In computer science, declarative programming is a programming paradigm, a style of building
the structure and elements of computer programs, 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 in terms of the problem domain, rather than describing
how to go about accomplishing it as a sequence of the programming language primitives.
This is in contrast with imperative programming, in which algorithms are implemented in terms
of explicit steps.
Declarative programming often considers programs as theories of a formal logic, and computations
as deductions in that logic space.
Declarative programming may greatly simplify writing parallel programs.
Common declarative languages include those of database query languages, regular expressions,
logic programming, functional programming, and configuration management systems.
Definition Declarative programming is often defined as
any style of programming that is not imperative.
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 effects A language with a clear correspondence to
mathematical logic.
These definitions overlap substantially.
Declarative programming contrasts with imperative and procedural programming.
Declarative programming is a non-imperative style of programming in which programs describe
their desired results without explicitly listing commands or steps that must be performed.
Functional and logical programming languages are characterized by a declarative programming
style.
In logical programming languages, programs consist of logical statements, and the program
executes by searching for proofs of the statements.
In a pure functional language, such as Haskell, all functions are without side effects, and
state changes are only represented as functions that transform the state.
Although pure functional languages are non-imperative, they often provide a facility for describing
the effect of a function as a series of steps.
Other functional languages, such as Lisp, OCaml and Erlang, support a mixture of procedural
and functional programming.
Some logical programming languages, such as Prolog, and database query languages, such
as SQL, while declarative in principle, also support a procedural style of programming.
Subparadigms Declarative programming is an umbrella term
that includes a number of better-known programming paradigms.
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 solved 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 include the yacc parser generator input language, the Make build specification
language, Puppet's configuration management language, regular expressions, and a subset
of SQL.
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, XAML, XSLT or other user-interface markup
languages are often declarative.
HTML, for example, only describes what should appear on a webpage - it does not specify
the possible interactions with that webpage.
As of 2013 some software systems combine traditional user-interface markup languages with declarative
markup that defines what the back-end server systems should do to support the declared
interface.
Such systems, typically using a domain-specific XML namespace, may include abstractions of
SQL database syntax or parameterised calls to web services using representational state
transfer and SOAP.
Functional programming Functional programming, and in particular
purely functional programming, attempts to minimize or eliminate side effects, and is
therefore considered declarative.
Most functional languages, such as Scheme, Clojure, Haskell, OCaml, Standard ML, 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 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, yacc specifies a context free grammar declaratively, but includes code snippets
from a host language, which is usually imperative.
Logic programming Logic programming languages such as Prolog
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.
Modeling
Models, or mathematical representations, of physical systems may be implemented in computer
code that is declarative.
The code contains a number of equations, not imperative assignments, that describe the
behavioral relationships.
When a model is expressed in this formalism, a computer is able to perform algebraic manipulations
to best formulate the solution algorithm.
The mathematical causality is typically imposed at the boundaries of the physical system,
while the behavioral description of the system itself is declarative or acausal.
Declarative modeling languages and environments include Modelica and Simile.
See also Comparison of programming paradigms
Inductive programming References
External links Frans Coenen.
Characteristics of declarative programming languages.
1999.
Robert Harper.
What, If Anything, Is A Declarative Language?.
2013.
Olof Torgersson.
A Note on Declarative Programming Paradigms and the Future of Definitional Programming.
1996.