View source for Modular programming

{{Short description |Organizing code into modules}}
'''Modular programming''' is a [[Programming paradigm|programming paradigm]] that emphasizes organizing the functions of a [[Codebase|codebase]] into independent modules {{endash}} each providing an [[Separation of concerns|aspect]] of a [[Computer program|computer program]] in its entirety without providing other aspects.

A module [[Interface (computing)|interface]] expresses the elements that are provided and required by the module. The elements defined in the interface are detectable by other modules. The [[Implementation|implementation]] contains the working code that corresponds to the elements declared in the interface. 

==Related==
Modular programming differs from but is related to other programming paradigms, including:

; [[Structured programming]]: Concerned with relatively low-level code structures for [[Control flow|control flow]].

; [[Object-oriented programming]]: Like modular programming, concerned with facilitating software construction via [[Decomposition (computer science)|decomposition]] but with specific focus on organizing code as [[Object (computer science)|objects]] that combine state with associated functionality.

; [[Interface-based programming]]: The use of interfaces as an architectural pattern to construct modules.

==History==
Modular programming, in the form of subsystems (particularly for I/O) and software libraries, dates to early software systems, where it was used for [[Code reuse|code reuse]]. Modular programming per se, with a goal of modularity, developed in the late 1960s and 1970s, as a larger-scale analog of the concept of [[Structured programming|structured programming]] (1960s). The term "modular programming" dates at least to the National Symposium on Modular Programming, organized at the Information and Systems Institute in July 1968 by Larry Constantine; other key concepts were [[Information hiding|information hiding]] (1972) and [[Separation of concerns|separation of concerns]] (SoC, 1974).

Modules were not included in the original specification for [[ALGOL 68]] (1968), but were included as extensions in early implementations, ALGOL 68-R (1970) and [[Software:ALGOL 68C|ALGOL 68C]] (1970), and later formalized.<ref>{{cite journal |last=Lindsey |first=Charles H. |date=Feb 1976 |title=Proposal for a Modules Facility in ALGOL 68 |url=http://archive.computerhistory.org/resources/text/algol/ACM_Algol_bulletin/1061719/p19-lindsey.pdf |url-status=dead |journal=ALGOL Bulletin |issue=39 |pages=20–29 |archive-url=https://web.archive.org/web/20160303230037/http://archive.computerhistory.org/resources/text/algol/ACM_Algol_bulletin/1061719/p19-lindsey.pdf |archive-date=2016-03-03 |access-date=2014-12-01}}</ref> One of the first languages designed from the start for modular programming was the short-lived [[Modula]] (1975), by [[Biography:Niklaus Wirth|Niklaus Wirth]]. Another early modular language was [[Mesa (programming language)|Mesa]] (1970s), by Xerox PARC, and Wirth drew on Mesa as well as the original Modula in its successor, [[Modula-2]] (1978), which influenced later languages, particularly through its successor, [[Modula-3]] (1980s). Modula's use of dot-qualified names, like <code>M.a</code> to refer to object <code>a</code> from module <code>M</code>, coincides with notation to access a field of a record (and similarly for attributes or methods of objects), and is now widespread, seen in [[C++]], [[C Sharp (programming language)|C#]], [[Dart (programming language)|Dart]], [[Go (programming language)|Go]], [[Java (programming language)|Java]], [[OCaml]], and [[Python (programming language)|Python]], among others. Modular programming became widespread from the 1980s: the original [[Pascal (programming language)|Pascal]] language (1970) did not include modules, but later versions, notably [[UCSD Pascal]] (1978) and [[Software:Turbo Pascal|Turbo Pascal]] (1983) included them in the form of "units", as did the Pascal-influenced [[Ada (programming language)|Ada]] (1980). The Extended Pascal ISO 10206:1990 standard kept closer to Modula2 in its modular support. [[Standard ML]] (1984)<ref>{{cite conference |author=David MacQueen |date=August 1984 |title=Modules for Standard ML |url=https://dl.acm.org/doi/pdf/10.1145/800055.802036 |book-title=LFP '84 Proceedings of the 1984 ACM Symposium on LISP and functional programming |pages=198–207|doi=10.1145/800055.802036 |doi-access=free }}</ref> has one of the most complete module systems, including [[Standard ML#Module system|functors]] (parameterized modules) to map between modules.

In the 1980s and 1990s, modular programming was overshadowed by and often conflated with [[Object-oriented programming|object-oriented programming]], particularly due to the popularity of C++ and Java. For example, the C family of languages had support for [[Object (computer science)|objects]] and classes in C++ (originally C with Classes, 1980) and Objective-C (1983), only supporting modules 30 years or more later. Java (1995) supports modules in the form of [[Java package|packages]], though the primary unit of code organization is a class. However, Python (1991) prominently used both modules and objects from the start, using modules as the primary unit of code organization and "packages" as a larger-scale unit; and Perl 5 (1994) includes support for both modules and objects, with a vast array of modules being available from [[CPAN]] (1993). [[OCaml]] (1996) followed ML by supporting modules and functors.

Modular programming is now widespread, and found in virtually all major languages developed since the 1990s. The relative importance of modules varies between languages, and in class-based object-oriented languages there is still overlap and confusion with classes as a unit of organization and encapsulation, but these are both well-established as distinct concepts.

==Terminology==
The term ''assembly'' (as in [[.NET|.NET]] languages like [[C Sharp (programming language)|C#]], [[F Sharp (programming language)|F#]], or [[Visual Basic (.NET)|Visual Basic]]) or ''package'' (as in [[Dart (programming language)|Dart]], [[Go (programming language)|Go]], or [[Java (programming language)|Java]]) is sometimes used instead of ''module''. In other implementations, these are distinct concepts; in [[Python (programming language)|Python]] a package is a set of modules, while in Java 9 the introduction of the [[Java Platform Module System]], in which a new module concept, involving a set of packages with enhanced access control, was implemented. (These packages are not the same as other sorts of packages in software, such as [[Package manager|package manager]] packages.)

In [[Java (programming language)|Java]], the term ''[[Java package|package]]'' is used for the module concept in the Java language specification.<ref>{{cite book |author1=James Gosling |author2=Bill Joy |author3=Guy Steele |author4=Gilad Bracha |author4-link=Gilad Bracha |title=The Java Language Specification, Third Edition |isbn=0-321-24678-0 |date=2005}} In the Introduction, it is stated "Chapter 7 describes the structure of a program, which is organized into packages similar to the modules of Modula." The word ''module'' has no special meaning in Java.</ref> The ''[[Java Platform Module System|module]]'', a kind of set of a package, was introduced in Java 9. 

In some [[Pascal (programming language)|Pascal]] dialects, the term ''unit'' is used for the module concept.

A component is a similar concept, but typically refers to a higher level; a component is a piece of a whole [[Software system|system]], while a module is a piece of an individual program. The scale of the term "module" varies significantly between languages; in Python it is very small-scale and each file is a module, while in Java 9 it is large-scale, where a module is a set of packages, which are in turn sets of files.

==Language support==
Languages that formally support the module concept include [[Ada (programming language)|Ada]], [[ALGOL]], BlitzMax, [[C++]], [[C Sharp (programming language)|C#]], [[Clojure]], [[COBOL]], [[Common Lisp]], [[D (programming language)|D]], [[Dart (programming language)|Dart]], eC, [[Erlang (programming language)|Erlang]], [[Elixir (programming language)|Elixir]], [[Elm (programming language)|Elm]], [[F (programming language)|F]], [[F Sharp (programming language)|F#]], [[Fortran]], [[Go (programming language)|Go]], [[Haskell]], IBM/360 [[IBM Basic assembly language and successors|Assembler]], IBM System/38 and AS/400 [[Control Language]] (CL), [[IBM RPG]], [[Java (programming language)|Java]], [[Julia (programming language)|Julia]], [[Software:MATLAB|MATLAB]], [[ML (programming language)|ML]], [[Modula]], [[Modula-2]], [[Modula-3]], Morpho, [[NEWP]], [[Oberon (programming language)|Oberon]], [[Oberon-2]], [[Objective-C]], [[OCaml]], several [[Pascal (programming language)|Pascal]] derivatives ([[Component Pascal]], [[Object Pascal]], [[Software:Turbo Pascal|Turbo Pascal]], [[UCSD Pascal]]), [[Perl]], [[Organization:PHP|PHP]], PL/I, [[PureBasic]], [[Python (programming language)|Python]], [[R (programming language)|R]], [[Ruby (programming language)|Ruby]],<ref>{{Cite web|url=https://docs.ruby-lang.org/en/master/Module.html|title=class Module - Documentation for Ruby 3.5}}</ref> [[Rust (programming language)|Rust]], [[JavaScript]],<ref>[http://www.ecma-international.org/ecma-262/6.0/#sec-modules ECMAScript® 2015 Language Specification, 15.2 Modules]</ref> [[Visual Basic (.NET)]] and WebDNA.

Conspicuous examples of languages that lack support for modules are [[C (programming language)|C]], and in their original forms, C++ and Pascal. C and C++ do, however, allow separate compilation and declarative interfaces to be specified using header files which is commonly considered modularization. Modules were added to Objective-C in [[Software:IOS 7|iOS 7]] (2013); to C++ with [[C++20]],<ref>{{cite web|url=https://isocpp.org/files/papers/n4720.pdf|title=N4720: Working Draft, Extensions to C++ for Modules}}</ref> and Pascal was superseded by [[Modula]] and [[Oberon (programming language)|Oberon]], which included modules from the start, and various derivatives that included modules. [[JavaScript]] has had native modules since [[ECMAScript]] 2015. C++ modules have allowed backwards compatibility with headers (with "header units"). Dialects of C allow for modules, for example [[Software:Clang|Clang]] supports modules for the C language,<ref>{{Cite web|url=https://clang.llvm.org/docs/Modules.html|title=Modules|website=clang.llvm.org}}</ref> though the syntax and semantics of Clang C modules differ from C++ modules.

Modular programming can be performed even where the programming language lacks explicit syntactic features to support named modules, like, for example, in C. This is done by using existing language features, together with, for example, [[Coding conventions|coding conventions]], programming idioms and the physical code structure. [[Software:IBM i|IBM i]] also uses modules when programming in the Integrated Language Environment (ILE).

==Key aspects==
With modular programming, [[Separation of concerns|concerns are separated]] such that modules perform logically discrete functions, interacting through well-defined interfaces. Often modules form a [[Directed acyclic graph|directed acyclic graph]] (DAG); in this case a cyclic dependency between modules is seen as indicating that these should be a single module. In the case where modules do form a DAG they can be arranged as a hierarchy, where the lowest-level modules are independent, depending on no other modules, and higher-level modules depend on lower-level ones. A particular program or library is a top-level module of its own hierarchy, but can in turn be seen as a lower-level module of a higher-level program, library, or system.

When creating a modular system, instead of creating a monolithic application (where the smallest component is the whole), several smaller modules are written separately so when they are composed together, they construct the executable application program. Typically, these are also [[Compiler|compiled]] separately, via separate compilation, and then linked by a [[Linker (computing)|linker]]. A [[Just-in-time compilation|just-in-time compiler]] may perform some of this construction "on-the-fly" at [[Run time (program lifecycle phase)|run time]].

These independent functions are commonly classified as either program control functions or specific task functions. Program control functions are designed to work for one program. Specific task functions are closely prepared to be applicable for various programs. 

This makes modular designed systems, if built correctly, far more reusable than a traditional monolithic design, since all (or many) of these modules may then be reused (without change) in other projects. This also facilitates the "breaking down" of projects into several smaller projects. Theoretically, a modularized software project will be more easily assembled by large teams, since no team members are creating the whole system, or even need to know about the system as a whole. They can focus just on the assigned smaller task.

==See also==
* {{Annotated link|Architecture description language}}
* {{Annotated link|Cohesion (computer science)}}
* {{Annotated link|Component-based software engineering}}
* {{Annotated link|Conway's law}}
* {{Annotated link|Coupling (computer science)}}
* {{Annotated link|Cross-cutting concern}}
* {{Annotated link|David Parnas}}
* {{Annotated link|Information hiding}}
* {{Annotated link|Library (computing)}}
* {{Annotated link|List of system quality attributes}}
* {{Annotated link|Modular design}}
* {{Annotated link|Plug-in (computing)}}
* {{Annotated link|Snippet (programming)}}
* {{Annotated link|Structured analysis}}

==References==
{{Reflist}}

==External links==
* [https://medium.com/@wrong.about/how-to-decompose-a-system-into-modules-796bd941f036 How To Decompose a System into Modules]
* [http://www.smcsystem.ru/ SMC Platform]

{{Programming paradigms navbox}}

{{DEFAULTSORT:Modular Programming}}
[[Category:Programming paradigms]]
[[Category:Modularity|Programming]]

{{Sourceattribution|Modular programming}}