Named parameter

A function call using named parameters differs from a regular function call in that the arguments are passed by associating each one with a parameter name, instead of providing an ordered list of arguments.

For example, consider this Java or C# method call that doesn't use named parameters:

window.addNewControl("Title", 20, 50, 100, 50, true);

Using named parameters in Python, 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 that name parameters are more explicit. Depending on circumstance, a programmer may find one or the other to be easier to read.

Named parameters are supported explicitly in many languages. A non-exhaustive selection of examples includes Ada,[1] C# 4.0+,[2] Ceylon, ColdFusion Markup Language (CFML), Common Lisp,[3] Fortran, IDL, Kotlin,[4] Mathematica, PL/SQL, PowerShell, Python,[5] R,[6] PHP,[7] Ruby,[8] Scala,[9] Smalltalk, Swift[10] and Visual Basic.[11] Note that Objective-C does not have named parameters (even though parts of the method name may look like named parameters).[12]

In C++, you can achieve named parameters by using designated initializers since C++20,[13] like so:

struct A{int a{}, int b{}}; 

void
foo(A bar)
{
   std::cout << bar.a << bar.b;
}
foo({.a = 1, .b = 3});

In languages that do not support named parameters, the order of arguments in a function call is necessarily fixed, since it is the only way that the language can identify which argument is intended to be used for which parameter.

With named parameters, it is usually possible to provide the arguments in any order, since the parameter name attached to each argument identifies its purpose. This reduces the connascence between parts of the program. A few languages support named parameters but still require the arguments to be provided in a specific order.

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 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 argument after all optional named-parameter arguments: 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, the codes (variables) {{{1}}}, {{{2}}} 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 positional parameters.

In languages that do not support named parameters, some of the same benefits can be achieved in other ways.

MyFunctionCall(
    20,  /* x coordinate */
    50,  /* y coordinate */
    100, /* width */
    5,   /* height */
    TRUE /* drawing now? */
);

MyFunctionCall({ xPosition: 20, yPosition: 50, width: 100, height: 5,
                 drawingNow: true });

MyFunctionCall({ width: 100, height: 5, xPosition: 20, yPosition: 50,
                 drawingNow: true });

struct MyParam {
    int xPosition;
    int yPosition;
    int width;
    int height;
    unsigned char drawingNow;
};
…
MyParam parameters = { .xPosition = 20, .yPosition = 50,
        .width = 100, .height = 5, .drawingNow  = TRUE };
MyFunctionCall(&parameters);

• Help:Template for named and positional parameters.
• Fluent interface
• Tag (programming)

• 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