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computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...
, function composition is an act or mechanism to combine simple functions to build more complicated ones. Like the usual composition of functions in
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, the result of each function is passed as the argument of the next, and the result of the last one is the result of the whole. Programmers frequently apply functions to results of other functions, and almost all programming languages allow it. In some cases, the composition of functions is interesting as a function in its own right, to be used later. Such a function can always be defined but languages with first-class functions make it easier. The ability to easily compose functions encourages factoring (breaking apart) functions for maintainability and code reuse. More generally, big systems might be built by composing whole programs. Narrowly speaking, function composition applies to functions that operate on a finite amount of data, each step sequentially processing it before handing it to the next. Functions that operate on potentially infinite data (a
stream A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long large streams ...
or other
codata The Committee on Data of the International Science Council (CODATA) was established in 1966 as the Committee on Data for Science and Technology, originally part of the International Council of Scientific Unions, now part of the International ...
) are known as filters, and are instead connected in a pipeline, which is analogous to function composition and can execute concurrently.


Composing function calls

For example, suppose we have two functions and , as in and . Composing them means we first compute , and then use to compute . Here is the example in the
C language C (''pronounced like the letter c'') is a general-purpose computer programming language. It was created in the 1970s by Dennis Ritchie, and remains very widely used and influential. By design, C's features cleanly reflect the capabilities ...
: float x, y, z; // ... y = g(x); z = f(y); The steps can be combined if we don't give a name to the intermediate result: z = f(g(x)); Despite differences in length, these two implementations compute the same result. The second implementation requires only one line of code and is colloquially referred to as a "highly composed" form. Readability and hence maintainability is one advantage of highly composed forms, since they require fewer lines of code, minimizing a program's "surface area". DeMarco and Lister empirically verify an inverse relationship between surface area and maintainability. On the other hand, it may be possible to overuse highly composed forms. A nesting of too many functions may have the opposite effect, making the code less maintainable. In a stack-based language, functional composition is even more natural: it is performed by
concatenation In formal language theory and computer programming, string concatenation is the operation of joining character strings end-to-end. For example, the concatenation of "snow" and "ball" is "snowball". In certain formalisations of concatenat ...
, and is usually the primary method of program design. The above example in
Forth Forth or FORTH may refer to: Arts and entertainment * ''forth'' magazine, an Internet magazine * ''Forth'' (album), by The Verve, 2008 * ''Forth'', a 2011 album by Proto-Kaw * Radio Forth, a group of independent local radio stations in Scotla ...
: g f Which will take whatever was on the stack before, apply g, then f, and leave the result on the stack. See postfix composition notation for the corresponding mathematical notation.


Naming the composition of functions

Now suppose that the combination of calling f() on the result of g() is frequently useful, and which we want to name foo() to be used as a function in its own right. In most languages, we can define a new function implemented by composition. Example in C: float foo(float x) (the long form with intermediates would work as well.) Example in
Forth Forth or FORTH may refer to: Arts and entertainment * ''forth'' magazine, an Internet magazine * ''Forth'' (album), by The Verve, 2008 * ''Forth'', a 2011 album by Proto-Kaw * Radio Forth, a group of independent local radio stations in Scotla ...
: : foo g f ; In languages such as C, the only way to create a new function is to define it in the program source, which means that functions can't be composed at run time. An evaluation of an arbitrary composition of ''predefined'' functions, however, is possible: #include typedef int FXN(int); int f(int x) int g(int x) int h(int x) int eval(FXN *fs[], int size, int x) int main()


First-class composition

In functional programming languages, function composition can be naturally expressed as a higher-order function or operator. In other programming languages you can write your own mechanisms to perform function composition.


Haskell

In Haskell (programming language), Haskell, the example given above becomes: foo = f . g using the built-in composition operator (.) which can be read as ''f after g'' or ''g composed with f''. The composition operator itself can be defined in Haskell using a lambda expression: (.) :: (b -> c) -> (a -> b) -> a -> c f . g = \x -> f (g x) The first line describes the type of (.) - it takes a pair of functions, and returns a function (the lambda expression on the second line). Note that Haskell doesn't require specification of the exact input and output types of f and g; the a, b, c, and x are placeholders; only the relation between matters (f must accept what g returns). This makes (.) a polymorphic operator.


Lisp

Variants of Lisp, especially
Scheme A scheme is a systematic plan for the implementation of a certain idea. Scheme or schemer may refer to: Arts and entertainment * ''The Scheme'' (TV series), a BBC Scotland documentary series * The Scheme (band), an English pop band * ''The Schem ...
, the interchangeability of code and data together with the treatment of functions lend themselves extremely well for a recursive definition of a
variadic In computer science, an operator or function is variadic if it can take a varying number of arguments; that is, if its arity is not fixed. For specific articles, see: * Variadic function * Variadic macro in the C preprocessor The C preproces ...
compositional operator. (define (compose . fs) (if (null? fs) (lambda (x) x) ; if no argument is given, evaluates to the identity function (lambda (x) ((car fs) ((apply compose (cdr fs)) x))))) ; examples (define (add-a-bang str) (string-append str "!")) (define givebang (compose string->symbol add-a-bang symbol->string)) (givebang 'set) ;

> set! ; anonymous composition ((compose sqrt negate square) 5) ;

> 0+5i


APL

Many dialects of APL feature built in function composition using the symbol . This higher-order function extends function composition to dyadic application of the left side function such that A f∘g B is A f g B. foo←f∘g Additionally, you can define function composition: o← In dialect that does not support inline definition using braces, the traditional definition is available: ∇ r←(f o g)x r←f g x ∇


Raku

Raku like Haskell (programming language), Haskell has a built in function composition operator, the main difference is it is spelled as or o. my &foo = &f ∘ &g; Also like Haskell (programming language), Haskell you could define the operator yourself. In fact the following is the Raku code used to define it in the Rakudo implementation. # the implementation has a slightly different line here because it cheats proto sub infix:<∘> (&?, &?) is equiv(& is assoc multi sub infix:<∘> () # allows ` ��@array` to work when `@array` is empty multi sub infix:<∘> (&f) # allows ` ��@array` to work when `@array` has one element multi sub infix:<∘> (&f, &g --> Block) # alias it to the "Texas" spelling ( everything is bigger, and ASCII in Texas ) my &infix: := &infix:<∘>;


Python

In Python, a way to define the composition for any group of functions, is using reduce function (use functools.reduce in Python 3): # Available since Python v2.6 from functools import reduce def compose(*funcs) -> int: """Compose a group of functions (f(g(h(...)))) into a single composite func.""" return reduce(lambda f, g: lambda x: f(g(x)), funcs) # Example f = lambda x: x + 1 g = lambda x: x * 2 h = lambda x: x - 3 # Call the function x=10 : ((x-3)*2)+1 = 15 print(compose(f, g, h)(10))


JavaScript

In
JavaScript JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
we can define it as a function which takes two functions f and g, and produces a function: function o(f, g) // Alternatively, using the rest operator and lambda expressions in ES2015 const compose = (...fs) => (x) => fs.reduceRight((acc, f) => f(acc), x)


C#

In C# we can define it as a Func which takes two Funcs f and g, and produces a Func: // Call example: // var c = Compose(f, g); // // Func g = _ => ... // Func f = _ => ... Func Compose(params Func[] xs) => xs.Aggregate((accum, item) => x => accum(item(x)));


Ruby

Languages like Ruby (programming language), Ruby let you construct a binary operator yourself: class Proc def compose(other_fn) ->(*as) end alias_method :+, :compose end f = ->(x) g = ->(x) (f + g).call(12) # => 13824 However, a native function composition operator was introduced in Ruby 2.6: f = proc g = proc (f << g).call(3) # -> 11; identical to f(g(3)) (f >> g).call(3) # -> 15; identical to g(f(3))


Research survey

Notions of composition, including the principle of compositionality and
composability Composability is a system design principle that deals with the inter-relationships of components. A highly composable system provides components that can be selected and assembled in various combinations to satisfy specific user requirements. In i ...
, are so ubiquitous that numerous strands of research have separately evolved. The following is a sampling of the kind of research in which the notion of composition is central. * directly applied function composition to the assemblage of building blocks known as ' monads' in the Haskell programming language. * addressed the software reuse problem in terms of composability. * formally defined a proof rule for functional composition that assures a program's safety and liveness. * identified a strengthened form of compositionality by placing it into a
semiotic Semiotics (also called semiotic studies) is the systematic study of sign processes ( semiosis) and meaning making. Semiosis is any activity, conduct, or process that involves signs, where a sign is defined as anything that communicates something ...
system and applying it to the problem of structural
ambiguity Ambiguity is the type of meaning in which a phrase, statement or resolution is not explicitly defined, making several interpretations plausible. A common aspect of ambiguity is uncertainty. It is thus an attribute of any idea or statement ...
frequently encountered in computational linguistics. * examined the role of compositionality in analog aspects of natural language processing. *According to a review by , formal treatment of composition underlies validation of component assembly in visual programming languages like IBM's Visual Age for the
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
language.


Large-scale composition

Whole programs or systems can be treated as functions, which can be readily composed if their inputs and outputs are well-defined. Pipelines allowing easy composition of
filters Filter, filtering or filters may refer to: Science and technology Computing * Filter (higher-order function), in functional programming * Filter (software), a computer program to process a data stream * Filter (video), a software component that ...
were so successful that they became a design pattern of operating systems. Imperative procedures with side effects violate referential transparency and therefore are not cleanly composable. However if one considers the "state of the world" before and after running the code as its input and output, one gets a clean function. Composition of such functions corresponds to running the procedures one after the other. The monad formalism uses this idea to incorporate side effects and input/output (I/O) into functional languages.


See also

*
Currying In mathematics and computer science, currying is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument. For example, currying a function f tha ...
* Functional decomposition * Implementation inheritance * Inheritance semantics * Iteratee * Pipeline (Unix) * Principle of compositionality * Virtual inheritance


Notes


References

*. *. *. *. *. *. *. *. *. *. *. *. *{{citation , last = Steele , first = Guy L., Jr. , author-link = Guy L. Steele, Jr. , contribution = Building interpreters by composing monads , doi = 10.1145/174675.178068 , pages = 472–492 , title = Proc. 21st ACM Symposium on Principles of Programming Languages , contribution-url = http://groups.csail.mit.edu/mac/~dae/related-papers/steele.ps.Z , year = 1994. Programming language topics Higher-order functions