in a 3GL deck.

Consolidation and growth

The 1980s were years of relative consolidation. C++ combined object-oriented and systems programming. The United States government standardized Ada, a systems programming language derived from Pascal and intended for use by defense contractors. In Japan and elsewhere, vast sums were spent investigating so-called "fifth generation" languages that incorporated logic programming constructs.[59] The functional languages community moved to standardize ML and Lisp. Rather than inventing new paradigms, all of these movements elaborated upon the ideas invented in the previous decade.

One important trend in language design during the 1980s was an increased focus on programming for large-scale systems through the use of modules, or large-scale organizational units of code. Modula-2, Ada, and ML all developed notable module systems in the 1980s, although other languages, such as PL/I, already had extensive support for modular programming. Module systems were often wedded to generic programming constructs.[60]

The rapid growth of the Internet in the mid-1990s created opportunities for new languages. Perl, originally a Unix scripting tool first released in 1987, became common in dynamic websites. Java came to be used for server-side programming. These developments were not fundamentally novel, rather they were refinements to existing languages and paradigms, and largely based on the C family of programming languages.

Programming language evolution continues, in both industry and research. Current directions include security and reliability verification, new kinds of modularity (mixins, delegates, aspects), and database integration such as Microsoft's LINQ.

The 4GLs are examples of languages which are domain-specific, such as SQL, which manipulates and returns sets of data rather than the scalar values which are canonical to most programming languages. Perl, for example, with its 'here document' can hold multiple 4GL programs, as well as multiple JavaScript programs, in part of its own perl code and use variable interpolation in the 'here document' to support multi-language programming.[61]

See also

References

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  2. ^ In mathematical terms, this means the programming language is Turing-complete MacLennan, Bruce J. (1987). Principles of Programming Languages. Oxford University Press. p. 1. ISBN 0-19-511306-3. 
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  28. ^ Michael Lee Scott, Programming language pragmatics, Edition 2, Morgan Kaufmann, 2006, ISBN 0126339511, p. 18-19
  29. ^ a b c d e f g Andrew Cooke. "An Introduction to Programming Languages". http://www.acooke.org/andrew/writing/lang.html#sec-types. Retrieved June 30, 2006. 
  30. ^ Specifically, instantiations of generic types are inferred for certain expression forms. Type inference in Generic Java—the research language that provided the basis for Java 1.5's bounded parametric polymorphism extensions—is discussed in two informal manuscripts from the Types mailing list: Generic Java type inference is unsound (Alan Jeffrey, 17 December 2001) and Sound Generic Java type inference (Martin Odersky, 15 January 2002). C#'s type system is similar to Java's, and uses a similar partial type inference scheme.
  31. ^ "Revised Report on the Algorithmic Language Scheme (February 20, 1998)". http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-4.html. Retrieved June 9, 2006. 
  32. ^ Luca Cardelli and Peter Wegner. "On Understanding Types, Data Abstraction, and Polymorphism". Manuscript (1985). http://citeseer.ist.psu.edu/cardelli85understanding.html. Retrieved June 9, 2006. 
  33. ^ Jing Huang. "Artificial Language vs. Natural Language". http://www.cs.cornell.edu/info/Projects/Nuprl/cs611/fall94notes/cn2/subsection3_1_3.html. 
  34. ^ IBM in first publishing PL/I, for example, rather ambitiously titled its manual The universal programming language PL/I (IBM Library; 1966). The title reflected IBM's goals for unlimited subsetting capability: PL/I is designed in such a way that one can isolate subsets from it satisfying the requirements of particular applications. ("Encyclopaedia of Mathematics » P  » PL/I". SpringerLink. http://eom.springer.de/P/p072885.htm. Retrieved June 29, 2006. ). Ada and UNCOL had similar early goals.
  35. ^ Frederick P. Brooks, Jr.: The Mythical Man-Month, Addison-Wesley, 1982, pp. 93-94
  36. ^ Dijkstra, Edsger W. On the foolishness of "natural language programming." EWD667.
  37. ^ Perlis, Alan, Epigrams on Programming. SIGPLAN Notices Vol. 17, No. 9, September 1982, pp. 7-13
  38. ^ Milner, R.; M. Tofte, R. Harper and D. MacQueen. (1997). The Definition of Standard ML (Revised). MIT Press. ISBN 0-262-63181-4. 
  39. ^ Kelsey, Richard; William Clinger and Jonathan Rees (February 1998). "Section 7.2 Formal semantics". Revised5 Report on the Algorithmic Language Scheme. http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-10.html#%_sec_7.2. Retrieved 2006-06-09. 
  40. ^ ANSI — Programming Language Rexx, X3-274.1996
  41. ^ "HOPL: an interactive Roster of Programming Languages". Australia: Murdoch University. http://hopl.murdoch.edu.au/. Retrieved 2009-06-01. "This site lists 8512 languages." 
  42. ^ Abelson, Sussman, and Sussman. "Structure and Interpretation of Computer Programs". http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-10.html. Retrieved 2009-03-03. 
  43. ^ Survey of Job advertisements mentioning a given language
  44. ^ Counting programming languages by book sales
  45. ^ Bieman, J.M.; Murdock, V., Finding code on the World Wide Web: a preliminary investigation, Proceedings First IEEE International Workshop on Source Code Analysis and Manipulation, 2001
  46. ^ Programming Language Popularity
  47. ^ Carl A. Gunter, Semantics of Programming Languages: Structures and Techniques, MIT Press, 1992, ISBN 0262570955, p. 1
  48. ^ "TUNES: Programming Languages". http://tunes.org/wiki/programming_20languages.html. 
  49. ^ Wirth, Niklaus (1993). "Recollections about the development of Pascal". Proc. 2nd ACM SIGPLAN conference on history of programming languages: 333–342. doi:10.1145/154766.155378. http://portal.acm.org/citation.cfm?id=155378. Retrieved 2006-06-30. 
  50. ^ Benjamin C. Pierce writes:
    "... the lambda calculus has seen widespread use in the specification of programming language features, in language design and implementation, and in the study of type systems."
    Pierce, Benjamin C. (2002). Types and Programming Languages. MIT Press. p. 52. ISBN 0-262-16209-1. 
  51. ^ Rojas, Raúl, et al. (2000). "Plankalkül: The First High-Level Programming Language and its Implementation". Institut für Informatik, Freie Universität Berlin, Technical Report B-3/2000. (full text)
  52. ^ Linda Null, Julia Lobur, The essentials of computer organization and architecture, Edition 2, Jones & Bartlett Publishers, 2006, ISBN 0763737690, p. 435
  53. ^ a b O'Reilly Media. "History of programming languages" (PDF). http://www.oreilly.com/news/graphics/prog_lang_poster.pdf. Retrieved October 5, 2006. 
  54. ^ Frank da Cruz. IBM Punch Cards Columbia University Computing History.
  55. ^ Richard L. Wexelblat: History of Programming Languages, Academic Press, 1981, chapter XIV.
  56. ^ François Labelle. "Programming Language Usage Graph". SourceForge. http://www.cs.berkeley.edu/~flab/languages.html. Retrieved June 21, 2006. . This comparison analyzes trends in number of projects hosted by a popular community programming repository. During most years of the comparison, C leads by a considerable margin; in 2006, Java overtakes C, but the combination of C/C++ still leads considerably.
  57. ^ Hayes, Brian (2006), "The Semicolon Wars", American Scientist 94 (4): 299–303 
  58. ^ Dijkstra, Edsger W. (March 1968). "Go To Statement Considered Harmful". Communications of the ACM 11 (3): 147–148. doi:10.1145/362929.362947. http://www.acm.org/classics/oct95/. Retrieved 2006-06-29. 
  59. ^ Tetsuro Fujise, Takashi Chikayama Kazuaki Rokusawa, Akihiko Nakase (December 1994). "KLIC: A Portable Implementation of KL1" Proc. of FGCS '94, ICOT Tokyo, December 1994. KLIC is a portable implementation of a concurrent logic programming language KL1.
  60. ^ Jim Bender (March 15, 2004). "Mini-Bibliography on Modules for Functional Programming Languages". ReadScheme.org. http://readscheme.org/modules/. Retrieved 2006-09-27. 
  61. ^ Wall, Programming Perl ISBN 0-596-00027-8 p.66

Further reading

  • Daniel P. Friedman, Mitchell Wand, Christopher Thomas Haynes: Essentials of Programming Languages, The MIT Press 2001.
  • David Gelernter, Suresh Jagannathan: Programming Linguistics, The MIT Press 1990.
  • Shriram Krishnamurthi: Programming Languages: Application and Interpretation, online publication.
  • Bruce J. MacLennan: Principles of Programming Languages: Design, Evaluation, and Implementation, Oxford University Press 1999.
  • John C. Mitchell: Concepts in Programming Languages, Cambridge University Press 2002.
  • Benjamin C. Pierce: Types and Programming Languages, The MIT Press 2002.
  • Ravi Sethi: Programming Languages: Concepts and Constructs, 2nd ed., Addison-Wesley 1996.
  • Michael L. Scott: Programming Language Pragmatics, Morgan Kaufmann Publishers 2005.
  • Richard L. Wexelblat (ed.): History of Programming Languages, Academic Press 1981.

External links


Wikibooks

Up to date as of January 23, 2010

From Wikibooks, the open-content textbooks collection

< Computer programming

About Computer Programming Languages
Computer programming languages are machine-readable artificial languages designed to express computations that can be performed by a machine, particularly a computer. Computer programming languages can be used to create programs that specify the behavior of a machine, to express algorithms precisely, or as a mode of human communication.

Simple English

Contents

A programming language is a type of written language that tells computers what to do. Programming languages are used to make all computer programs and computer software. A programming language is like a set of instructions that the computer follows to do something.

A programmer writes source code text in the programming language to create programs. Usually the programming language uses real words for some of the commands, so that the language is easier for a human to read. Many programming languages use punctuation just like a normal language. Many programs now are compiled. This means that the computer translates the source code into new languages such as assembly language or machine language, which are much faster and easier for the computer to read, but much harder for a person to read.

Computer programs must be written very carefully. If the programmer makes mistakes, or the program tries to do something the programmer did not design it to do, the program might then "crash" or stop working. When a program has a problem because of how the code was written, this is called a bug. A very small mistake can cause a very big problem. For example, forgetting a period or typing a plus sign instead of a minus sign can cause a bad bug.

Types of programming languages

There are many types of programming languages. Most programming languages do not follow one type alone, so it is difficult to assign a type for each language. The examples of each type are given in each section below because they are the best well-known examples of that type.

Declarative vs. Imperative programming

Declarative programming languages describe a "problem" but they usually do not say how the problem should be solved. The problem description uses logic, "solving" the problem often looks like automatically proving a system of logical axioms. Examples for such programming languages are Prolog, XSLT, LISP and SQL

Imperative programming languages describe a system of state changes. At the start, the program is in a certain state, and the computer is given steps to follow, in order to perform an action. Following the steps causes the program to "change state".

In general, declarative programming languages are more readable, and easier to understand. Imperative programming languages are more common, because they are easier to use.

Functional vs. Procedural

Functional programming looks at programming like a function in mathematics. The program receives input, some information, and uses this information to create output. It will not have a state in between, and it will also not change things that are not related to the computation.

Procedural programs are a set of steps or state changes.

Stack based

Stack based languages look at the some of the program's memory like a stack of cards. There are very few things that can be done with a stack. A data item can be put on the top of the stack. This operation is generally called "push". A data item can be removed from the top of the stack. This is called a "pop". You can look at the item at the top of the stack without removing it. This is called a "peek".

If a program is written as "push 5; push 3; add; pop;" it will put 5 on the top of the stack, put 3 on top of the 5, add the top two values (3 + 5 = 8), replace the 3 and 5 with the 8, and print the top (8). An example for a programming language that is stack-based is the language Postscript

Object-oriented

Object-oriented programming languages place data and functions that change data into a single unit. This unit is called an "object". Objects can interact with each other, but one object can not change another object's data. This is usually called encapsulation or information hiding. Most modern programming languages are object-oriented.

Flow-oriented

Flow oriented programming sees programming as connecting different components. These components send messages back and forth. A single component can be part of different "programs", without the need to be changed internally.

Rules

Every programming language has rules about what it can and can not do. These include:

  • Correct numbers (types of numbers, and how large or small the numbers can go)
  • Words (reserved words, case-sensitivity)
  • Limits on what the programming language can do

Most languages have official standards that define the rules of how to write the source code. Some programming languages have two or more standards. This can happen when a new standard replaces an old one. For example, the Perl 5 standard replaced Perl 4 in 1993. It can happen because two people made two standards at the same time. For example, there are several standards for APL.

Object-Oriented Programming

Object-Oriented Programming (sometimes shortened to OOP) is a form of programming where all parts of the program are objects. Objects are pieces of memory with the same structure that can be used again and again. A bank account, bitmap, or hero from a video game could all be objects within a program. Objects are made up of properties (pieces of information the object stores) and methods which are things the object can do. A Dog object might have properties like height and hairColor. Its methods might include bark() and wagTail().

All objects are created from templates called classes. You can think of a class as a mold from which objects are made. The class defines all the properties and methods that its objects will have. Objects created from a class are called instances of the class. A class can extend another class, which means that it takes all the properties and methods of the class but can add its own.

Here is an example of what a class might look like in a programming language:

class Dog extends Mammal{

  //These are properties:
  String breed = "Collie"
  String type = "Herding Dog"

  //These are methods
  void wagTail(){
    //Do some wagging
  }

  void bark(){
    //Do the barking here
  }

}

Notice that the Dog class extends the Mammal class, so all dogs will have the properties of a mammal, like hairLength, and methods, like eat() or sleep().

Object-oriented programming is used in many of today's most popular programming languages, such as Java, Objective-C, C++, Python, Ruby, Javascript, and ActionScript.

Examples

Example of Visual Basic

Here is a simple program written in Visual Basic:

Dim Input
Input = InputBox("How old are you?")
If Not IsNumeric(Input) Then
  MsgBox "That's not a number!"
ElseIf Input < 0 Then
  MsgBox "You cannot be less than zero!"
ElseIf Input > 100 Then
  MsgBox "That's old!"
Else
  MsgBox "You're " & Input & " years old."
End If

This program asks the user his or her age and responds based on what the user typed. If the user typed something that is not a number, the program says so. If the user typed a number less than zero, the program says so. If the user says he or she is older than 100 years old, the program says "That's old!" If the user typed a correct age the program says back to the user how old he or she is.

Example of Python

Here is a program that does the same thing as the program above, but in Python:

try:
    age = int(raw_input("How old are you? "))
except ValueError:
    print "That's not a number!"
else:
    if age < 0:
        print "You cannot be less than zero!"
    elif age > 100:
        print "That's old!"
    else:
        print "You're %s years old." % age

Getting started

If you want to learn how to program, the Scratch programming language is a great way to start. You do not have to type anything, and all programming is done by dragging blocks together to form commands. You can build games, stories, and much more and share them on the Scratch website with other users. Best of all it is available in many different languages. Scratch is free to use and make changes to.

Other pages

rue:Язык проґрамованя


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