Named parameters: Difference between revisions

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You wrap the parameters in a class, and make the function a friend of the parameter class.
You wrap the parameters in a class, and make the function a friend of the parameter class.


<lang c++>class foo_params{
<lang cpp>class foo_params{
friend void foo(foo_params p);
friend void foo(foo_params p);
public:
public:
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Declare the function to take the parameter class as its only parameter.
Declare the function to take the parameter class as its only parameter.


<lang c++>void foo(foo_params p){ . . .}</lang>
<lang cpp>void foo(foo_params p){ . . .}</lang>


Call the function using the parameter object constructor with the required parameters and chaining the optional parameters.
Call the function using the parameter object constructor with the required parameters and chaining the optional parameters.


<lang c++>foo(foo_params(42).x(7).z(23.54));</lang>
<lang cpp>foo(foo_params(42).x(7).z(23.54));</lang>




Revision as of 20:56, 4 January 2012

Task
Named parameters
You are encouraged to solve this task according to the task description, using any language you may know.

Create a function which takes in a number of arguments which are specified by name rather than (necessarily) position, and show how to call the function. If the language supports reordering the arguments or optionally omitting some of them, note this.

Note:

Named parameters relies on being able to use the names given to function parameters when the function is defined, when assigning arguments when the function is called.
For example, if f a function were to be defined as define func1( paramname1, paramname2); then it could be called normally as func1(argument1, argument2) and in the called function paramname1 would be associated with argument1 and paramname2 with argument2.
func1 must also be able to be called in a way that visually binds each parameter to its respective argument, irrespective of argument order, for example: func1(paramname2=argument2, paramname1=argument1) which explicitly makes the same parameter/argument bindings as before.
Named parameters are often a feature of languages used in safety critical areas such as Verilog and VHDL.

See also:

Ada

All callable entities (procedures, functions, entries) require named arguments. All of them can be called using either positional or keyed association of the actual arguments. The arguments supplied with default values can be omitted. <lang Ada>procedure Foo (Arg_1 : Integer; Arg_2 : Float := 0.0);</lang> It can be equivalently called as: <lang Ada>Foo (1, 0.0); Foo (1); Foo (Arg_2 => 0.0, Arg_1 => 1); Foo (Arg_1 => 1);</lang>

ALGOL 68

Translation of: Lua
Works with: ALGOL 68 version Revision 1 - no extensions to language used.
Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny.

Note: this method of implementing Named Parameters is experimental and to date has never been used outside of rosettacode.org. <lang algol68>#!/usr/local/bin/a68g --script #

MODE OPTNAME = STRUCT(STRING name), 

    OPTSPECIES = STRUCT(STRING species),
    OPTBREED = STRUCT(STRING breed),
    OWNER=STRUCT(STRING first name, middle name, last name);
  1. due to the Yoneda ambiguity simple arguments must have an unique operator defined #

OP NAME = (STRING name)OPTNAME: (OPTNAME opt; name OF opt := name; opt), 

  SPECIES = (STRING species)OPTSPECIES: (OPTSPECIES opt; species OF opt := species; opt),
  BREED = (STRING breed)OPTBREED: (OPTBREED opt; breed OF opt := breed; opt);

PROC print pet = ([]UNION(OPTNAME,OPTSPECIES,OPTBREED,OWNER) option)VOID: ( 

 STRING name:="Rex", species:="Dinosaur", breed:="Tyrannosaurus"; # Defaults #
 OWNER owner := ("George","W.","Bush");
 FOR i TO UPB option DO
   CASE option[i] IN
     (OPTNAME option): name := name OF option,
     (OPTSPECIES option): species := species OF option,
     (OPTBREED option): breed := breed OF option,
     (OWNER option): owner := option
   ESAC
 OD;
 printf(($gx$,"Details: a",breed,species,"named",name,"owned by",owner,$l$))

);

print pet((NAME "Mike", SPECIES "Dog", BREED "Irish Setter", OWNER("Harry", "S.", "Truman"))); print pet(())</lang> Output: 

Details: a Irish Setter Dog named Mike owned by Harry S. Truman 
Details: a Tyrannosaurus Dinosaur named Rex owned by George W. Bush

AppleScript

AppleScript does not implement default or optional parameters, but they can be simulated using records. <lang AppleScript>on getName(x) set {firstName, lastName} to {"?", "?"} try set firstName to x's firstName end try try set lastName to x's lastName end try end getName</lang> Examples: <lang AppleScript >getName({firstName:"John", lastName:"Doe"}) --> Returns: "John, Doe" getName({lastName:"Doe"}) --> Returns: "?, Doe"</lang>

AutoHotkey

AutoHotkey doesn't have named parameters, but they can be simulated as follows. ahk discussion <lang AutoHotkey>MyFunc( "Val=0, w=1024, Text=The Quick Brown Fox, newVar=I'm New" )

MyFunc( _overrides="" ) { 

Static x=5, y=5, w=100, h=100, Count
Name:="AutoHotkey", Type:="Scripting", Text:="qwerty", Val:=True

Loop, Parse, _overrides,`,=, %A_Space%  ; Override routine for Local/Static variables
  A_Index & 1  ? (_:=A_LoopField) : (%_%:=A_LoopField)

Listvars 

WinWaitClose, %A_ScriptFullPath%

}</lang>

C++

C++ does not support name parameters, but they can be simulated with the "Named Parameter Idiom" (http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.18).

You wrap the parameters in a class, and make the function a friend of the parameter class.

<lang cpp>class foo_params{ friend void foo(foo_params p); public: 

   foo_params(int r):
       required_param_(r),

optional_x_(0), optional_y_(1), optional_z_(3.1415) {} 

    foo_params& x(int i){

optional_x_=i; return *this; 

    }
    foo_params& y(int i){

optional_y_=i; return *this; 

    }
    foo_params& z(float f){

optional_z_=f; return *this; 

    }

private: 

       int 	required_param_;

int optional_x_; int optional_y_; float optional_z_; };</lang>

Declare the function to take the parameter class as its only parameter.

<lang cpp>void foo(foo_params p){ . . .}</lang>

Call the function using the parameter object constructor with the required parameters and chaining the optional parameters.

<lang cpp>foo(foo_params(42).x(7).z(23.54));</lang>


Library: Boost

If you want real named parameters you can use The Boost Parameter Library. <lang cpp>#include <boost/parameter/name.hpp>

  1. include <boost/parameter/preprocessor.hpp>
  2. include <string>

BOOST_PARAMETER_NAME(foo) BOOST_PARAMETER_NAME(bar) BOOST_PARAMETER_NAME(baz) BOOST_PARAMETER_NAME(bonk)

BOOST_PARAMETER_FUNCTION( 

   (int),  // the return type of the function, the parentheses are required.
   function_with_named_parameters, // the name of the function.
   tag,  // part of the deep magic. If you use BOOST_PARAMETER_NAME you need to put "tag" here.
   (required // names and types of all required parameters, parentheses are required.
       (foo, (int)) 
       (bar, (float))
   )
   (optional // names, types, and default values of all optional parameters.
       (baz, (bool) , false)
       (bonk, (std::string), "default value")
   )

) { 

   if (baz && (bar > 1.0)) return foo;
   return bonk.size();

}</lang>

Once the definition is written, using it is easy, by name or position.

<lang cpp>function_with_named_parameters(1, 10.0); function_with_named_parameters(7, _bar = 3.14); function_with_named_parameters( _bar = 0.0, _foo = 42); function_with_named_parameters( _bar = 2.5, _bonk= "Hello", _foo = 9); function_with_named_parameters(9, 2.5, true, "Hello");</lang>

C#

Works with: C# version 4.0

Named parameters were added in C# 4.0. The examples below demonstrate how named parameters and optional parameters are a single concept in the language.

<lang csharp>using System;

namespace NamedParams { 

   class Program
   {
       static void AddWidget(string parent, float x = 0, float y = 0, string text = "Default")
       {
           Console.WriteLine("parent = {0}, x = {1}, y = {2}, text = {3}", parent, x, y, text);
       }

       static void Main(string[] args)
       {
           AddWidget("root", 320, 240, "First");
           AddWidget("root", text: "Origin");
           AddWidget("root", 500);
           AddWidget("root", text: "Footer", y: 400);
       }
   }

}</lang>

Output: 

parent = root, x = 320, y = 240, text = First
parent = root, x = 0, y = 0, text = Origin
parent = root, x = 500, y = 0, text = Default
parent = root, x = 0, y = 400, text = Footer

Clojure

Clojure doesn't have built-in support for named or keyword arguments, but you can use destructuring to achieve a similar effect.

<lang clojure>(defn foo [& opts] 

 (let [opts (merge {:bar 1 :baz 2} (apply hash-map opts))
       {:keys [bar baz]} opts]
   [bar baz]))</lang>

Clojure 1.2 supports destructuring of trailing arguments as a map:

<lang clojure>(defn foo [& {:keys [bar baz] :or {bar 1, baz 2}}] 

 [bar baz])</lang>

You can also use defnk from clojure.contrib.def, which is a macro that works similarly.

<lang clojure>(use 'clojure.contrib.def) (defnk foo [:bar 1 :baz 2] 

 [bar baz])</lang>

Sample output for all variants: 

user> (foo :baz 123)
[1 123]

user> (foo :bar 456)
[456 2]

Common Lisp

<lang lisp>(defun print-name (&key first (last "?")) 

 (princ last)
 (when first
   (princ ", ")
   (princ first))
 (values))</lang>

&key indicates that further parameters are named (keyword parameters); a bare symbol (e.g. first) has a default of NIL, whereas a list (e.g. (last "?")) gives the variable name and the default value (which is evaluated when the function is called, unlike Python).

<lang lisp>CL-USER> (print-name) ? CL-USER> (print-name :first "John") ?, John CL-USER> (print-name :first "John" :last "Doe") Doe, John CL-USER> (print-name :last "Doe") Doe</lang>

In Common Lisp, the arguments to a function are always a simple list of values; the &key facility merely defines an interpretation of that list by the function: alternating keys and values. (As a result of this, mixing varargs (&rest) with named parameters is not recommended as it requires some additional means of distinguishing them. On the other hand, functions which pass their arguments on to other functions need not handle named arguments distinctly.)

E

Since E supports arbitrary pattern matching (in the sense of Pattern Matching in parameter lists, a map-pattern can be used to provide named parameters, though the syntax is sufficiently noisy that this is not used casually.

<lang e>def printName([=> first := null, => last := null]) { 

   if (last == null) {
       print("?")
   } else {
       print(last)
   }
   if (first != null) {
       print(", ")
       print(first)
   }

}</lang>

(Note: In map literals and map patterns, “=> x” is shorthand for “"x" => x”.)

<lang e>? printName(["first" => "John"]) ?, John

? printName(["last" => "Doe"]) Doe

? printName(["first" => "John", "last" => "Doe"]) Doe, John</lang>

Fortran

Works with: Fortran version 95 and later

Fortran accepts named parameter and optional parameter. Arguments are always named: if the name is omitted, the order used in the definition of the function / subroutine must be used.

<lang fortran>subroutine a_sub(arg1, arg2, arg3) 

 integer, intent(in) :: arg1, arg2
 integer, intent(out), optional :: arg3
 ! ...

end subroutine a_sub</lang>

<lang fortran>! usage 

 integer :: r
 call a_sub(1,2, r)
 call a_sub(arg2=2, arg1=1)</lang>

The presence of an optional argument can be tested with PRESENT; if optional arguments come before a non optional argument, the usage of the name of the argument is mandatory.

<lang fortran>subroutine b_sub(arg1, arg2) 

  integer, intent(in), optional :: arg1
  integer, intent(in) :: arg2
  !...

end subroutine b_sub</lang>

<lang fortran>call b_sub(1)  ! error: missing non optional arg2 call b_sub(arg2=1)  ! ok call b_sub(1, 2)  ! ok: arg1 is 1, arg2 is 2 call b_sub(arg2=2, arg1=1) ! the same as the previous line</lang>

Haskell

We can simulate named, but not moveable arguments, with nullary data constructors:

<lang Haskell>data X = X data Y = Y data Point = Point Int Int deriving Show

createPointAt :: X -> Int -> Y -> Int -> Point createPointAt X x Y y = Point x y

main = print $ createPointAt X 5 Y 3</lang>

We can also emulate named, moveable, optional arguments with record syntax:

<lang Haskell>data Point = Point {x, y :: Int} deriving Show defaultPoint = Point {x = 0, y = 0}

createPointAt :: Point -> Point createPointAt = id main = print $ createPointAt (defaultPoint { y = 3, x = 5 })</lang>

Though this is cumbersome without using template Haskell, as the call site must supply the defaults.

Icon and Unicon

Icon and Unicon do not support named parameters. There are a couple of approaches that could be adapted to provide this kind of functionality. The basic considerations are:

  • writing a string like "parm1=value" would be limiting as the value would have to be parsed and this form would be challenged to represent all data types
  • using two parameters like this (...,"parm1:=",x,"parm2:=",y,...) removes this limitation

The test procedure below includes a list of valid parameter names to check against


<lang Icon>procedure main() 

  testproc("x:=",1,"y:=",2,"z:=",3)
  testproc("x:=",3,"y:=",1,"z:=",2)
  testproc("z:=",4,"x:=",2,"y:=",3)
  testproc("i:=",1,"y:=",2,"z:=",3)

end

procedure testproc(A[]) #: demo to test named parameters 

  write("Calling testproc")

  while a := get(A) do               # implement named parameters here
     (( a ? (v := =!["x","y","z"], =":=")  |   #  valid parameter name?
        stop("No parameter ",a)) &             #  . . no
           ((variable(a[1:-2]) := get(A))  |   #  assign
              runerr(205,a)))                  #  . . problem
  
  write("   x:=",x)
  write("   y:=",y)
  write("   z:=",z)

end</lang>

This example is in need of improvement:

there is currently a bug in Icon/Unicon that causes assignments via variable("x") to fail for local and static variables. Remove this when this is fixed.

Output:

Calling testproc
   x:=1
   y:=2
   z:=3
Calling testproc
   x:=3
   y:=1
   z:=2
Calling testproc
   x:=2
   y:=3
   z:=4
Calling testproc
No parameter i:=

J

J is similar to Perl in that all arguments to functions come in as separate elements in an array. But it is possible to emulate more complex calling conventions. For example, using the calling convention J script, one could write:

<lang j>NB. Strand notation myFunc['c:\file.txt' 906 'blue' fs]

NB. Commas, like other langs myFunc['c:\file.txt', 906, 'blue' fs]

NB. Unspecified args are defaulted ("optional") myFunc['c:\file.txt' fs]

NB. Can use named arguments, like eg VB myFunc[color='blue' fs]

NB. Often values needn't be quoted myFunc[color= blue fs]

NB. Combination of comma syntax and name=value myFunc[max=906, color=blue fs]

NB. Spelling of names is flexible myFunc[MAX=906, COLOR=blue fs]

NB. Order of names is flexible myFunc[COLOR=blue, MAX=906 fs]

NB. Even the delimiters are flexible... myFunc<MAX=906, COLOR=blue fs></lang>

For further discussion, see the corresponding thread in the J Forums.

Java

Like C++, Java also does not support named parameters. Named parameters can however be simulated simply with the "Builder pattern". (Joshua Bloch: Builder Pattern) <lang java>processNutritionFacts(new NutritionFacts.Builder(240, 8).calories(100).sodium(35).carbohydrate(27).build());</lang> Follow the link for extra details about the 'NutritionFacts' class example.

JavaScript

JavaScript only has positional parameters, but named parameters can be emulated by passing an object with the appropriate properties: <lang javascript>function example(options) { 

 // assign some defaults where the user's has not provided a value
 opts = {}
 opts.foo = options.foo || 0;
 opts.bar = options.bar || 1;
 opts.grill = options.grill || 'pork chops'

 alert("foo is " + opts.foo + ", bar is " + opts.bar + ", and grill is " + opts.grill);

}

example({grill: "lamb kebab", bar: 3.14}); // => "foo is 0, bar is 3.14, and grill is lamb kebab"</lang>

Lua

<lang Lua> function CreatePet(options) 

 local name=options.name
 local species=options.species
 local breed=options.breed
 print('Created a '..breed..' '..species..' named '..name)

end CreatePet{name='Rex',species='Dog',breed='Irish Setter'} --position does not matter here. </lang>

Mathematica

<lang Mathematica>Options[fn]={Add->False,Offset-> 0}; fn[x_,y_,OptionsPattern[]]:=If[OptionValue[Add]==True,x+y+OptionValue[Offset],{x,y,OptionValue[Offset]}]

fn[3,4,{Add->True,Offset->2}] ->9 fn[3,4,{Offset->2,Add->True}] ->9</lang>

Modula-3

Much like Ada, Modula-3 allows either positional or keyed association of actual parameters. Defaults can also be ignored.

<lang modula3>PROCEDURE Foo(Arg1: INTEGER; Arg2: REAL := 0.0);</lang> It can be equivalently called as: <lang modula3>Foo(1, 0.0); Foo(1); Foo(Arg2 := 0.0, Arg1 := 1); Foo(Arg1 := 1);</lang>

Nemerle

<lang Nemerle>Foo(number : int, word = "Default", option = true) : void // note type inference with default values

Foo(word = "Bird", number = 3) // an argument with a default value can be omitted from function call Foo(3, option = false, word = "Bird") // unnamed arguments must be in same order as function definition and precede named arguments </lang>

Objective-C

Objective-C, like Smalltalk, has a method call syntax that is visually identical to named arguments, but they are not optional and may not be reordered. (Optional arguments may be simulated by defining multiple methods with the same leading name part.) <lang objc>@interface Demo : Object { 

   // Omitted ...

}

- (double) hypotenuseOfX: (double)x andY: (double)y; - (double) hypotenuseOfX: (double)x andY: (double)y andZ: (double)z;

@end</lang> <lang objc>@implementation Demo

- (double) hypotenuseOfX: (double)x andY: (double)y { 

   return hypot(x,y);

} - (double) hypotenuseOfX: (double)x andY: (double)y andZ: (double)z { 

   return hypot(hypot(x, y), z);

}

@end</lang> <lang objc>Demo *example = [[Demo alloc] init]; double h = [example hypotenuseOfX:1.23 andY:3.79];</lang> Note in the example above that the name of the method, the selector; is actually “hypotenuseOfX:andY:”.

OCaml

You can make a named argument (called labels in OCaml) by putting a tilde (~) before the name: <lang ocaml># let foo ~arg1 ~arg2 = arg1 + arg2;; val foo : arg1:int -> arg2:int -> int = <fun>

  1. let foo ~arg1:x ~arg2:y = x + y;; (* you can also use different variable names internally if you want *)

val foo : arg1:int -> arg2:int -> int = <fun>

  1. foo ~arg2:17 ~arg1:42;;

- : int = 59</lang>

Named arguments can be re-ordered, but two arguments of the same label cannot be re-ordered relative to each other.

Named arguments can be curried. If you partially apply a function on a named argument (even if the named argument is not first in the function declaration), it will evaluate to a function of the remaining arguments.

Named arguments can be made optional, with the ?(arg = value) syntax in the parameter declaration. See the optional parameters task for more details.

Oz

For methods, Oz does support named parameters with default values. The named parameters can be freely reordered. <lang oz>declare class Foo 

  meth init skip end
  meth bar(PP %% positional parameter

named1:N1 named2:N2 namedWithDefault:NWD <= 42) 

     {System.showInfo "PP: "#PP#", N1: "#N1#", N2: "#N2#", NWD: "#NWD}
  end

end

O = {New Foo init} {O bar(1 named1:2 named2:3 namedWithDefault:4)} %% ok {O bar(1 named2:2 named1:3)} %% ok {O bar(1 named1:2)} %% not ok, "missing message feature in object application"</lang>

For procedures only positional parameters are supported. However, you can emulate named parameters by using records: <lang oz> declare proc {Foo PP Other=unit(named1:N1 named2:N2 ...)} 

  NWD = {CondSelect Other namedWithDefault 42}

in 

     {System.showInfo "PP: "#PP#", N1: "#N1#", N2: "#N2#", NWD: "#NWD}

end

{Foo 1 unit(named1:2 named2:3 namedWithDefault:4)} {Foo 1 unit(named2:2 named1:3)} {Foo 1 unit(named1:2)} %% not ok...</lang>

The procedure Foo is defined with pattern matching in the argument list. The ellipsis means that additional record fields are allowed. To access optional record fields, we have to explicitly try to select a field and provide a default value in case it is missing ("CondSelect").

Perl

Perl has no formal parameters. Instead, Perl subroutines access all of their arguments through the special array @_. You can easily implement named arguments by making your function interpret @_ (or part of it) as a hash.

<lang perl>sub funkshun 

  {my %h = @_;
   # Print every argument and its value.
   print qq(Argument "$_" has value "$h{$_}".\n)
       foreach sort keys %h;
   # If a 'verbosity' argument was passed in, print its value,
   # whatever that value may be.
   exists $h{verbosity}
       and print "Verbosity specified as $h{verbosity}.\n";
   # Say that safe mode is on if 'safe' is set to a true value.
   # Otherwise, say that it's off.
   print "Safe mode ", ($h{safe} ? 'on' : 'off'), ".\n";}</lang>

The semantics of calling such a function follow directly from the semantics of using a hash. For instance, if you provide multiple values for the same named argument, only the last one will be used. An example call:

<lang perl>funkshun 

   verbosity => 3, password => 'foobie blech',
   extra_lives => 3, '42' => 'answer', password => 'joshua';</lang>

Its output: 

Argument "42" has value "answer".
Argument "extra_lives" has value "3".
Argument "password" has value "joshua".
Argument "verbosity" has value "3".
Verbosity specified as 3.
Safe mode off.

Perl 6

Works with: Rakudo version #22 "Thousand Oaks"

Perl 6's support for optional parameters is much like Python's. Consider this declaration:

<lang perl6>sub funkshun ($a, $b?, $c = 15, :$d, *@e, *%f) { 

  ...

}</lang>

In the above signature:

  • $a is a mandatory parameter that can be passed by position (funkshun 15, ...) or by name (funkshun a => 15, ...).
  • $b is an optional parameter that can be passed by position or by name. By default, it's undefined.
  • $c is an optional parameter that can be passed by position or by name. Its default value is 15.
  • $d is an optional parameter that can only be passed by name. By default, it's undefined.
  • @e is a slurpy array: it receives any leftover positional arguments.
  • %f is a slurpy hash: it receives any leftover named arguments.

So, if we defined the function like this:

<lang perl6>sub funkshun ($a, $b?, $c = 15, :$d, *@e, *%f) { 

  say "$a $b $c $d";
  say join ' ', @e;
  say join ' ', keys %f;

}</lang>

this particularly thorny call:

<lang perl6>funkshun 

   'Alfa', k1 => 'v1', c => 'Charlie', 'Bravo', 'e1',
   d => 'Delta', 'e2', k2 => 'v2';</lang>

would print this: 

Alfa Bravo Charlie Delta
e1 e2
k1 k2

PicoLisp

PicoLisp uses normally positional parameters, but 'bind' can be used to establish bindings to passed names.

Passing symbol-value pairs

<lang PicoLisp>(de foo @ 

  (bind (rest)  # Bind symbols in CARs to values in CDRs
     (println 'Bar 'is Bar)
     (println 'Mumble 'is Mumble) ) )

(foo '(Bar . 123) '(Mumble . "def"))</lang>

Passing a name list followed by values

<lang PicoLisp>(de foo @ 

  (bind (next)                # Save all symbols in first argument
     (mapc set (arg) (rest))  # then bind them to remaining arguments
     (println 'Bar 'is Bar)
     (println 'Mumble 'is Mumble) ) )

(foo '(Bar Mumble) 123 "def")</lang> Output in both cases: 

Bar is 123
Mumble is "def"

PowerShell

Positional parameters

When writing a function and not stating any parameters explicitly, such as the following function <lang powershell>function Test { 

   Write-Host Argument 1 is $args[0]

}</lang> the only option are positional parameters using the $args array.

Named parameters

Stating any number of parameters directly in the function definition, such as <lang powershell>function Test ($SomeArgument, $AnotherArgument, $ThirdArgument) { 

   Write-Host "Some argument:    $SomeArgument"
   Write-Host "Another argument: $AnotherArgument"
   Write-Host "Third argument:   $ThirdArgument"

}</lang> will cause them to be named automatically which enables the caller to state the arguments in any order. The syntax follows the convention used with cmdlets as well: 

PS> Test -ThirdArgument foo -AnotherArgument bar -SomeArgument baz
Some argument:    baz
Another argument: bar
Third argument:   foo

However, one can still just give the arguments in order without explicitly having to state the names of the parameters.

Arbitrary arguments can be omitted as well: 

PS> Test -ThirdArgument foo -AnotherArgument bar
Some argument:
Another argument: bar
Third argument:   foo

This will cause the omitted arguments to have the value $null

Switch parameters

Functions can have so-called switch parameters which are always boolean and either present or not. There is no need to give a value for them. <lang powershell>function SwitchTest ([switch] $on) { 

   Write-Host Switched $(if ($on) { "on" } else { "off" })

}</lang> When calling a function with such a parameter the switch is simply given directly (which sets its value to true) or omitted (which causes it to evaluate to false): 

PS> SwitchTest
Switched off
PS> SwitchTest -on
Switched on

Optional parameters and default values

Usually all parameters can be omitted. In the case of switch parameters this will cause them to assume the value false, for normal parameters they will have the value $null. This is not always the desired value, though. Default values can be given too: <lang powershell>function Greeting ($Name = "Nobody") { 

   Write-Host Hello, $Name!

}</lang> If the Name argument is omitted now, its value will be "Nobody" instead of $null: 

PS> Greeting
Hello Nobody!
PS> Greeting John
Hello John!

Python

Basic explanation

A more detailed explanation of parameters, arguments, and how they are used is in the sections below. This is a simplified explanation:

In Python, a regular parameter of a function can be used as either a positional or a named parameter. The variable name that you use for the parameter when you declare the function becomes the "name" for the parameter, should you use it as a named parameter. When you call a function, you use the "name = value" syntax to provide the argument to a named parameter. The named arguments must come after all the positional arguments.

<lang python>def subtract(x, y): 

   return x - y

subtract(5, 3) # used as positional parameters; evaluates to 2 subtract(y = 3, x = 5) # used as named parameters; evaluates to 2</lang>

Parameters can be made optional by providing a default argument, as described in the optional parameters article.

Detailed Explanation

Function Definition Parameters

Function definitions in Python allow for the following parameter types:

  • Optional default parameter types which are explicitly specified by name, and may have an optional default value.
  • An optional positional parameter which is an identifier preceded by "*".
  • And an optional keyword parameter which is an identifier preceded by "**".

If any of the parameter types are given then they must appear in the order specified above.

The syntax of function parameter declarations is more formally defined as: 

funcdef        ::=  "def" funcname "(" [parameter_list] ")" ":" suite
dotted_name    ::=  identifier ("." identifier)*
parameter_list ::=  (defparameter ",")*
                    (  posparameter [, keyparameter]
                    | keyparameter
                    | defparameter [","] )
defparameter   ::=  parameter ["=" expression]
posparameter   ::=  "*" identifier
keyparameter   ::=  "**" identifier
sublist        ::=  parameter ("," parameter)* [","]
parameter      ::=  identifier | "(" sublist ")"

Function Call Arguments

The call of a function in python can use the following argument types:

  • Positional arguments that are mapped by their position in the call argument list to the defparameter name in the corresponding position of the function definition.
  • Sequence arguments that are the character "*" followed by an expression evaluating to a sequence (such as a list or tuple). The values from the sequence are unpacked and mapped like individual positional arguments to defparameters of the function definition. Sequence arguments are “evaluated before any keyword argument, irrespecctive of their relative positions in an argument list”.
  • All positional arguments must appear before any keyword argument.
  • Keyword arguments of the form parameter_name "=" value will map the value to the defparameter in the definition of the same name.
  • Mapping arguments that are the characters "**" followed by an expression evaluating to a mapping (such as a dict/hash). The key, value pairs from the mapping are unpacked and mapped like individual keyword arguments to defparameters of the function definition.
  • If the function definition includes a positional parameter, then if the assignment of positional arguments and sequence arguments in the call gives more values than the defparameters of the definition, then these extra arguments are assembled, in order, into a tuple that is assigned to the posparameter of the definition.
  • If the function definition includes a keyword parameter, then if the parameter name of any keyword arguments and mapping arguments in the call is unknown in the defparameters of the function definition, then these extra keyword/value pairs are assembled into a dict that is assigned to the keyparameter of the definition.
  • Any default parameter of the function definition that is not assigned a value at this point, but which has a default value, will be aassigned this default value, without re-evaluating the default value.
  • Any default parameter of the function definition that is still un-assigned will cause a TypeError exception to be raised.
  • In addition, multiple mappings to any parameter will raise a TypeError exception. (This includes multiple mappings into a keyparameter or keyword arguments clashing with positional/sequence arguments).

The more formal definition of a function call's syntax is 

call                 ::=  primary "(" [argument_list [","]
                          | expression genexpr_for] ")"
argument_list        ::=  positional_arguments ["," keyword_arguments]
                            ["," sequence_argument] ["," keyword_arguments]
                            ["," mapping_argument]
                          | keyword_arguments ["," sequence_argument]
                            ["," mapping_argument]
                          | sequence_argument ["," sequence_argument] ["," mapping_argument]
                          | mapping_argument
positional_arguments ::=  expression ("," expression)*
keyword_arguments    ::=  keyword_item ("," keyword_item)*
sequence_argument    ::=  "*" expression
mapping_argument     ::=  "**" expression
keyword_item         ::=  identifier "=" expression

Examples

<lang python>>>> from __future__ import print_function >>> >>> def show_args(defparam1, defparam2 = 'default value', *posparam, **keyparam): 

 "Straight-forward function to show its arguments"
 print ("  Default Parameters:")
 print ("    defparam1 value is:", defparam1)
 print ("    defparam2 value is:", defparam2)

 print ("  Positional Arguments:")
 if posparam:
   n = 0
   for p in posparam:
     print ("    positional argument:", n, "is:", p)
     n += 1
 else:
   print ("    <None>")

 print ("  Keyword Arguments (by sorted key name):")
 if keyparam:
   for k,v in sorted(keyparam.items()):
     print ("    keyword argument:", k, "is:", v)
 else:
   print ("    <None>")


>>> show_args('POSITIONAL', 'ARGUMENTS') 

 Default Parameters:
   defparam1 value is: POSITIONAL
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args(defparam2='ARGUMENT', defparam1='KEYWORD') 

 Default Parameters:
   defparam1 value is: KEYWORD
   defparam2 value is: ARGUMENT
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args( *('SEQUENCE', 'ARGUMENTS') ) 

 Default Parameters:
   defparam1 value is: SEQUENCE
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args( **{'defparam2':'ARGUMENTS', 'defparam1':'MAPPING'} ) 

 Default Parameters:
   defparam1 value is: MAPPING
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args('ONLY DEFINE defparam1 ARGUMENT') 

 Default Parameters:
   defparam1 value is: ONLY DEFINE defparam1 ARGUMENT
   defparam2 value is: default value
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args('POSITIONAL', 'ARGUMENTS', 

             'EXTRA', 'POSITIONAL', 'ARGUMENTS')
 Default Parameters:
   defparam1 value is: POSITIONAL
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   positional argument: 0 is: EXTRA
   positional argument: 1 is: POSITIONAL
   positional argument: 2 is: ARGUMENTS
 Keyword Arguments (by sorted key name):
   <None>

>>> show_args('POSITIONAL', 'ARGUMENTS', 

             kwa1='EXTRA', kwa2='KEYWORD', kwa3='ARGUMENTS')
 Default Parameters:
   defparam1 value is: POSITIONAL
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   <None>
 Keyword Arguments (by sorted key name):
   keyword argument: kwa1 is: EXTRA
   keyword argument: kwa2 is: KEYWORD
   keyword argument: kwa3 is: ARGUMENTS

>>> show_args('POSITIONAL', 

             'ARGUMENTS', 'EXTRA', 'POSITIONAL', 'ARGUMENTS', 
             kwa1='EXTRA', kwa2='KEYWORD', kwa3='ARGUMENTS')
 Default Parameters:
   defparam1 value is: POSITIONAL
   defparam2 value is: ARGUMENTS
 Positional Arguments:
   positional argument: 0 is: EXTRA
   positional argument: 1 is: POSITIONAL
   positional argument: 2 is: ARGUMENTS
 Keyword Arguments (by sorted key name):
   keyword argument: kwa1 is: EXTRA
   keyword argument: kwa2 is: KEYWORD
   keyword argument: kwa3 is: ARGUMENTS

>>> # But note: >>> show_args('POSITIONAL', 'ARGUMENTS', 

             kwa1='EXTRA', kwa2='KEYWORD', kwa3='ARGUMENTS', 
             'EXTRA', 'POSITIONAL', 'ARGUMENTS')

SyntaxError: non-keyword arg after keyword arg >>></lang>

R

R parameters are all named; arguments can be passed either positionally or with explicit naming. The named arguments are matched to their parameters first, then the unnamed arguments fill in remaining slots. A parameter whose name begins with a period will not be matched to unnamed arguments. R allows abbreviated names to be used as long as they match uniquely to an argument.

<lang r>divide <- function(numerator, denominator) { 

 numerator / denominator

}

divide(3, 2) # 1.5 divide(numerator=3, denominator=2) # 1.5 divide(n=3, d=2) # 1.5 divide(den=3, num=2) # 0.66 divide(den=3, 2) # 0.66 divide(3, num=2) # 0.66</lang>

Ruby

Ruby, like Tcl, has no direct support for named parameters. Ruby can fake the effect with a Hash. When the caller passes name/value pairs, Ruby passes them as one Hash in the last positional parameter.

  • This parameter should default to an empty Hash {}.
  • By convention, each hash key is a Symbol like :foo.

<lang ruby>def example(opts = {}) 

 # Assign some defaults, then apply the caller's options.
 # Hash#merge raises TypeError if _opts_ is not a Hash.
 opts = {:foo => 0, :bar => 1, :grill => "pork chops"}.merge(opts)
 puts "foo is #{opts[:foo]}, bar is #{opts[:bar]}, " \
   "and grill is #{opts[:grill]}"

end

  1. Note that -foo is omitted and -grill precedes -bar

example(:grill => "lamb kebab", :bar => 3.14)

  1. => "foo is 0, bar is 3.14, and grill is lamb kebab"</lang>

Users of Ruby 1.9 can also use the new shortcut syntax for Symbols as keys.

Works with: Ruby version 1.9

<lang ruby>example(grill: "lamb kebab", bar: 3.14)</lang>

Ruby's future

Ruby 2.0 will add keyword arguments, which are named parameters that have default values.

Works with: MRI version 2.0.0dev (2012-01-04 trunk 34211)

<lang ruby>def example(foo: 0, bar: 1, grill: "pork chops") 

 puts "foo is #{foo}, bar is #{bar}, and grill is #{grill}"

end

  1. Note that -foo is omitted and -grill precedes -bar

example(grill: "lamb kebab", bar: 3.14)</lang>

Scala

Scala 2.8 utilizes named parameters and default values:

<lang scala> def add(x: Int, y: Int = 1) = x + y </lang>

<lang scala> scala> add(5) 6

scala> add(y=10, x=4) 14 </lang>

Scheme

<lang scheme> (define (keyarg-parser argdefs args kont) 

 (apply kont

(map (lambda (argdef) (let loop ((args args)) (cond ((null? args) (cadr argdef)) ((eq? (car argdef) (car args)) (cadr args)) (else (loop (cdr args)))))) argdefs)))

(define (print-name . args) 

 (keyarg-parser '((first #f)(last "?"))

args (lambda (first last) (display last) (cond (first (display ", ") (display first))) (newline)))) </lang>

<lang scheme> => (print-name) ? => (print-name 'first "John") ?, John =>(print-name 'first "John" 'last "Doe") Doe, John =>(print-name 'last "Doe") Doe </lang>

Smalltalk

As for Objective-C the methods signature is made of "Symbol:[OtherSymbol:]*" (* stands for 0 or more repetition of the part in the brackets), without the possibility to reorder them (it would be another signature) or to make them optional.

Works with: GNU Smalltalk

<lang smalltalk>Object subclass: AnotherClass [ 

  "..."
  initWithArray: anArray [ "single argument" ]
  initWithArray: anArray andString: aString [ 
       "two args; these two methods in usage resemble
        a named argument, with optional andString argument"
  ]
  "..."

]</lang>

Suneido

Suneido can handle named and unnamed parameters. When using a combination, unnamed parameters must come before named ones and must be in the correct order. Named parameters can be in any order. Named parameters are given a default value so they are not mandatory. <lang Suneido> test = function (one, two, three = , four = , five = ) 

   {
   Print('one: ' $ one $ ', two: ' $ two $ ', three: ' $ three $ 
       ', four: ' $ four $ ', five: ' $ five)
   }

test('1', '2', five: '5', three: '3') </lang> Output: <lang Suneido>one: 1, two: 2, three: 3, four: , five: 5</lang>

Tcl

The simplest way of passing named parameters is to use the Tcl language's strong support for variadic commands together with its arrays. By convention (originally from Tk) the named parameters names start with a hyphen (“-”) and are called options. <lang tcl>proc example args { 

   # Set the defaults
   array set opts {-foo 0 -bar 1 -grill "hamburger"}
   # Merge in the values from the caller
   array set opts $args
   # Use the arguments
   return "foo is $opts(-foo), bar is $opts(-bar), and grill is $opts(-grill)"

}

  1. Note that -foo is omitted and -grill precedes -bar

example -grill "lamb kebab" -bar 3.14

  1. => ‘foo is 0, bar is 3.14, and grill is lamb kebab’</lang>

More complex option parsing is possible, e.g., with the opt package (of which only a small fraction of the functionality is shown here). This package also allows you to specify type constraints, though that is usually not necessary, and will generate a standard help message that can be obtained with the -help option: <lang tcl>package require opt tcl::OptProc example { 

   {-foo   -int   0           "The number of foos"}
   {-bar   -float 1.0         "How much bar-ness"}
   {-grill -any   "hamburger" "What to cook on the grill"}

} { 

   return "foo is $foo, bar is $bar, and grill is $grill"

} example -grill "lamb kebab" -bar 3.14

  1. => ‘foo is 0, bar is 3.14, and grill is lamb kebab’

example -help

  1. Usage information:
  2. Var/FlagName Type Value Help
  3. ------------ ---- ----- ----
  4. ( -help gives this help )
  5. -foo int (0) The number of foos
  6. -bar float (1.0) How much bar-ness
  7. -grill any (hamburger) What to cook on the grill</lang>

According to wiki.tcl.tk discussions, ::tcl::OptProc is deprecated. The recommended replacement is cmdline in tcllib. "This is probably the most standard and widely-used of these packages."

Visual Basic

<lang vb>'the function Sub whatever(foo As Long, bar As Integer, baz As Byte, qux As String) 

   '...

End Sub 'calling the function -- note the Pascal-style assignment operator Sub crap() 

   whatever bar:=1, baz:=2, foo:=-1, qux:="Why is ev'rybody always pickin' on me?"

End Sub</lang>

XSLT

XSLT only allows specification of template parameters by name, not position. <lang xml><xsl:template name="table-header"> 

   <xsl:param name="title"/>
   ...

</xsl:template></lang>

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