In
computer programming, named parameters, named argument or keyword arguments refer to a computer language's support for function calls that clearly state the name of each
parameter within the function call.
Overview
A function call using named parameters differs from a regular function call in that the values are passed by associating each one with a parameter name, instead of providing an ordered list of values.
For example, consider this
Java or
C# method call using no named parameters:
window.addNewControl("Title", 20, 50, 100, 50, true);
Using named parameters in
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pr ...
, the call can be written as:
window.addNewControl(title="Title",
xPosition=20,
yPosition=50,
width=100,
height=50,
drawingNow=True)
Using named parameters in
PHP, the call can be written as:
$window->addNewControl(title: "Title",
xPosition: 20,
yPosition: 50,
width: 100,
height: 50,
drawingNow: True);
The version with positional arguments is more implicit. The versions with named arguments is more explicit. Depending on a given instance, a programmer may find one or the other easier to read.
Use in programming languages
Named parameters are supported explicitly in many languages. A non-exhaustive selection of examples includes
Ada,
C# 4.0+,
Ceylon
Sri Lanka (, ; si, ශ්රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...
,
ColdFusion Markup Language
ColdFusion Markup Language, more commonly known as CFML, is a scripting language for web development that runs on the JVM, the .NET framework, and Google App Engine. Multiple commercial and open source implementations of CFML engines are availa ...
(CFML),
Common Lisp,
Fortran,
IDL,
Kotlin,
Mathematica,
PL/SQL,
PowerShell
PowerShell is a task automation and configuration management program from Microsoft, consisting of a command-line shell and the associated scripting language. Initially a Windows component only, known as Windows PowerShell, it was made open-sou ...
,
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pr ...
,
R,
PHP,
Ruby,
Scala,
Smalltalk
Smalltalk is an object-oriented, dynamically typed reflective programming language. It was designed and created in part for educational use, specifically for constructionist learning, at the Learning Research Group (LRG) of Xerox PARC by Alan K ...
,
Swift and
Visual Basic. Note that
Objective-C
Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXTS ...
does not have named parameters (even though parts of the method name may look like named parameters).
Order of parameters
In languages with no named parameters, the ''order'' of parameters in a function call is necessarily fixed, since it is the only way that the language can identify which value is intended to be used for which purpose.
With named parameters, it is usually possible to provide the values in any arbitrary order, since the name attached to each value identifies its purpose. This reduces the
connascence between parts of the program. A few languages use named parameters but still require the parameters to be provided in a specific order.
Optional parameters and positional parameters
Named parameters are often used in conjunction with optional parameters. Without named parameters, optional parameters can only appear at the end of the parameter list, since there is no other way to determine which values have been omitted. In languages that support named optional parameters, however, programs may supply any subset of the available parameters, and the names are used to determine which values have been provided.
An added complication arises in languages such as
OCaml
OCaml ( , formerly Objective Caml) is a general-purpose, multi-paradigm programming language which extends the Caml dialect of ML with object-oriented features. OCaml was created in 1996 by Xavier Leroy, Jérôme Vouillon, Damien Doligez, Di ...
that support both optional named parameters and
partial application. It is impossible in general to distinguish between a function partly applied, and a function to which a subset of parameters have been provided. OCaml resolves this ambiguity by requiring a positional parameter after all optional named parameters: its presence or absence is used to decide if the function has been fully or partly applied. If all parameters are optional, the implementor may solve the issue by adding a dummy positional parameter of type
unit.
In
MediaWiki
MediaWiki is a free and open-source wiki software. It is used on Wikipedia and almost all other Wikimedia websites, including Wiktionary, Wikimedia Commons and Wikidata; these sites define a large part of the requirement set for MediaWik ...
, the codes (variables)
,
in templates and so on, will be replaced by the first, second, and so on ''unnamed parameter'' (or the value of a parameter named
1
,
2
, etc.); these are known as ''s.
Emulating
In languages without named parameters, some of the same benefits can be achieved in other ways.
With documentation
Their value as documentation can be replicated by tooltips in
integrated development environments (IDEs) for languages such as
Java, or with comments (in
C):
MyFunctionCall(
20, /* x coordinate */
50, /* y coordinate */
100, /* width */
5, /* height */
TRUE /* drawing now? */
);
But this provides no checking, and the order of arguments remains important.
With data structures
Removing the argument order restriction, and the ability to leave some values unspecified, can be achieved by passing a
record or
associative array
In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms an a ...
.
For example, in
JavaScript, these two calls are equivalent:
MyFunctionCall();
MyFunctionCall();
Compare to C99:
struct MyParam ;
…
MyParam parameters = ;
MyFunctionCall(¶meters);
Special Support
In
Perl and pre-2.0
Ruby a similar convention exists (generally called a ''hash'' or ''options hash''), with special support for omitting the delimiters within function calls. As an example, the core module's Net::FTP ''new'' function accepts a hash of optional arguments.
With chained method calls
In
object-oriented programming languages, it is possible to use
method chaining
Method chaining, also known as named parameter idiom, is a common syntax for invoking multiple method calls in object-oriented programming languages. Each method returns an object, allowing the calls to be chained together in a single statement w ...
to simulate named parameters, as a form of
fluent interface. Each named parameter is replaced with a method on a parameter object that modifies and then returns the object. In C++, this is termed the ''named parameter idiom''.
[C++ FAQ]
10.20 What is the "Named Parameter Idiom"?
/ref> The object may then be passed to a function that uses the parameters it contains.
Method chaining
Method chaining, also known as named parameter idiom, is a common syntax for invoking multiple method calls in object-oriented programming languages. Each method returns an object, allowing the calls to be chained together in a single statement w ...
is often used in conjunction with the builder pattern The builder pattern is a design pattern designed to provide a flexible solution to various object creation problems in object-oriented programming. The intent of the Builder design pattern is to separate the construction of a complex object from it ...
as a way to override default values provided by the builder class.
See also
* Help:Template for named and positional parameters.
* Fluent interface
* Tag (programming)
References
{{Reflist
External links
* https://web.archive.org/web/20070502112455/http://plg.uwaterloo.ca/~rgesteve/cforall/named_pars.html
* In C++ this paradigm can be easily implemented
Boost Parameter Library
Named Parameters in various programming languages
at Rosetta Code
Rosetta Code is a wiki-based programming website with implementations of common algorithms and solutions to various programming problems in many different programming languages. It is named for the Rosetta Stone, which has the same text inscrib ...
Subroutines
Articles with example code