Higher-order functions
You are encouraged to solve this task according to the task description, using any language you may know.
Pass a function as an argument to another function.
ActionScript
<lang actionscript>package {
public class MyClass {
public function first(func:Function):String {
return func.call();
}
public function second():String {
return "second";
}
public static function main():void {
var result:String = first(second);
trace(result);
result = first(function() { return "third"; });
trace(result);
}
}
}</lang>
Ada
Simple Example
<lang ada>with Ada.Text_Io; use Ada.Text_Io;
procedure Subprogram_As_Argument is
type Proc_Access is access procedure;
procedure Second is
begin
Put_Line("Second Procedure");
end Second;
procedure First(Proc : Proc_Access) is
begin
Proc.all;
end First;
begin
First(Second'Access);
end Subprogram_As_Argument;</lang>
Complex Example
<lang ada>with Ada.Text_Io; use Ada.Text_Io;
procedure Subprogram_As_Argument_2 is
-- Definition of an access to long_float
type Lf_Access is access Long_Float; -- Definition of a function returning Lf_Access taking an -- integer as a parameter
function Func_To_Be_Passed(Item : Integer) return Lf_Access is
Result : Lf_Access := new Long_Float;
begin
Result.All := 3.14159 * Long_Float(Item);
return Result;
end Func_To_Be_Passed;
-- Definition of an access to function type matching the function
-- signature above
type Func_Access is access function(Item : Integer) return Lf_Access; -- Definition of an integer access type
type Int_Access is access Integer; -- Define a function taking an instance of Func_Access as its -- parameter and returning an integer access type
function Complex_Func(Item : Func_Access; Parm2 : Integer) return Int_Access is
Result : Int_Access := new Integer;
begin
Result.All := Integer(Item(Parm2).all / 3.14149);
return Result;
end Complex_Func;
-- Declare an access variable to hold the access to the function
F_Ptr : Func_Access := Func_To_Be_Passed'access; -- Declare an access to integer variable to hold the result
Int_Ptr : Int_Access;
begin
-- Call the function using the access variable
Int_Ptr := Complex_Func(F_Ptr, 3); Put_Line(Integer'Image(Int_Ptr.All));
end Subprogram_As_Argument_2;</lang>
ALGOL 68
<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
AmigaE
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>
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>
Brat
<lang brat>add = { a, b | a + b }
doit = { f, a, b | f a, b }
p doit ->add 1 2 #prints 3</lang>
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
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.
<lang cpp>
- 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
<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)
{
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#.
<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.
<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:
<lang coffeescript>(-> -> -> -> 2 )()()()() # => 2</lang>
A function that takes a function that takes a function argument:
<lang coffeescript>((x)->
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>
D
D's function/delegate will be investigated by passing each type of function/delegate to _test_ as argument. <lang d>module functest ; 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) ;
// leteral function/delegate
"Literal".test(function string() { return "literal.F"; })
.test(delegate string() { return "literal.D"; }) ;
}</lang>
Delphi
- See Pascal
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>
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>
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
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
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 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 (N - number) multiplied 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
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.
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>
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>
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>
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>
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>
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
<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;
function second(x: real): real;
begin second := x/2.0; end;
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();
}
$result = first(function() { return 'second'; });</lang>
PicoLisp
<lang PicoLisp>: (de first (Fun)
(Fun) )
-> first
- (de second ()
"second" )
-> second
- (first second)
-> "second"
- (de add (A B)
(+ A B) )
-> add
- (add 1 2)
-> 3
- (de call-it (Fun X Y)
(Fun X Y) )
-> call-it
- (call-it add 1 2)
-> 3
- (mapcar inc (1 2 3 4 5))
-> (2 3 4 5 6)
- (mapcar + (1 2 3) (4 5 6))
-> (5 7 9)
- (mapcar add (1 2 3) (4 5 6))
-> (5 7 9)</lang>
PL/I
<lang PL/I> f: procedure (g) returns (float);
declare g entry (float);
get (x); put (g(x));
end f;
x = f(p); /* where "p" is the name of a function. */
</lang>
Pop11
<lang pop11>;;; Define a function define x_times_three_minus_1(x);
return(3*x-1);
enddefine;
- Pass it as argument to built-in function map and print the result
mapdata({0 1 2 3 4}, x_times_three_minus_1) =></lang>
PostScript
<lang> /x_times_3_sub_1 {3 * 1 sub}. [0 1 2 3 4] {x_times_3_sub_1} map </lang>
Prolog
<lang prolog> first(Predicate):-Predicate. second(Argument):-print(Argument).
- -first(second('Hello World!')).
</lang>
PureBasic
<lang PureBasic>Prototype.d func(*text$)
Procedure NumberTwo(arg$)
Debug arg$
EndProcedure
Procedure NumberOne(*p, text$)
Define MyFunc.func=*p MyFunc(@text$)
EndProcedure
NumberOne(@NumberTwo(),"Hello Worldy!")</lang>
Python
<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>
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]
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.)
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>
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.)
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>
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
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