Higher-order functions

Pass a function as an argument to another function.

ALGOL 68

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

<lang algol68>PROC first = (PROC(LONG REAL)LONG REAL f) LONG REAL: (

 f(1) + 2

);

PROC second = (LONG REAL x)LONG REAL: (

x/2

);

main: (

 printf(($xg(5,2)l$,first(second)))

)</lang> Output:

+2.50

The {} takes the pointer to an object (code/functions, variables and so on); Amiga E does not distinguish nor check anything, so it is up to the programmer to use the pointer properly. For this reason, a warning is always raised when a variable (func, holding a pointer to a real function in our case) is used like a function. <lang amigae>PROC compute(func, val)

 DEF s[10] : STRING
 WriteF('\s\n', RealF(s,func(val),4))

ENDPROC

PROC sin_wrap(val) IS Fsin(val) PROC cos_wrap(val) IS Fcos(val)

PROC main()

 compute({sin_wrap}, 0.0)
 compute({cos_wrap}, 3.1415)

ENDPROC</lang>

AppleScript

<lang applescript>-- This handler takes a script object (singer) -- with another handler (call). on sing about topic by singer call of singer for "Of " & topic & " I sing" end sing

-- Define a handler in a script object, -- then pass the script object. script cellos on call for what say what using "Cellos" end call end script sing about "functional programming" by cellos

-- Pass a different handler. This one is a closure -- that uses a variable (voice) from its context. on hire for voice script on call for what say what using voice end call end script end hire sing about "closures" by (hire for "Pipe Organ")</lang>

AutoHotkey

<lang AutoHotkey> f(x) { return x } g(x, y) { msgbox %x% msgbox %y% } g(f("RC Function as an Argument AHK implementation"), "Non-function argument") return </lang>

BBC BASIC

Works with: BBC BASIC for Windows

<lang bbcbasic> REM Test passing a function to a function:

     PRINT FNtwo(FNone(), 10, 11)
     END
     
     REM Function to be passed:
     DEF FNone(x, y) = (x + y) ^ 2
     
     REM Function taking a function as an argument:
     DEF FNtwo(RETURN f%, x, y) = FN(^f%)(x, y)</lang>

Output:

Bracmat

<lang bracmat>( (plus=a b.!arg:(?a.?b)&!a+!b) & ( print

 =   text a b func
   .   !arg:(?a.?b.(=?func).?text)
     & out$(str$(!text "(" !a "," !b ")=" func$(!a.!b)))
 )

& print$(3.7.'$plus.add) & print

 $ ( 3
   . 7
   . (=a b.!arg:(?a.?b)&!a*!b)
   . multiply
   )

);</lang> Output:

add(3,7)=10
multiply(3,7)=21

Brat

<lang brat>add = { a, b | a + b }

doit = { f, a, b | f a, b }

p doit ->add 1 2 #prints 3</lang>

Burlesque

Burlesque doesn't have functions in the usual sense. One can think of blocks in Burlesque as anonymous functions. The function "m[" (map) takes a block (a 'function') as it's argument. Add 5 to every element in a list (like map (+5) [1,2,3,4] in haskell):

C

Simple example

The pointer to the function to be passed as an argument is the only involved pointer.

Definition of a function whose only parameter is a pointer to a function with no parameters and no return value:

<lang c>void myFuncSimple( void (*funcParameter)(void) ) {

   /* ... */
  
   (*funcParameter)();  /* Call the passed function. */
   funcParameter();     /* Same as above with slight different syntax. */

   /* ... */

}</lang>

Note that you can't call the passed function by " *funcParameter() ", since that would mean "call funcParameter and than apply the * operator on the returned value".

Call:

<lang c>void funcToBePassed(void);

/* ... */

myFuncSimple(&funcToBePassed);</lang>

Complex example

Definition of a function whose return value is a pointer to int and whose only parameter is a pointer to a function, whose (in turn) return value is a pointer to double and whose only parameter is a pointer to long.

<lang c>int* myFuncComplex( double* (*funcParameter)(long* parameter) ) {

    long inLong;
    double* outDouble;
    long *inLong2 = &inLong;

    /* ... */

    outDouble = (*funcParameter)(&inLong);  /* Call the passed function and store returned pointer. */
    outDouble = funcParameter(inLong2);     /* Same as above with slight different syntax. */

    /* ... */

}</lang> Call:

<lang c>double* funcToBePassed(long* parameter);

/* ... */

int* outInt;

outInt = myFuncComplex(&funcToBePassed);</lang>

Pointer

Finally, declaration of a pointer variable of the proper type to hold such a function as myFunc:

<lang c>int* (*funcPointer)( double* (*funcParameter)(long* parameter) );

/* ... */

funcPointer = &myFuncComplex;</lang>

Of course, in a real project you shouldn't write such a convoluted code, but use some typedef instead, in order to break complexity into steps.

C++

Function Pointer

Works with: g++ version 3.4.2 (mingw-special)

C++ can pass function pointers in the same manner as C.

Function class template

Using the std::tr1::function class template allows more powerful usage. function<> can be used to pass around arbitrary function objects. This permits them to be used as closures.

For C++11 this is now std::function.

Works with: gcc version 4.4

<lang cpp> // Use <functional> for C++11

include <tr1/functional>
include <iostream>

using namespace std; using namespace std::tr1;

void first(function<void()> f) {

 f();

}

void second() {

 cout << "second\n";

}

int main() {

 first(second);

} </lang>

Template and Inheritance

Works with: Visual C++ version 2005

<lang cpp>#include <iostream>

include <functional>

template<class Func> typename Func::result_type first(Func func, typename Func::argument_type arg) {

 return func(arg);

}

class second : public std::unary_function<int, int> { public:

 result_type operator()(argument_type arg) const
 {
   return arg * arg;
 }

};

int main() {

 std::cout << first(second(), 2) << std::endl;
 return 0;

}</lang>

C#

Each example below does the same thing and has the same output:

f=Add, f(6, 2) = 8
f=Mul, f(6, 2) = 12
f=Div, f(6, 2) = 3

Delegates: Named methods

This works for all standard versions of C#.

<lang csharp>using System; delegate int Func2(int a, int b); class Program {

   static int Add(int a, int b)
   {
       return a + b;
   }
   
   static int Mul(int a, int b)
   {
       return a * b;
   }
   
   static int Div(int a, int b)
   {
       return a / b;
   }
   
   static int Call(Func2 f, int a, int b)
   {
       return f(a, b);
   }

   static void Main()
   {
       int a = 6;
       int b = 2;
       
       Func2 add = new Func2(Add);
       Func2 mul = new Func2(Mul);
       Func2 div = new Func2(Div);
       
       Console.WriteLine("f=Add, f({0}, {1}) = {2}", a, b, Call(add, a, b));
       Console.WriteLine("f=Mul, f({0}, {1}) = {2}", a, b, Call(mul, a, b));
       Console.WriteLine("f=Div, f({0}, {1}) = {2}", a, b, Call(div, a, b));
   }

}</lang>

Delegates: Anonymous functions

Anonymous functions added closures to C#.

Works with: C# version 2+

<lang csharp>using System; delegate int Func2(int a, int b); class Program {

   static int Call(Func2 f, int a, int b)
   {
       return f(a, b);
   }

   static void Main()
   {
       int a = 6;
       int b = 2;
       
       Console.WriteLine("f=Add, f({0}, {1}) = {2}", a, b, Call(delegate(int x, int y) { return x + y; }, a, b));
       Console.WriteLine("f=Mul, f({0}, {1}) = {2}", a, b, Call(delegate(int x, int y) { return x * y; }, a, b));
       Console.WriteLine("f=Div, f({0}, {1}) = {2}", a, b, Call(delegate(int x, int y) { return x / y; }, a, b));
   }

}</lang>

Lambdas

Lambda functions are syntactic sugar for anonymous functions. The System namespace also gained some common delegates, such as Func<T0, T1, T2>, which refers to a function that returns a value of type T2 and has two parameters of types T0 and T1.

Works with: C# version 3+

<lang csharp>using System; class Program {

   static int Call(Func<int, int, int> f, int a, int b)
   {
       return f(a, b);
   }

   static void Main()
   {
       int a = 6;
       int b = 2;
       
       Console.WriteLine("f=Add, f({0}, {1}) = {2}", a, b, Call((x, y) => x + y, a, b));
       Console.WriteLine("f=Mul, f({0}, {1}) = {2}", a, b, Call((x, y) => x * y, a, b));
       Console.WriteLine("f=Div, f({0}, {1}) = {2}", a, b, Call((x, y) => x / y, a, b));
   }

}</lang>

Clean

Take a function as an argument and apply it to all elements in a list: <lang clean>map f [x:xs] = [f x:map f xs] map f [] = []</lang> Pass a function as an argument: <lang clean>incr x = x + 1

Start = map incr [1..10]</lang> Do the same using a anonymous function: <lang clean>Start = map (\x -> x + 1) [1..10]</lang> Do the same using currying: <lang clean>Start = map ((+) 1) [1..10]</lang>

Clojure

<lang lisp> (defn append-hello [s]

 (str "Hello " s))

(defn modify-string [f s]

 (f s))

(println (modify-string append-hello "World!")) </lang>

CoffeeScript

Passing an anonymous function to built-in map/reduce functions:

<lang coffeescript>double = [1,2,3].map (x) -> x*2</lang>

Using a function stored in a variable:

<lang coffeescript>fn = -> return 8 sum = (a, b) -> a() + b() sum(fn, fn) # => 16 </lang>

List comprehension with a function argument:

<lang coffeescript>bowl = ["Cheese", "Tomato"]

smash = (ingredient) ->

   return "Smashed #{ingredient}"

contents = smash ingredient for ingredient in bowl

=> ["Smashed Cheese", "Smashed Tomato"]

</lang>

Nested function passing:

<lang coffeescript>double = (x) -> x*2 triple = (x) -> x*3 addOne = (x) -> x+1

addOne triple double 2 # same as addOne(triple(double(2)))</lang>

A function that returns a function that returns a function that returns a function that returns 2, immediately executed:

A function that takes a function that takes a function argument:

   2 + x(-> 5)

)((y) -> y()+3)

result: 10</lang>

Common Lisp

In Common Lisp, functions are first class objects, so you can pass function objects as arguments to other functions:

<lang lisp>CL-USER> (defun add (a b) (+ a b)) ADD CL-USER> (add 1 2) 3 CL-USER> (defun call-it (fn x y)

           (funcall fn x y))

CALL-IT CL-USER> (call-it #'add 1 2) 3</lang> The Common Lisp library makes extensive use of higher-order functions:

CL-USER> (funcall #'+ 1 2 3)
6
CL-USER> (apply #'+ (list 1 2 3))
6
CL-USER> (sort (string-downcase "Common Lisp will bend your mind!") #'string<)
"     !bcddeiiilllmmmnnnoooprsuwy"
CL-USER> (reduce #'/ '(1 2 3 4 5))
1/120
CL-USER> (mapcar #'(lambda (n) (expt 2 n)) '(0 1 2 3 4 5))
(1 2 4 8 16 32)
CL-USER>

D

<lang d>int hof(int a, int b, int delegate(int, int) f) {

   return f(a, b);

}

void main() {

   import std.stdio;
   writeln("Add: ", hof(2, 3, (a, b) => a + b));
   writeln("Multiply: ", hof(2, 3, (a, b) => a * b));

}</lang>

Output:

Add: 5
Multiply: 6

This longer and more systematic example shows D functions/delegates by passing each type of function/delegate to _test_ as argument. <lang d>import std.stdio;

// Test the function argument. string test(U)(string scopes, U func) {

   string typeStr = typeid(typeof(func)).toString();

   string isFunc = (typeStr[$ - 1] == '*') ? "function" : "delegate";
   writefln("Hi, %-13s : scope: %-8s (%s) : %s",
            func(), scopes, isFunc, typeStr );
   return scopes;

}

// Normal module level function. string aFunction() { return "Function"; }

// Implicit-Function-Template-Instantiation (IFTI) Function. T tmpFunc(T)() { return "IFTI.function"; }

// Member in a template. template tmpGroup(T) {

   T t0(){ return "Tmp.member.0"; }
   T t1(){ return "Tmp.member.1"; }
   T t2(){ return "Tmp.member.2"; }

}

// Used for implementing member function at class & struct. template Impl() {

   static string aStatic() { return "Static Method";  }
   string aMethod() { return "Method"; }

}

class C { mixin Impl!(); } struct S { mixin Impl!(); }

void main() {

   // Nested function.
   string aNested() {
       return "Nested";
   }

 // Bind to a variable.
 auto variableF = function string() { return "variable.F"; };
 auto variableD = delegate string() { return "variable.D"; };

 C c = new C;
 S s;

     "Global".test(&aFunction);
     "Nested".test(&aNested);
      "Class".test(&C.aStatic)
             .test(&c.aMethod);
     "Struct".test(&S.aStatic)
             .test(&s.aMethod);
   "Template".test(&tmpFunc!(string))
             .test(&tmpGroup!(string).t2);
    "Binding".test(variableF)
             .test(variableD);
   // Literal function/delegate.
    "Literal".test(function string() { return "literal.F"; })
             .test(delegate string() { return "literal.D"; });

}</lang>

Output:

}

Hi, Function      : scope: Global   (function) : immutable(char)[]()*
Hi, Nested        : scope: Nested   (delegate) : immutable(char)[] delegate()
Hi, Static Method : scope: Class    (function) : immutable(char)[]()*
Hi, Method        : scope: Class    (delegate) : immutable(char)[] delegate()
Hi, Static Method : scope: Struct   (function) : immutable(char)[]()*
Hi, Method        : scope: Struct   (delegate) : immutable(char)[] delegate()
Hi, IFTI.function : scope: Template (function) : immutable(char)[]()*
Hi, Tmp.member.2  : scope: Template (function) : immutable(char)[]()*
Hi, variable.F    : scope: Binding  (function) : immutable(char)[]()*
Hi, variable.D    : scope: Binding  (delegate) : immutable(char)[] delegate()
Hi, literal.F     : scope: Literal  (function) : immutable(char)[]()*
Hi, literal.D     : scope: Literal  (delegate) : immutable(char)[] delegate()

Delphi

See Pascal

Déjà Vu

<lang dejavu>map f lst:

   ]
   for item in lst:
       f item
   [

twice:

* 2

!. map @twice [ 1 2 5 ]</lang>

Output:

[ 2 4 10 ]

DWScript

<lang delphi>type TFnType = function(x : Float) : Float;

function First(f : TFnType) : Float; begin

  Result := f(1) + 2;

end;

function Second(f : Float) : Float; begin

  Result := f/2;

end;

PrintLn(First(Second));</lang>

E

<lang e>def map(f, list) {

 var out := []
 for x in list {
   out with= f(x)
 }
 return out

}

? map(fn x { x + x }, [1, "two"])

value: [2, "twotwo"]

? map(1.add, [5, 10, 20])

value: [6, 11, 21]

? def foo(x) { return -(x.size()) } > map(foo, ["", "a", "bc"])

value: [0, -1, -2]</lang>

ECL

<lang>//a Function prototype: INTEGER actionPrototype(INTEGER v1, INTEGER v2) := 0;

INTEGER aveValues(INTEGER v1, INTEGER v2) := (v1 + v2) DIV 2; INTEGER addValues(INTEGER v1, INTEGER v2) := v1 + v2; INTEGER multiValues(INTEGER v1, INTEGER v2) := v1 * v2;

//a Function prototype using a function prototype: INTEGER applyPrototype(INTEGER v1, actionPrototype actionFunc) := 0;

//using the Function prototype and a default value: INTEGER applyValue2(INTEGER v1,

                   actionPrototype actionFunc = aveValues) :=
                   actionFunc(v1, v1+1)*2;

//Defining the Function parameter inline, witha default value: INTEGER applyValue4(INTEGER v1,

                   INTEGER actionFunc(INTEGER v1,INTEGER v2) = aveValues)
              := actionFunc(v1, v1+1)*4;

INTEGER doApplyValue(INTEGER v1,

                    INTEGER actionFunc(INTEGER v1, INTEGER v2))
       := applyValue2(v1+1, actionFunc);

//producing simple results: OUTPUT(applyValue2(1)); // 2 OUTPUT(applyValue2(2)); // 4 OUTPUT(applyValue2(1, addValues)); // 6 OUTPUT(applyValue2(2, addValues)); // 10 OUTPUT(applyValue2(1, multiValues)); // 4 OUTPUT(applyValue2(2, multiValues)); // 12 OUTPUT(doApplyValue(1, multiValues)); // 12 OUTPUT(doApplyValue(2, multiValues)); // 24

//A definition taking function parameters which themselves //have parameters that are functions...

STRING doMany(INTEGER v1,

             INTEGER firstAction(INTEGER v1,
                                 INTEGER actionFunc(INTEGER v1,INTEGER v2)),
             INTEGER secondAction(INTEGER v1,
                                  INTEGER actionFunc(INTEGER v1,INTEGER v2)),
             INTEGER actionFunc(INTEGER v1,INTEGER v2)) 
      := (STRING)firstAction(v1, actionFunc) + ':' + (STRING)secondaction(v1, actionFunc);

OUTPUT(doMany(1, applyValue2, applyValue4, addValues));

    // produces "6:12"

OUTPUT(doMany(2, applyValue4, applyValue2,multiValues));

    // produces "24:12"</lang>

Efene

<lang efene>first = fn (F) {

F()

}

second = fn () {

 io.format("hello~n")

}

@public run = fn () {

   # passing the function specifying the name and arity
   # arity: the number of arguments it accepts
   first(fn second:0)

   first(fn () { io.format("hello~n") })

   # holding a reference to the function in a variable
   F1 = fn second:0
   F2 = fn () { io.format("hello~n") }

   first(F1)
   first(F2)

} </lang>

Erlang

Erlang functions are atoms, and they're considered different functions if their arity (the number of arguments they take) is different. As such, an Erlang function must be passed as fun Function/Arity, but can be used as any other variable: <lang erlang>-module(test). -export([first/1, second/0]).

first(F) -> F(). second() -> hello.</lang> Testing it: <lang erlang>1> c(tests). {ok, tests} 2> tests:first(fun tests:second/0). hello 3> tests:first(fun() -> anonymous_function end). anonymous_function</lang>

Euler Math Toolbox

<lang Euler Math Toolbox> >function f(x,a) := x^a-a^x >function dof (f$:string,x) := f$(x,args()); >dof("f",1:5;2)

[ -1  0  1  0  -7 ]

>plot2d("f",1,5;2): </lang>

Euphoria

<lang euphoria>procedure use(integer fi, integer a, integer b)

   print(1,call_func(fi,{a,b}))

end procedure

function add(integer a, integer b)

   return a + b

end function

use(routine_id("add"),23,45)</lang>

Factor

Using words (factor's functions) : <lang factor>USING: io ; IN: rosetacode

argument-function1 ( -- ) "Hello World!" print ;

argument-function2 ( -- ) "Goodbye World!" print ;

! normal words have to know the stack effect of the input parameters they execute

calling-function1 ( another-function -- ) execute( -- ) ;

! unlike normal words, inline words do not have to know the stack effect.

calling-function2 ( another-function -- ) execute ; inline

! Stack effect has to be written for runtime computed values :

calling-function3 ( bool -- ) \ argument-function1 \ argument-function2 ? execute( -- ) ;

</lang>

   ( scratchpad )
   \ argument-function1 calling-function1
   \ argument-function1 calling-function2
   t calling-function3
   f calling-function3

   Hello World!
   Hello World!
   Hello World!
   Goodbye World!

FALSE

Anonymous code blocks are the basis of FALSE control flow and function definition. These blocks may be passed on the stack as with any other parameter. <lang false>[f:[$0>][@@\f;!\1-]#%]r: { reduce n stack items using the given basis and binary function }

1 2 3 4 0 4[+]r;!." " { 10 } 1 2 3 4 1 4[*]r;!." " { 24 } 1 2 3 4 0 4[$*+]r;!. { 30 }</lang>

Fantom

<lang fantom> class Main {

 // apply given function to two arguments
 static Int performOp (Int arg1, Int arg2, |Int, Int -> Int| fn)
 {
   fn (arg1, arg2)
 }

 public static Void main ()
 {
   echo (performOp (2, 5, |Int a, Int b -> Int| { a + b }))
   echo (performOp (2, 5, |Int a, Int b -> Int| { a * b }))
 }

} </lang>

Forth

Forth words can be referenced on the stack via their execution token or XT. An XT is obtained from a word via the quote operator, and invoked via EXECUTE. Anonymous functions may be defined via :NONAME (returning an XT) instead of a standard colon definition.

<lang forth>: square dup * ;

cube dup dup * * ;

map. ( xt addr len -- )

 0 do  2dup i cells + @ swap execute .  loop 2drop ;

create array 1 , 2 , 3 , 4 , 5 , ' square array 5 map. cr \ 1 4 9 16 25 ' cube array 5 map. cr \ 1 8 27 64 125

noname 2* 1+ ; array 5 map. cr \ 3 5 7 9 11</lang>

Fortran

Works with: Fortran version 90 and later

use the EXTERNAL attribute to show the dummy argument is another function rather than a data object. i.e. <lang fortran>FUNCTION FUNC3(FUNC1, FUNC2, x, y)

 REAL, EXTERNAL :: FUNC1, FUNC2 
 REAL :: FUNC3
 REAL :: x, y

 FUNC3 = FUNC1(x) * FUNC2(y)

END FUNCTION FUNC3</lang>

Another way is to put the functions you want to pass in a module:

<lang fortran>module FuncContainer

 implicit none

contains

 function func1(x)
   real :: func1
   real, intent(in) :: x

   func1 = x**2.0
 end function func1

 function func2(x)
   real :: func2
   real, intent(in) :: x

   func2 = x**2.05
 end function func2

end module FuncContainer

program FuncArg

 use FuncContainer
 implicit none

 print *, "Func1"
 call asubroutine(func1)

 print *, "Func2"
 call asubroutine(func2)

contains

 subroutine asubroutine(f)
   ! the following interface is redundant: can be omitted
   interface
      function f(x)
        real, intent(in) :: x
        real :: f
      end function f
   end interface
   real :: px

   px = 0.0
   do while( px < 10.0 )
      print *, px, f(px)
      px = px + 1.0
   end do
 end subroutine asubroutine

end program FuncArg</lang>

F#

We define a function that takes another function f as an argument and applies that function twice to the argument x: <lang fsharp>> let twice f x = f (f x);;

val twice : ('a -> 'a) -> 'a -> 'a

> twice System.Math.Sqrt 81.0;; val it : float = 3.0</lang>

Another example, using an operator as a function: <lang fsharp>> List.map2 (+) [1;2;3] [3;2;1];; val it : int list = [4; 4; 4]</lang>

GAP

<lang gap>Eval := function(f, x)

 return f(x);

end;

Eval(x -> x^3, 7);

343</lang>

Go

<lang go>package main import "fmt"

func func1(f func(string) string) string { return f("a string") } func func2(s string) string { return "func2 called with " + s } func main() { fmt.Println(func1(func2)) }</lang>

Groovy

As closures: <lang groovy>first = { func -> func() } second = { println "second" }

first(second)</lang>

As functions: <lang groovy>def first(func) { func() } def second() { println "second" }

first(this.&second)</lang>

Haskell

Works with: GHCi version 6.6

A function is just a value that wants arguments: <lang haskell>func1 f = f "a string" func2 s = "func2 called with " ++ s

main = putStrLn $ func1 func2</lang> Or, with an anonymous function: <lang haskell>func f = f 1 2

main = print $ func (\x y -> x+y) -- output: 3</lang> Note that func (\x y -> x+y) is equivalent to func (+). (Operators are functions too.)

Icon and Unicon

<lang icon> procedure main()

  local lst
  lst := [10, 20, 30, 40]
  myfun(callback, lst)

end

procedure myfun(fun, lst)

  every fun(!lst)

end

procedure callback(arg)

  write("->", arg)

end</lang>

Inform 6

As in C, functions in Inform 6 are not first-class, but pointers to functions can be used. <lang Inform6>[ func;

 print "Hello^";

];

[ call_func x;

 x();

];

[ Main;

 call_func(func);

];</lang>

Inform 7

Phrases usually aren't defined with names, only with invocation syntax. A phrase must be given a name (here, "addition" and "multiplication") in order to be passed as a phrase value. <lang inform7>Higher Order Functions is a room.

To decide which number is (N - number) added to (M - number) (this is addition): decide on N + M.

To decide which number is multiply (N - number) by (M - number) (this is multiplication): decide on N * M.

To demonstrate (P - phrase (number, number) -> number) as (title - text): say "[title]: [P applied to 12 and 34]."

When play begins: demonstrate addition as "Add"; demonstrate multiplication as "Mul"; end the story.</lang>

J

Adverbs take a single verb or noun argument and conjunctions take two. For example, / (insert) \ (prefix) and \. (suffix) are adverbs and ^: (power) is a conjunction. The following expressions illustrate their workings.

<lang j> + / 3 1 4 1 5 9 NB. sum 23

  >./ 3 1 4 1 5 9   NB. max

9

  *./ 3 1 4 1 5 9   NB. lcm

180

  +/\ 3 1 4 1 5 9   NB. sum prefix (partial sums)

3 4 8 9 14 23

  +/\. 3 1 4 1 5 9  NB. sum suffix

23 20 19 15 14 9

  f=: -:@(+ 2&%)    NB. one Newton iteration
  f 1

1.5

  f f 1

1.41667

  f^:(i.5) 1        NB. first 5 Newton iterations

1 1.5 1.41667 1.41422 1.41421

  f^:(i.5) 1x       NB. rational approximations to sqrt 2

1 3r2 17r12 577r408 665857r470832</lang>

Adverbs and conjunctions may also be user defined

<lang J> + conjunction def 'u' - +

  + conjunction def 'v' -

-

  * adverb def '10 u y' 11

110

  ^ conjunction def '10 v 2 u y' * 11

20480</lang>

Java

There is no real callback in Java like in C or C++, but we can do the same as swing does for executing an event. We need to create an interface that has the method we want to call or create one that will call the method we want to call. The following example uses the second way.

<lang java>public class NewClass {

  public NewClass() {
      first(new AnEventOrCallback() {
          public void call() {
              second();
          }
      });
  }
  
  public void first(AnEventOrCallback obj) {
      obj.call();
  }
  
  public void second() {
      System.out.println("Second");
  }
  
  public static void main(String[] args) {
      new NewClass();
  }

}

interface AnEventOrCallback {

  public void call();

}</lang>

JavaScript

<lang javascript>function first (func) {

 return func();

}

function second () {

 return "second";

}

var result = first(second); result = first(function () { return "third"; });</lang>

An example with anonymous functions and uses in the core library

Works with: Firefox version 1.5

for methods filter and map.

<lang javascript>>>> var array = [2, 4, 5, 13, 18, 24, 34, 97]; >>> array [2, 4, 5, 13, 18, 24, 34, 97]

// return all elements less than 10 >>> array.filter(function (x) { return x < 10 }); [2, 4, 5]

// return all elements less than 30 >>> array.filter(function (x) { return x < 30 }); [2, 4, 5, 13, 18, 24]

// return all elements less than 100 >>> array.filter(function (x) { return x < 100 }); [2, 4, 5, 13, 18, 24, 34, 97]

// multiply each element by 2 and return the new array >>> array.map(function (x) { return x * 2 }); [4, 8, 10, 26, 36, 48, 68, 194]

// sort the array from smallest to largest >>> array.sort(function (a, b) { return a > b }); [2, 4, 5, 13, 18, 24, 34, 97]

// sort the array from largest to smallest >>> array.sort(function (a, b) { return a < b }); [97, 34, 24, 18, 13, 5, 4, 2]</lang>

Joy

This example is taken from V. Define first as multiplying two numbers on the stack. <lang joy>DEFINE first == *.</lang> There will be a warning about overwriting builtin first. Define second as interpreting the passed quotation on the stack. <lang joy>DEFINE second == i.</lang> Pass first enclosed in quotes to second which applies it on the stack. <lang joy>2 3 [first] second.</lang> The program prints 6.

jq

The examples given in this section closely follow the exposition in the Julia section of this page.

To understand these examples, it is helpful to remember that:

jq functions are filters that can participate in a left-to-right pipeline, just as in most modern command shells;

"." on the right of a pipe ("|") refers to the output from the filter on the left.

Example 1: "hello blue world"

<lang jq>def foo( filter ):

 ("world" | filter) as $str
 | "hello \($str)" ;

blue is defined here as a filter that adds blue to its input:

def blue: "blue \(.)";

foo( blue ) # prints "hello blue world" </lang>

Example 2: g(add; 2; 3)

<lang jq> def g(f; x; y): [x,y] | f;

g(add; 2; 3) # => 5</lang>

Example: Built-in higher-order functions

In the following sequence of interactions, we pass the function *is_even/0* to some built-in higher order functions. *is_even/0* is defined as follows: <lang jq>def is_even:

 if floor == . then (. % 2) == 0 
 else error("is_even expects its input to be an integer")
 end;</lang><lang jq>

Are all integers between 1 and 5 even?
For this example, we will use all/2 even
though it requires a release of jq after jq 1.4;
we do so to highlight the fact that all/2
terminates the generator once the condition is satisfied:

all( range(1;6); is_even ) false

Display the even integers in the given range:

range(1;6) | select(is_even) 2 4

Evaluate is_even for each integer in an array

[range(1;6)] | map(is_even) [false, true, false, true, false]

Note that in jq, there is actually no need to call
a higher-order function in cases like this.
For example one can simply write:

range(1;6) | is_even false true false true false</lang>

Julia

<lang Julia> function foo(x)

 str = x("world")
 println("hello $(str)!")

end foo(y -> "blue $y") # prints "hello blue world" </lang>

The above code snippet defines a named function, foo, which takes a single argument, which is a Function. foo calls this function on the string literal "world", and then interpolates the result into the "hello ___!" string literal, and prints it. In the final line, foo is called with an anonymous function that takes a string, and returns a that string with "blue " preppended to it.

<lang Julia> function g(x,y,z)

 x(y,z)

end println(g(+,2,3)) # prints 5 </lang>

This code snippet defines a named function g that takes three arguments: x is a function to call, and y and z are the values to call x on. We then call g on the + function. Operators in Julia are just special names for functions.

In the following interactive session, we pass the function iseven to a few higher order functions. The function iseven returns true if its argument is an even integer, false if it is an odd integer, and throws an error otherwise. The second argument to the functions is a range of integers, specifically the five integers between 1 and 5 included. <lang julia>julia> all(iseven, 1:5) # not all integers between 1 and 5 are even. false

julia> findfirst(iseven, 1:5) # the first even integer is at index 2 in the range. 2

julia> count(iseven, 1:5) # there are two even integers between 1 and 5. 2

julia> filter(iseven, 1:5) # here are the even integers in the given range. 2-element Array{Int64,1}:

2
4

julia> map(iseven, 1:5) # we apply our function to all integers in range. 5-element Array{Bool,1}:

false
 true
false
 true
false

</lang>

Logo

You can pass the quoted symbol for the function and invoke it with RUN. <lang logo>to printstuff

 print "stuff

end to runstuff :proc

 run :proc

end runstuff "printstuff ; stuff runstuff [print [also stuff]] ; also stuff</lang>

Lua

Lua functions are first-class: <lang lua>a = function() return 1 end b = function(r) print( r() ) end b(a)</lang>

Mathematica

Passing 3 arguments and a value (could be a number, variable, graphic or a function as well, actually it could be anything), and composing them in an unusual way: <lang Mathematica>PassFunc[f_, g_, h_, x_] := f[g[x]*h[x]] PassFunc[Tan, Cos, Sin, x] % /. x -> 0.12 PassFunc[Tan, Cos, Sin, 0.12]</lang> gives back: <lang Mathematica>Tan[Cos[x] Sin[x]] 0.119414 0.119414</lang>

MATLAB / Octave

<lang MATLAB> F1=@sin; % F1 refers to function sin()

  F2=@cos;	% F2 refers to function cos()

  % varios ways to call the refered function 	
  F1(pi/4)
  F2(pi/4)
  feval(@sin,pi/4)
  feval(@cos,pi/4)
  feval(F1,pi/4)
  feval(F2,pi/4)

  % named functions, stored as strings	
  feval('sin',pi/4)
  feval('cos',pi/4)
  F3 = 'sin';
  F4 = 'cos';
  feval(F3,pi/4)
  feval(F4,pi/4)</lang>

Maxima

<lang maxima>callee(n) := (print(sconcat("called with ", n)), n + 1)$ caller(f, n) := sum(f(i), i, 1, n)$ caller(callee, 3); "called with 1" "called with 2" "called with 3"</lang>

MAXScript

<lang maxscript>fn second = (

   print "Second"

)

fn first func = (

   func()

)

first second</lang>

Metafont

We can simulate this by using scantokens, which digests a string as if it would be a source input.

<lang metafont>def calcit(expr v, s) = scantokens(s & decimal v) enddef;

t := calcit(100.4, "sind"); show t; end</lang>

МК-61/52

Note: as the receiver of argument used register Р7; the result is "1" on the indicator.

Modula-3

<lang modula3>MODULE Proc EXPORTS Main;

IMPORT IO;

TYPE Proc = PROCEDURE();

PROCEDURE Second() =

 BEGIN
   IO.Put("Second procedure.\n");
 END Second;

PROCEDURE First(proc: Proc) =

 BEGIN
   proc();
 END First;

BEGIN

 First(Second);

END Proc.</lang>

Nemerle

Functions must declare the types of their parameters in Nemerle. Function types in Nemerle are written params type -> return type, as seen in the simple example below. <lang Nemerle>Twice[T] (f : T -> T, x : T) : T { f(f(x)) }</lang>

NewLISP

<lang NewLISP>> (define (my-multiply a b) (* a b)) (lambda (a b) (* a b)) > (define (call-it f x y) (f x y)) (lambda (f x y) (f x y)) > (call-it my-multiply 2 3) 6 </lang>

Nimrod

<lang nimrod>proc first(fn): auto =

 return fn()

proc second(): string =

 return "second"

echo first(second)</lang>

Objeck

<lang objeck> bundle Default {

 class HighOrder {
   function : Main(args : String[]) ~ Nil {
     f := GetSize(String) ~ Int;
     Print(f);
   }

   function : GetSize(s : String) ~ Int {
     return s->Size();
   }

   function : Print(func : (String)~Int) ~ Nil {
     func("Hello World!")->PrintLine();
   }
 }

} </lang>

OCaml

A function is just a value that wants arguments: <lang ocaml># let func1 f = f "a string";; val func1 : (string -> 'a) -> 'a = <fun>

let func2 s = "func2 called with " ^ s;;

val func2 : string -> string = <fun>

print_endline (func1 func2);;

func2 called with a string - : unit = ()</lang>

Or, with an anonymous function: <lang ocaml># let func f = f 1 2;; val func : (int -> int -> 'a) -> 'a = <fun>

Printf.printf "%d\n" (func (fun x y -> x + y));;

3 - : unit = ()</lang> Note that func (fun x y -> x + y) is equivalent to func (+). (Operators are functions too.)

Octave

We can pass a function handle (@function_name)

<lang octave>function r = computeit(f, g, v)

 r = f(g(v));

endfunction

computeit(@exp, @sin, pi/3) computeit(@log, @cos, pi/6)</lang>

Or pass the string name of the function and use the feval primitive.

<lang octave>function r = computeit2(f, g, v)

 r = f(feval(g, v));

endfunction

computeit2(@exp, "sin", pi/3)</lang>

ooRexx

routines are first class ooRexx objects that can be passed to other routines or methods and invoked. <lang ooRexx> say callit(.routines~fib, 10) say callit(.routines~fact, 6) say callit(.routines~square, 13) say callit(.routines~cube, 3) say callit(.routines~reverse, 721) say callit(.routines~sumit, 1, 2) say callit(.routines~sumit, 2, 4, 6, 8)

-- call the provided routine object with the provided variable number of arguments

routine callit

 use arg function
 args = arg(2, 'a')   -- get all arguments after the first to pass along
 return function~callWith(args)  -- and pass along the call

routine cube

 use arg n
 return n**3

routine square

 use arg n
 return n**2

routine reverse

 use arg n
 return reverse(n)

routine fact

  use arg n
  accum = 1
  loop j = 2 to n
    accum = accum * j
  end
  return accum

routine sumit

 use arg n
 accum = 0
 do i over arg(1, 'a')  -- iterate over the array of args
    accum += i
 end
 return accum

routine fib

 use arg n
 if n == 0 then
    return n
 if n == 1 then
    return n
 last = 0
 next = 1
 loop j = 2 to n;
   current = last + next
   last = next
   next = current
 end
 return current

</lang>

Order

Functions in Order can accept any other named function, local variable, or anonymous function as arguments:

include <order/interpreter.h>

define ORDER_PP_DEF_8func1 ORDER_PP_FN ( \

8fn(8F, \

   8ap(8F, 8("a string")) ))

define ORDER_PP_DEF_8func2 ORDER_PP_FN ( \

8fn(8S, \

   8adjoin(8("func2 called with "), 8S ) ))

ORDER_PP(

 8func1(8func2)

) // -> "func2 called with ""a string"

define ORDER_PP_DEF_8func3 ORDER_PP_FN ( \

8fn(8F, \

   8ap(8F, 1, 2) ))

ORDER_PP(

 8func3(8plus)

) // -> 3

ORDER_PP(

 8ap( 8fn(8X, 8Y, 8mul(8add(8X, 8Y), 8sub(8X, 8Y))), 5, 3)

) // -> 16 </lang>

The only difference between toplevel function definitions, and variables or literals, is that variables and anonymous functions must be called using the 8ap syntactic form rather than direct argument application syntax. This is a limitation of the C preprocessor.

OxygenBasic

<lang oxygenbasic> 'FUNCTION TO BE PASSED '=====================

function f(double d,e) as double

 return (d+e)*2

end function

'FUNCTION TAKING A FUNCTION AS AN ARGUMENT '=========================================

function g(sys p) as string

 declare function x(double d,e) as double at p

 return x(10,11)

end function

'TEST: PASSING ADDRESS OF FUNCTION f '===================================

'the name 'f' is combined with the prototype signature '#double#double' '@' signifies the address of the function is being passed

print g(@f#double#double) 'result '42'

</lang>

Oz

Functions are just regular values in Oz. <lang oz>declare

 fun {Twice Function X}
    {Function {Function X}}
 end

in

 {Show {Twice Sqrt 81.0}}  %% prints 3.0</lang>

PARI/GP

Works with: PARI/GP version 2.4.2 and above

<lang parigp>secant_root(ff,a,b)={ e = eps() * 2; aval=ff(a); bval=ff(b); while (abs(bval) > e, oldb = b; b = b - (b - a)/(bval - aval) * bval; aval = bval; bval = ff(b); a = oldb ); b }; addhelp(secant_root, "secant_root(ff,a,b): Finds a root of ff between a and b using the secant method.");

eps()={ precision(2. >> (32 * ceil(default(realprecision) * 38539962 / 371253907)), 9) }; addhelp(eps,"Returns machine epsilon for the current precision.");</lang>

Pascal

Standard Pascal (will not work with Turbo Pascal): <lang pascal>program example(output);

function first(function f(x: real): real): real;

begin
 first := f(1.0) + 2.0;
end;

function second(x: real): real;

begin
 second := x/2.0;
end;

begin

writeln(first(second));

end.</lang>

Turbo Pascal (will not work with Standard Pascal):

<lang pascal>program example;

type

  FnType = function(x: real): real;

function first(f: FnType): real; begin

  first := f(1.0) + 2.0;

end;

{$F+} function second(x: real): real; begin

  second := x/2.0;

end; {$F-}

begin

  writeln(first(second));

end.</lang>

Perl

<lang perl>sub another {

   # take a function and a value
   my $func = shift;
   my $val  = shift;

   # call the function with the value as argument
   return $func->($val);

};

sub reverser {

   return scalar reverse shift;

};

pass named coderef

print another \&reverser, 'data';

pass anonymous coderef

print another sub {return scalar reverse shift}, 'data';

if all you have is a string and you want to act on that,
set up a dispatch table

my %dispatch = (

   square => sub {return shift() ** 2},
   cube   => sub {return shift() ** 3},
   rev    => \&reverser,

);

print another $dispatch{$_}, 123 for qw(square cube rev);</lang>

<lang perl>sub apply (&@) { # use & as the first item in a prototype to take bare blocks like map and grep

   my ($sub, @ret) = @_;   # this function applies a function that is expected to modify $_ to a list
   $sub->() for @ret;      # it allows for simple inline application of the s/// and tr/// constructs
   @ret

}

print join ", " => apply {tr/aeiou/AEIOU/} qw/one two three four/;

OnE, twO, thrEE, fOUr</lang>

<lang perl>sub first {shift->()}

sub second {'second'}

print first \&second;

print first sub{'sub'};</lang>

Perl 6

The best type to use for the parameter of a higher-order function is Callable (implied by the & sigil), a role common to all function-like objects. For an example of defining and calling a second-order function, see Functional Composition.

Convenient syntax is provided for anonymous functions, either a bare block, or a parameterized block introduced with ->, which serves as a "lambda":

<lang Perl 6>sub twice(&todo) {

  todo(); todo(); # declaring &todo also defines bare function

} twice { say "Boing!" }

output:
Boing!
Boing!

sub twice-with-param(&todo) {

   todo(0); todo(1);

} twice-with-param -> $time {

  say "{$time+1}: Hello!"

}

output:
1: Hello!
2: Hello!</lang>

PHP

<lang php>function first($func) {

 return $func();

}

function second() {

 return 'second';

}

$result = first('second');</lang> Or, with an anonymous function in PHP 5.3+: <lang php>function first($func) {

 return $func();

}

$result = first(function() { return 'second'; });</lang>

PicoLisp

<lang PicoLisp>: (de first (Fun)

  (Fun) )

-> first

(de second ()

  "second" )

-> second

(first second)

-> "second"

(de add (A B)

  (+ A B) )

-> add

(add 1 2)

-> 3

(de call-it (Fun X Y)

  (Fun X Y) )

-> call-it

(call-it add 1 2)

-> 3

(mapcar inc (1 2 3 4 5))

-> (2 3 4 5 6)

(mapcar + (1 2 3) (4 5 6))

-> (5 7 9)

(mapcar add (1 2 3) (4 5 6))

-> (5 7 9)</lang>

PL/I

<lang PL/I> f: procedure (g) returns (float);

  declare g entry (float);

  get (x);
  put (g(x));

end f;

  x = f(p); /* where "p" is the name of a function. */

</lang>

Pop11

<lang pop11>;;; Define a function define x_times_three_minus_1(x);

 return(3*x-1);

enddefine;

Pass it as argument to built-in function map and print the result

mapdata({0 1 2 3 4}, x_times_three_minus_1) =></lang>

PostScript

Library: initlib

<lang> /x_times_3_sub_1 {3 * 1 sub}. [0 1 2 3 4] {x_times_3_sub_1} map </lang>

Prolog

Works with: SWI Prolog

<lang prolog> first(Predicate):-Predicate. second(Argument):-print(Argument).

-first(second('Hello World!')).

</lang>

PureBasic

<lang PureBasic>Prototype.d func(*text$)

Procedure NumberTwo(arg$)

 Debug arg$

EndProcedure

Procedure NumberOne(*p, text$)

 Define MyFunc.func=*p
 MyFunc(@text$)

EndProcedure

NumberOne(@NumberTwo(),"Hello Worldy!")</lang>

Python

Works with: Python version 2.5

<lang python>def first(function):

   return function()

def second():

   return "second"

result = first(second)</lang>

or

<lang python> result = first(lambda: "second")</lang>

Functions are first class objects in Python. They can be bound to names ("assigned" to "variables"), associated with keys in dictionaries, and passed around like any other object.

R

<lang R>f <- function(f0) f0(pi) # calc. the function in pi tf <- function(x) x^pi # a func. just to test

print(f(sin)) print(f(cos)) print(f(tf))</lang>

Racket

 #lang racket/base
 (define (add f g x)
   (+ (f x) (g x)))
 (add sin cos 10)

</lang>

Raven

This is not strictly passing a function, but the string representing the function name. <lang Raven>define doit use $v1

  "doit called with " print $v1 print "\n" print

define callit use $v2

  "callit called with " print $v2 print "\n" print
  $v2 call

23.54 "doit" callit</lang>

Output:

callit called with doit
doit called with 23.54

REBOL

<lang REBOL>REBOL [ Title: "Function Argument" Author: oofoe Date: 2009-12-19 URL: http://rosettacode.org/wiki/Function_as_an_Argument ]

map: func [ "Apply function to contents of list, return new list." f [function!] "Function to apply to list." data [block! list!] "List to transform." /local result i ][ result: copy [] repeat i data [append result f i] result]

square: func [ "Calculate x^2." x [number!] ][x * x]

cube: func [ "Calculate x^3." x [number!] ][x * x * x]

Testing

x: [1 2 3 4 5] print ["Data: " mold x] print ["Squared:" mold map :square x] print ["Cubed: " mold map :cube x] print ["Unnamed:" mold map func [i][i * 2 + 1] x]</lang>

Output:

Data:    [1 2 3 4 5]
Squared: [1 4 9 16 25]
Cubed:   [1 8 27 64 125]
Unnamed: [3 5 7 9 11]

Retro

<lang Retro>: disp ( nq- )

 do putn ;

31 [ ( n-n ) 100 * ] disp </lang>

REXX

<lang rexx>/*REXX program demonstrates passing a function as a name to a function.*/ n=3735928559 funcName='fib' ; q= 10; call someFunk funcName, q; call tell funcName='fact' ; q= 6; call someFunk funcName, q; call tell funcName='square' ; q= 13; call someFunk funcName, q; call tell funcName='cube' ; q= 3; call someFunk funcName, q; call tell

                    q=721;  call someFunk 'reverse',q;  call tell

say copies('─',30) /*display a nice separator fence.*/ say 'done as' d2x(n)"." /*prove that var N still intact. */ exit /*stick a fork in it, we're done.*/

/*──────────────────────────────────subroutines─────────────────────────*/ cube: return n**3 fact: !=1; do j=2 to n; !=!*j; end; return ! reverse: return 'REVERSE'(n) someFunk: procedure; arg ?,n; signal value (?); say result 'result'; return square: return n**2 tell: say right(funcName'('q") = ",20) result; return

fib: if n==0 | n==1 then return n; _=0; a=0; b=1

                  do j=2 to n;   _=a+b;   a=b;   b=_;   end;      return _</lang>

output

          fib(10) =  55
          fact(6) =  720
       square(13) =  169
          cube(3) =  27
     reverse(721) =  127
──────────────────────────────
done as DEADBEEF.

Ruby

With a proc (procedure): <lang ruby>succ = proc{|x| x+1} def to2(&f)

 f[2]

end

to2(&succ) #=> 3 to2{|x| x+1} #=> 3</lang>

With a method: <lang ruby>def succ(n)

n+1

end def to2(m)

 m[2]

end

meth = method(:succ) to2(meth) #=> 3</lang>

Scala

<lang scala>def functionWithAFunctionArgument(x : int, y : int, f : (int, int) => int) = f(x,y)</lang> Call: <lang scala>functionWithAFunctionArgument(3, 5, {(x, y) => x + y}) // returns 8</lang>

Scheme

A function is just a value that wants arguments: <lang scheme>> (define (func1 f) (f "a string")) > (define (func2 s) (string-append "func2 called with " s)) > (begin (display (func1 func2)) (newline)) func2 called with a string</lang>

Or, with an anonymous function: <lang scheme>> (define (func f) (f 1 2)) > (begin (display (func (lambda (x y) (+ x y)))) (newline)) 3</lang> Note that (func (lambda (x y) (+ x y))) is equivalent to (func +). (Operators are functions too.)

Sidef

<lang ruby>func first(f) {

 return f.call();

}

func second {

 return "second";

}

say first(second); # => "second" say first(func { "third" }); # => "third"</lang>

Slate

Methods and blocks can both be passed as arguments to functions (other methods and blocks): <lang slate>define: #function -> [| :x | x * 3 - 1].

(1 1 2 3 5 8) collect: function.</lang>

Smalltalk

<lang Smalltalk>first := [ :f | f value ]. second := [ 'second' ]. Transcript show: (first value: second).</lang> <lang Smalltalk>function := [:x | x * 3 - 1].

(1 1 2 3 5 8) collect: function.</lang>

Sparkling

<lang sparkling>function call_me(func, arg) {

   return func(arg);

}

let answer = call_me(function(x) { return 6 * x; }, 7); print(answer);</lang>

Standard ML

<lang sml>- fun func1 f = f "a string"; val func1 = fn : (string -> 'a) -> 'a - fun func2 s = "func2 called with " ^ s; val func2 = fn : string -> string

- print (func1 func2 ^ "\n"); func2 called with a string val it = () : unit</lang>

Or, with an anonymous function: <lang sml>- fun func f = f (1, 2); val func = fn : (int * int -> 'a) -> 'a

- print (Int.toString (func (fn (x, y) => x + y)) ^ "\n"); 3 val it = () : unit</lang> Note that func (fn (x, y) => x + y) is equivalent to func op+. (Operators are functions too.)

Swift

<lang swift>func func1(f: String->String) -> String { return f("a string") } func func2(s: String) -> String { return "func2 called with " + s } println(func1(func2)) // prints "func2 called with a string"</lang>

Or, with an anonymous function: <lang swift>func func3<T>(f: (Int,Int)->T) -> T { return f(1, 2) } println(func3 {(x, y) in x + y}) // prints "3"</lang> Note that {(x, y) in x + y} can also be written as {$0 + $1} or just +.

Tcl

<lang tcl># this procedure executes its argument: proc demo {function} {

   $function

}

for example:

demo bell</lang> It is more common to pass not just a function, but a command fragment or entire script. When used with the built-in list command (which introduces a very useful degree of quoting) this makes for a very common set of techniques when doing advanced Tcl programming. <lang tcl># This procedure executes its argument with an extra argument of "2" proc demoFrag {fragment} {

   {*}$fragment 2

}

This procedure executes its argument in the context of its caller, which is
useful for scripts so they get the right variable resolution context

proc demoScript {script} {

   uplevel 1 $script

}

Examples...

set chan stderr demoFrag [list puts $chan] demoFrag {

   apply {x {puts [string repeat ? $x]}}

} demoScript {

   parray tcl_platform

}</lang>

TI-89 BASIC

TI-89 BASIC does not have first-class functions; while function definitions as stored in variables are fully dynamic, it is not possible to extract a function value from a variable rather than calling it. In this case, we use the indirection operator #, which takes a string and returns the value of the named variable, to use the name of the function as something to be passed.

The function name passed cannot be that of a local function, because the local function map does not see the local variables of the enclosing function.

<lang ti89b>Local map Define map(f,l)=Func

 Return seq(#f(l[i]),i,1,dim(l))

EndFunc Disp map("sin", {0, π/6, π/4, π/3, π/2})</lang>

Toka

Toka allows obtaining a function pointer via the ` (backtick) word. The pointers are passed on the stack, just like all other data.

<lang toka>[ ." First\n" ] is first [ invoke ] is second ` first second</lang>

Trith

Due to the homoiconic program representation and the concatenative nature of the language, higher-order functions are as simple as: <lang trith>: twice 2 times ;

hello "Hello, world!" print ;

[hello] twice</lang>

TXR

lambda passed to mapcar with environment capture:

<lang txr>@(bind a @(let ((counter 0))

           (mapcar (lambda (x y) (list (inc counter) x y))
                   '(a b c) '(t r s))))

@(output) @ (repeat) @ (rep)@a:@(last)@a@(end) @ (end) @(end)</lang>

1:a:t
2:b:r
3:c:s

Ursala

Autocomposition is a user defined function that takes a function as an argument, and returns a function equivalent to the given functon composed with itself.

<lang Ursala>(autocomposition "f") "x" = "f" "f" "x"</lang> test program: <lang Ursala>#import flo

cast %e

example = autocomposition(sqrt) 16.0</lang> output:

2.000000e+00

V

Define first as multiplying two numbers on stack <lang v>[first *].</lang> Define second as applying the passed quote on stack <lang v>[second i].</lang> Pass the first enclosed in quote to second which applies it on stack. <lang v>2 3 [first] second</lang>

=6

Visual Prolog

<lang Prolog> domains

 intFunction = (integer In) -> integer Out procedure (i).

class predicates

 addone : intFunction.
 doTwice : (intFunction, integer) -> integer procedure (i, i).

clauses

 doTwice(Pred,X) = Y :- Y = Pred(Pred(X)).

 addone(X) = Y := Y = X + 1.

 run():-
   init(),
   write(dotwice(addone,2)),
   succeed().

</lang>

zkl

Everything is a first class object so <lang zkl>fcn f(g){g()} fcn g{"Hello World!".println()}</lang>

Output:

f(g)
"Hello World!"

or <lang zkl>fcn f(g){g()} f( fcn{"Hello World!".println()} )</lang>

Simple Example

Complex Example

Function Pointer

Function class template

Template and Inheritance

Delegates: Named methods

Delegates: Anonymous functions

Lambdas

Icon and Unicon

Example 1: "hello blue world"

Example 2: g(add; 2; 3)

Example: Built-in higher-order functions

MATLAB / Octave