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In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions. In functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function ''f'' twice with the same value for an argument ''x'' will produce the same result ''f(x)'' each time. Eliminating side effects, i.e. changes in state that do not depend on the function inputs, can make it much easier to understand and predict the behavior of a program, which is one of the key motivations for the development of functional programming. Functional programming has its roots in lambda calculus, a formal system developed in the 1930s to investigate computability, the Entscheidungsproblem, function definition, function application, and recursion. Many functional programming languages can be viewed as elaborations on the lambda calculus. Another well-known declarative programming paradigm, ''logic programming'', is based on relations.〔 In contrast, imperative programming changes state with commands in the source language, the most simple example being assignment. Imperative programming does have functions—not in the mathematical sense—but in the sense of subroutines. They can have side effects that may change the value of program state. Functions without return values therefore make sense. Because of this, they lack referential transparency, i.e. the same language expression can result in different values at different times depending on the state of the executing program. Functional programming languages, especially purely functional ones such as Hope and Rex, have largely been emphasized in academia rather than in commercial software development. However, prominent programming languages which support functional programming such as Common Lisp, Scheme,〔〔〔〔 Clojure,〔〔 Wolfram Language 〔(【引用サイトリンク】 url = http://reference.wolfram.com/language/guide/FunctionalProgramming.html )〕 (also known as Mathematica), Racket,〔 Erlang,〔〔〔 OCaml,〔〔 Haskell,〔〔 and F# have been used in industrial and commercial applications by a wide variety of organizations. Functional programming is also supported in some domain-specific programming languages like R (statistics),〔 J, K and Q from Kx Systems (financial analysis), XQuery/XSLT (XML),〔〔 and Opal.〔 Widespread domain-specific declarative languages like SQL and Lex/Yacc use some elements of functional programming, especially in eschewing mutable values.〔 Programming in a functional style can also be accomplished in languages that are not specifically designed for functional programming. For example, the imperative Perl programming language has been the subject of a book describing how to apply functional programming concepts. This is also true of the PHP programming language. C# 3.0 and Java 8 added constructs to facilitate the functional style. The Julia language also offers functional programming abilities. An interesting case is that of Scala〔 – it is frequently written in a functional style, but the presence of side effects and mutable state place it in a grey area between imperative and functional languages. == History == Lambda calculus provides a theoretical framework for describing functions and their evaluation. Although it is a mathematical abstraction rather than a programming language, it forms the basis of almost all functional programming languages today. An equivalent theoretical formulation, combinatory logic, is commonly perceived as more abstract than lambda calculus and preceded it in invention. Combinatory logic and lambda calculus were both originally developed to achieve a clearer approach to the foundations of mathematics. An early functional-flavored language was Lisp, developed by John McCarthy while at Massachusetts Institute of Technology (MIT) for the IBM 700/7000 series scientific computers in the late 1950s. Lisp introduced many features now found in functional languages, though Lisp is technically a multi-paradigm language. Scheme and Dylan were later attempts to simplify and improve Lisp. Information Processing Language (IPL) is sometimes cited as the first computer-based functional programming language.〔The memoir of Herbert A. Simon (1991), ''Models of My Life'' pp.189-190 ISBN 0-465-04640-1 claims that he, Al Newell, and Cliff Shaw are "commonly adjudged to be the parents of () artificial intelligence ()", for writing Logic Theorist, a program which proved theorems from ''Principia Mathematica'' automatically. In order to accomplish this, they had to invent a language and a paradigm which, which viewed retrospectively, embeds functional programming.〕 It is an assembly-style language for manipulating lists of symbols. It does have a notion of "generator", which amounts to a function accepting a function as an argument, and, since it is an assembly-level language, code can be used as data, so IPL can be regarded as having higher-order functions. However, it relies heavily on mutating list structure and similar imperative features. Kenneth E. Iverson developed APL in the early 1960s, described in his 1962 book ''A Programming Language'' (ISBN 9780471430148). APL was the primary influence on John Backus's FP. In the early 1990s, Iverson and Roger Hui created J. In the mid-1990s, Arthur Whitney, who had previously worked with Iverson, created K, which is used commercially in financial industries along with its descendant Q. John Backus presented FP in his 1977 Turing Award lecture "Can Programming Be Liberated From the von Neumann Style? A Functional Style and its Algebra of Programs".〔http://www.stanford.edu/class/cs242/readings/backus.pdf〕 He defines functional programs as being built up in a hierarchical way by means of "combining forms" that allow an "algebra of programs"; in modern language, this means that functional programs follow the principle of compositionality. Backus's paper popularized research into functional programming, though it emphasized function-level programming rather than the lambda-calculus style which has come to be associated with functional programming. In the 1970s, ML was created by Robin Milner at the University of Edinburgh, and David Turner initially developed the language SASL at the University of St. Andrews and later the language Miranda at the University of Kent. Also in Edinburgh in the 1970s, Burstall and Darlington developed the functional language NPL.〔R.M. Burstall. Design considerations for a functional programming language. Invited paper, Proc. Infotech State of the Art Conf. "The Software Revolution", Copenhagen, 45–57 (1977)〕 NPL was based on Kleene Recursion Equations and was first introduced in their work on program transformation.〔R.M. Burstall and J. Darlington. A transformation system for developing recursive programs. Journal of the Association for Computing Machinery 24(1):44–67 (1977)〕 Burstall, MacQueen and Sannella then incorporated the polymorphic type checking from ML to produce the language Hope.〔R.M. Burstall, D.B. MacQueen and D.T. Sannella. HOPE: an experimental applicative language. Proc. 1980 LISP Conference, Stanford, 136–143 (1980).〕 ML eventually developed into several dialects, the most common of which are now OCaml and Standard ML. Meanwhile, the development of Scheme (a partly functional dialect of Lisp), as described in the influential Lambda Papers and the 1985 textbook ''Structure and Interpretation of Computer Programs'', brought awareness of the power of functional programming to the wider programming-languages community. In the 1980s, Per Martin-Löf developed intuitionistic type theory (also called ''constructive'' type theory), which associated functional programs with constructive proofs of arbitrarily complex mathematical propositions expressed as dependent types. This led to powerful new approaches to interactive theorem proving and has influenced the development of many subsequent functional programming languages. The Haskell language began with a consensus in 1987 to form an open standard for functional programming research; implementation releases have been ongoing since 1990. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「functional programming」の詳細全文を読む スポンサード リンク
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