Declarative programming
Encyclopedia
In computer science
, declarative programming is a programming paradigm
that expresses the logic of a computation
without describing its control flow
. Many languages applying this style attempt to minimize or eliminate side effects
by describing what the program should accomplish, rather than describing how to go about accomplishing it. This is in contrast with imperative programming
, which requires an explicitly provided algorithm
.
Declarative programming often considers programs
as theories of a formal logic
, and computations as deductions in that logic space. Declarative programming has become of particular interest recently, as it may greatly simplify writing parallel programs
.
Common declarative languages include those of regular expression
s, logic programming
, and functional programming
.
. A number of other common definitions exist that attempt to give the term a definition other than simply contrasting it with imperative programming. For example:
These definitions overlap substantially.
s.
by giving a value to each variable so that the solution is consistent with the maximum number of constraints.
Constraint programming is often used as a complement to other paradigms: functional, logical or even imperative programming.
, regular expression
s, and a subset of SQL
(SELECT queries, for example). DSLs have the advantage of being useful while not necessarily needing to be Turing-complete, which makes it easier for a language to be purely declarative.
Many markup languages such as HTML
, MXML
, SVG, XAML
, XSLT
or other user interface markup language
s are often declarative. HTML, for example, only describes what should appear on a webpage and does not specify the possible interaction
s with it.
Some software systems now combine traditional user interface markup languages such as HTML with declarative markup that defines what (but not how) the back-end server systems should do to support the declared interface. Such systems, typically using a domain specific XML
namespace
, include abstractions of SQL
database syntax or parameterised calls to web services using REST
and SOAP
.
programming, attempts to minimize or eliminate side effects, and is therefore considered declarative. Most functional languages, such as Scheme, Objective Caml
and Unlambda
, however, do permit side effects in practice.
While functional languages typically do appear to specify "how", a compiler for a purely functional programming language is free to extensively rewrite the operational behavior of a function, so long as the same result is returned for the same inputs. This can be used to, for example, make a function compute its result in parallel
, or to perform substantial optimizations (such as deforestation
) that a compiler may not be able to safely apply to a language with side effects.
specifies a context free grammar declaratively, but includes code snippets from a host language, which is usually imperative (such as C
).
state and query relations. The specifics of how these queries are answered is up to the implementation and its theorem prover, but typically take the form of some sort of unification. Like functional programming, many logic programming languages permit side effects, and as a result are not strictly declarative.
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, declarative programming is a programming paradigm
Programming paradigm
A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program and the steps that compose a computation A programming paradigm is a fundamental style of computer programming. (Compare with a...
that expresses the logic of a computation
Computation
Computation is defined as any type of calculation. Also defined as use of computer technology in Information processing.Computation is a process following a well-defined model understood and expressed in an algorithm, protocol, network topology, etc...
without describing its control flow
Control flow
In computer science, control flow refers to the order in which the individual statements, instructions, or function calls of an imperative or a declarative program are executed or evaluated....
. Many languages applying this style attempt to minimize or eliminate side effects
Side effect (computer science)
In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world...
by describing what the program should accomplish, rather than describing how to go about accomplishing it. This is in contrast with imperative programming
Imperative programming
In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
, which requires an explicitly provided algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
.
Declarative programming often considers programs
Program (machine)
A program is a list of instructions written in a programming language that is used to control the behavior of a machine, often a computer ....
as theories of a formal logic
Formal logic
Classical or traditional system of determining the validity or invalidity of a conclusion deduced from two or more statements...
, and computations as deductions in that logic space. Declarative programming has become of particular interest recently, as it may greatly simplify writing parallel programs
Parallel computing
Parallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
.
Common declarative languages include those of regular expression
Regular expression
In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...
s, logic programming
Logic programming
Logic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
, and functional programming
Functional programming
In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
.
Definition
Declarative programming is often defined as any style of programming that is not imperativeImperative programming
In computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
. A number of other common definitions exist that attempt to give the term a definition other than simply contrasting it with imperative programming. For example:
- A program that describes what computation should be performed and not how to compute it
- Any programming language that lacks side effectsSide effect (computer science)In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world...
(or more specifically, is referentially transparent) - A language with a clear correspondence to mathematical logicMathematical logicMathematical logic is a subfield of mathematics with close connections to foundations of mathematics, theoretical computer science and philosophical logic. The field includes both the mathematical study of logic and the applications of formal logic to other areas of mathematics...
.
These definitions overlap substantially.
Subparadigms
Declarative programming is an umbrella term that includes a number of better-known programming paradigmProgramming paradigm
A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program and the steps that compose a computation A programming paradigm is a fundamental style of computer programming. (Compare with a...
s.
Constraint programming
In constraint programming relations between variables are stated in the form of constraints, specifying the properties of a solution to be found. The set of constraints is then solvedSolver (computer science)
A solver is a generic term indicating a piece of mathematical software, possibly in the form of a stand-alone computer program or as a software library, that 'solves' a mathematical problem. A solver takes problem descriptions in some sort of generic form and calculate their solution...
by giving a value to each variable so that the solution is consistent with the maximum number of constraints.
Constraint programming is often used as a complement to other paradigms: functional, logical or even imperative programming.
Domain-specific languages
Some well-known examples of declarative domain specific languages (DSLs) include CSSCSS
-Computing:*Cascading Style Sheets, a language used to describe the style of document presentations in web development*Central Structure Store in the PHIGS 3D API*Closed source software, software that is not distributed with source code...
, regular expression
Regular expression
In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...
s, and a subset of SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
(SELECT queries, for example). DSLs have the advantage of being useful while not necessarily needing to be Turing-complete, which makes it easier for a language to be purely declarative.
Many markup languages such as HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
, MXML
MXML
MXML is an XML-based user interface markup language first introduced by Macromedia in March 2004. Application developers use MXML in combination with ActionScript to develop rich Internet applications, with products such as Adobe Flex....
, SVG, XAML
XAML
Extensible Application Markup Language is a declarative XML-based language created by Microsoft used for initializing structured values and objects. It is available under Microsoft's Open Specification Promise...
, XSLT
XSLT
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
or other user interface markup language
User interface markup language
A user interface markup language is a markup language that renders and describes graphical user interfaces and controls. Many of these markup languages are dialects of XML and are dependent upon a pre-existing scripting language engine, usually a JavaScript engine, for rendering of controls and...
s are often declarative. HTML, for example, only describes what should appear on a webpage and does not specify the possible interaction
Interaction
Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect...
s with it.
Some software systems now combine traditional user interface markup languages such as HTML with declarative markup that defines what (but not how) the back-end server systems should do to support the declared interface. Such systems, typically using a domain specific XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
namespace
XML Namespace
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
, include abstractions of SQL
SQL
SQL is a programming language designed for managing data in relational database management systems ....
database syntax or parameterised calls to web services using REST
Representational State Transfer
Representational state transfer is a style of software architecture for distributed hypermedia systems such as the World Wide Web. The term representational state transfer was introduced and defined in 2000 by Roy Fielding in his doctoral dissertation...
and SOAP
SOAP
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
.
Functional programming
Functional programming, and in particular purely functionalPurely functional
Purely functional is a term in computing used to describe algorithms, data structures or programming languages that exclude destructive modifications...
programming, attempts to minimize or eliminate side effects, and is therefore considered declarative. Most functional languages, such as Scheme, Objective Caml
Objective Caml
OCaml , originally known as Objective Caml, is the main implementation of the Caml programming language, created by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Didier Rémy and others in 1996...
and Unlambda
Unlambda
Unlambda is a minimal, "nearly pure" functional programming language invented by David Madore. It is based on combinatory logic, a version of the lambda calculus that omits the lambda operator. It relies mainly on two built-in functions and an "apply" operator...
, however, do permit side effects in practice.
While functional languages typically do appear to specify "how", a compiler for a purely functional programming language is free to extensively rewrite the operational behavior of a function, so long as the same result is returned for the same inputs. This can be used to, for example, make a function compute its result in parallel
Parallel computing
Parallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
, or to perform substantial optimizations (such as deforestation
Deforestation (computer science)
In the theory of programming languages in computer science, deforestation is a program transformation to eliminate tree structures....
) that a compiler may not be able to safely apply to a language with side effects.
Hybrid languages
Makefiles, for example, specify dependencies in a declarative fashion, but include an imperative list of actions to take as well. Similarly, yaccYacc
The computer program yacc is a parser generator developed by Stephen C. Johnson at AT&T for the Unix operating system. The name is an acronym for "Yet Another Compiler Compiler." It generates a parser based on an analytic grammar written in a notation similar to BNF.Yacc used to be available as...
specifies a context free grammar declaratively, but includes code snippets from a host language, which is usually imperative (such as C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
).
Logic programming
Logic programming languages such as PrologProlog
Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics.Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of...
state and query relations. The specifics of how these queries are answered is up to the implementation and its theorem prover, but typically take the form of some sort of unification. Like functional programming, many logic programming languages permit side effects, and as a result are not strictly declarative.
See also
:Category:Declarative programming languages- Comparison of programming paradigmsComparison of programming paradigmsThis article attempts to set out the various similarities and differences between the various programming paradigms as a summary in both graphical and tabular format with links to the separate discussions concerning these similarities and differences in existing Wikipedia articles- Main paradigm...
- Constraint programmingConstraint programmingConstraint programming is a programming paradigm wherein relations between variables are stated in the form of constraints. Constraints differ from the common primitives of imperative programming languages in that they do not specify a step or sequence of steps to execute, but rather the properties...
- Domain-specific language
- Functional programmingFunctional programmingIn computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state...
- Imperative programmingImperative programmingIn computer science, imperative programming is a programming paradigm that describes computation in terms of statements that change a program state...
(contrast) - Logic programmingLogic programmingLogic programming is, in its broadest sense, the use of mathematical logic for computer programming. In this view of logic programming, which can be traced at least as far back as John McCarthy's [1958] advice-taker proposal, logic is used as a purely declarative representation language, and a...
External links
- Frans Coenen. Characteristics of declarative programming languages. 1999.
- Olof Torgersson. A Note on Declarative Programming Paradigms and the Future of Definitional Programming. 1996.