Higher-order functions
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Pass a function as an argument to another function.
- Related task
11l
<lang 11l>F first(function)
R function()
F second()
R ‘second’
V result = first(second) print(result)</lang>
- Output:
second
8th
<lang forth>
- pass-me
"I was passed\n" . ;
- passer
w:exec ;
\ pass 'pass-me' to 'passer' ' pass-me passer </lang>
- Output:
I was passed
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>
Aime
<lang aime>integer average(integer p, integer q) {
return (p + q) / 2;
}
void
out(integer p, integer q, integer (*f) (integer, integer))
{
o_integer(f(p, q));
o_byte('\n');
}
integer
main(void)
{
# display the minimum, the maximum and the average of 117 and 319 out(117, 319, min); out(117, 319, max); out(117, 319, average);
return 0;
}</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>
AntLang
<lang AntLang>twice:{x[x[y]]} echo twice "Hello!"</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>
As we can see above, AppleScript functions (referred to as 'handlers' in Apple's documentation) are not, in themselves, first class objects. They can only be applied within other functions, when passed as arguments, if wrapped in Script objects. If we abstract out this lifting of functions into objects by writing an mReturn or mInject function, we can then use it to write some higher-order functions which directly accept unadorned AppleScript handlers as arguments.
We could, for example, write map, fold/reduce and filter functions for AppleScript as follows:
<lang applescript>on run
-- PASSING FUNCTIONS AS ARGUMENTS TO
-- MAP, FOLD/REDUCE, AND FILTER, ACROSS A LIST
set lstRange to {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
map(squared, lstRange)
--> {0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100}
foldl(summed, 0, map(squared, lstRange))
--> 385
filter(isEven, lstRange)
--> {0, 2, 4, 6, 8, 10}
-- OR MAPPING OVER A LIST OF FUNCTIONS
map(testFunction, {doubled, squared, isEven})
--> {{0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20},
-- {0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100},
-- {true, false, true, false, true, false, true, false, true, false, true}}
end run
-- testFunction :: (a -> b) -> [b] on testFunction(f)
map(f, {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10})
end testFunction
-- MAP, REDUCE, FILTER
-- Returns a new list consisting of the results of applying the -- provided function to each element of the first list -- map :: (a -> b) -> [a] -> [b] on map(f, xs)
tell mReturn(f)
set lng to length of xs
set lst to {}
repeat with i from 1 to lng
set end of lst to |λ|(item i of xs, i, xs)
end repeat
return lst
end tell
end map
-- Applies a function against an accumulator and -- each list element (from left-to-right) to reduce it -- to a single return value
-- In some languages, like JavaScript, this is called reduce()
-- Arguments: function, initial value of accumulator, list -- foldl :: (a -> b -> a) -> a -> [b] -> a on foldl(f, startValue, xs)
tell mReturn(f)
set v to startValue
set lng to length of xs
repeat with i from 1 to lng
set v to |λ|(v, item i of xs, i, xs)
end repeat
return v
end tell
end foldl
-- Sublist of those elements for which the predicate
-- function returns true
-- filter :: (a -> Bool) -> [a] -> [a]
on filter(f, xs)
tell mReturn(f)
set lst to {}
set lng to length of xs
repeat with i from 1 to lng
set v to item i of xs
if |λ|(v, i, xs) then set end of lst to v
end repeat
return lst
end tell
end filter
-- Lift 2nd class handler function into 1st class script wrapper -- mReturn :: Handler -> Script on mReturn(f)
if class of f is script then
f
else
script
property |λ| : f
end script
end if
end mReturn
-- HANDLER FUNCTIONS TO BE PASSED AS ARGUMENTS
-- squared :: Number -> Number on squared(x)
x * x
end squared
-- doubled :: Number -> Number on doubled(x)
x * 2
end doubled
-- summed :: Number -> Number -> Number on summed(a, b)
a + b
end summed
-- isEven :: Int -> Bool on isEven(x)
x mod 2 = 0
end isEven</lang>
- Output:
<lang applescript>{{0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20}, {0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100}, {true, false, true, false, true, false, true, false, true, false, true}}</lang>
Alternative description
In plain English, one handler can be passed as a parameter to another either in a script object …
<lang applescript>script aScript
on aHandler(aParameter)
say aParameter
end aHandler
end script
on receivingHandler(passedScript)
passedScript's aHandler("Hello")
end receivingHandler
receivingHandler(aScript)</lang>
… or directly, with the passed pointer being assigned to a script object property upon receipt:
<lang applescript>on aHandler(aParameter)
say aParameter
end aHandler
on receivingHandler(passedHandler)
script o
property h : passedHandler
end script
o's h("Hello")
end receivingHandler
receivingHandler(aHandler)</lang>
It's not often that handlers actually need to be passed as parameters, but passing them in script objects is usually more flexible as additional information on which they depend can then be included if required which the receiving handler doesn't need to know.
An alternative to the script object property in the second case would be a global or property of the main script (the one containing the receiving handler), but this isn't recommended owing to the possibilities for conflict.
Arturo
<lang arturo>doSthWith: function [x y f][ f x y ]
print [ "add:" doSthWith 2 3 $[x y][x+y] ] print [ "multiply:" doSthWith 2 3 $[x y][x*y] ]</lang>
- Output:
add: 5 multiply: 6
ATS
<lang ATS>
- include
"share/atspre_staload.hats"
fun app_to_0 (f: (int) -> int): int = f (0)
implement main0 () = { // val () = assertloc (app_to_0(lam(x) => x+1) = 1) val () = assertloc (app_to_0(lam(x) => 10*(x+1)) = 10) // } (* end of [main0] *) </lang>
AutoHotkey
<lang AutoHotkey> f(x) {
return "This " . x
}
g(x) {
return "That " . x
}
show(fun) {
msgbox % %fun%("works")
}
show(Func("f")) ; either create a Func object show("g") ; or just name the function return </lang>
BBC BASIC
<lang bbcbasic> REM Test passing a function to a function:
PRINT FNtwo(FNone(), 10, 11)
END
REM Function to be passed:
DEF FNone(x, y) = (x + y) ^ 2
REM Function taking a function as an argument:
DEF FNtwo(RETURN f%, x, y) = FN(^f%)(x, y)</lang>
Output:
441
Bracmat
<lang bracmat>( (plus=a b.!arg:(?a.?b)&!a+!b) & ( print
= text a b func
. !arg:(?a.?b.(=?func).?text)
& out$(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):
<lang burlesque> blsq ) {1 2 3 4}{5.+}m[ {6 7 8 9} </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 then 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#
Each example below performs the same task and utilizes .NET delegates, which are objects that refer to a static method or to an instance method of a particular object instance.
- Output (for each example):
f=Add, f(6, 2) = 8 f=Mul, f(6, 2) = 12 f=Div, f(6, 2) = 3
Named methods
This implementation works in all standard versions of C#.
<lang csharp>using System;
// A delegate declaration. Because delegates are types, they can exist directly in namespaces. 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)
{
// Invoking a delegate like a method is syntax sugar; this compiles down to f.Invoke(a, b);
return f(a, b);
}
static void Main()
{
int a = 6;
int b = 2;
// Delegates must be created using the "constructor" syntax in C# 1.0; in C# 2.0 and above, only the name of the method is required (when a target type exists, such as in an assignment to a variable with a delegate type or usage in a function call with a parameter of a delegate type; initializers of implicitly typed variables must use the constructor syntax as a raw method has no delegate type). Overload resolution is performed using the parameter types of the target delegate type.
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>
C# 2.0: Anonymous methods
Anonymous methods were added in C# 2.0. Parameter types must be specified. Anonymous methods must be "coerced" to a delegate type known at compile-time; they cannot be used with a target type of Object or to initialize implicitly typed variables.
<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>
C# 3.0: Lambda expressions
Lambda expressions were added in C# 3.0 as a more concise replacement to anonymous methods. The target delegate type must also be known at compile-time.
With .NET Framework 3.5, the System namespace also gained the Func and Action "families" of generic delegate types. Action delegates are void-returning, while Func delegates return a value of a specified type. In both families, a separate type exists for every function arity from zero to sixteen, as .NET does not have variadic generics.
For instance, the Action delegate has no parameters, Action<T>, has one parameter of type T, Action<T1, T2> has two parameters of types T1 and T2, and so on. Similarly, Func<TResult> has no parameters and a return type of TResult, Func<T1, TResult> additionally has one parameter of type T, and so on.
<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;
// No lengthy delegate keyword.
Console.WriteLine("f=Add, f({0}, {1}) = {2}", a, b, Call((int x, int y) => { return x + y; }, a, b));
// Parameter types can be inferred.
Console.WriteLine("f=Mul, f({0}, {1}) = {2}", a, b, Call((x, y) => { return x * y; }, a, b));
// Expression lambdas are even shorter (and are most idiomatic).
Console.WriteLine("f=Div, f({0}, {1}) = {2}", a, b, Call((x, y) => x / y, a, b));
}
}</lang>
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.
For C++11 this is now std::function.
<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
<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>
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> 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>
Cowgol
<lang cowgol>include "cowgol.coh";
- In order to pass functions around, you must first define an interface.
- This is similar to a delegate in C#; it becomes a function pointer type.
- This interface takes two integers and returns one.
interface Dyadic(x: int32, y: int32): (r: int32);
- For a function to be able to be passed around, it must explicitly implement
- an interface. Then it has the same type as that interface.
- The interface replaces the method's parameter list entirely.
sub Add implements Dyadic is
r := x + y;
end sub;
- Here are the other basic operators.
sub Sub implements Dyadic is r := x - y; end sub; sub Mul implements Dyadic is r := x * y; end sub; sub Div implements Dyadic is r := x / y; end sub;
- An interface is just like any other type, and the functions that implement
- it are first-class values. For example, this code maps the operator
- characters to their functions.
record Operator is
char: uint8; func: Dyadic;
end record;
var operators: Operator[] := {
{'+', Add}, {'-', Sub}, {'*', Mul}, {'/', Div},
{0, Dyadic}
};
- This is a function that applies such a function to two values
sub apply(f: Dyadic, x: int32, y: int32): (r: int32) is
r := f(x,y); # the function can be called as normal
end sub;
- And this is a function that applies all the above operators to two values
sub showAll(ops: [Operator], x: int32, y: int32) is
while ops.char != 0 loop
print_i32(x as uint32);
print_char(' ');
print_char(ops.char);
print_char(' ');
print_i32(y as uint32);
print(" = ");
print_i32(apply(ops.func, x, y) as uint32);
print_nl();
ops := @next ops;
end loop;
end sub;
showAll(&operators[0], 84, 42); # example</lang>
- Output:
84 + 42 = 126 84 - 42 = 42 84 * 42 = 3528 84 / 42 = 2
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
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>
Dyalect
<lang Dyalect>func call(f, a, b) {
f(a, b)
}
let a = 6 let b = 2
print("f=add, f(\(a), \(b)) = \(call((x, y) => x + y, a, b))") print("f=mul, f(\(a), \(b)) = \(call((x, y) => x * y, a, b))") print("f=div, f(\(a), \(b)) = \(call((x, y) => x / y, a, b))")</lang>
- Output:
f=add, f(6, 2) = 8 f=mul, f(6, 2) = 12 f=div, f(6, 2) = 3
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 ]
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>
Elena
ELENA 4.1 : <lang elena>import extensions;
public program() {
var first := (f => f());
var second := {"second"};
console.printLine(first(second))
}</lang>
- Output:
second
Elixir
<lang elixir>iex(1)> defmodule RC do ...(1)> def first(f), do: f.() ...(1)> def second, do: :hello ...(1)> end {:module, RC,
<<70, 79, 82, 49, 0, 0, 4, 224, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 142,
131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115, 95 , 118, 49, 108, 0, 0, 0, 2, 104, 2, ...>>,
{:second, 0}}
iex(2)> RC.first(fn -> RC.second end)
- hello
iex(3)> RC.first(&RC.second/0) # Another expression
- hello
iex(4)> f = fn -> :world end # Anonymous function
- Function<20.54118792/0 in :erl_eval.expr/5>
iex(5)> RC.first(f)
- world</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>
ERRE
ERRE function are limited to one-line FUNCTION, but you can write: <lang ERRE> PROGRAM FUNC_PASS
FUNCTION ONE(X,Y)
ONE=(X+Y)^2
END FUNCTION
FUNCTION TWO(X,Y)
TWO=ONE(X,Y)+1
END FUNCTION
BEGIN
PRINT(TWO(10,11))
END PROGRAM </lang> Answer is 442
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>
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>
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>
FreeBASIC
<lang freebasic>' FB 1.05.0 Win64
Function square(n As Integer) As Integer
Return n * n
End Function
Function cube(n As Integer) As Integer
Return n * n * n
End Function
Sub doCalcs(from As Integer, upTo As Integer, title As String, func As Function(As Integer) As Integer)
Print title; " -> "; For i As Integer = from To upTo Print Using "#####"; func(i); Next Print
End Sub
doCalcs 1, 10, "Squares", @square doCalcs 1, 10, "Cubes ", @cube Print Print "Press any key to quit" Sleep</lang>
- Output:
Squares -> 1 4 9 16 25 36 49 64 81 100 Cubes -> 1 8 27 64 125 216 343 512 729 1000
Frink
The following defines an anonymous function and passes it to another function. In this case, the anonymous function is a comparison function that sorts by string length.
<lang frink> cmpFunc = {|a,b| length[a] <=> length[b]}
a = ["tree", "apple", "bee", "monkey", "z"] sort[a, cmpFunc] </lang>
You can also look up functions by name and number of arguments. The following is equivalent to the previous example.
<lang frink> lengthCompare[a,b] := length[a] <=> length[b]
func = getFunction["lengthCompare", 2] a = ["tree", "apple", "bee", "monkey", "z"] sort[a, func] </lang>
FutureBasic
<lang futurebasic> include "ConsoleWindow"
dim as pointer functionOneAddress
def fn FunctionOne( x as long, y as long ) as long = (x + y) ^ 2 functionOneAddress = @fn FunctionOne
def fn FunctionTwo( x as long, y as long ) using functionOneAddress
print fn FunctionTwo( 12, 12 ) </lang>
Output:
576
Fōrmulæ
In this page you can see the solution of this task.
Fōrmulæ programs are not textual, visualization/edition of programs is done showing/manipulating structures but not text (more info). Moreover, there can be multiple visual representations of the same program. Even though it is possible to have textual representation —i.e. XML, JSON— they are intended for transportation effects more than visualization and edition.
The option to show Fōrmulæ programs and their results is showing images. Unfortunately images cannot be uploaded in Rosetta Code.
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 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 @ (atop), & (bond or compose) and ^: (power) are conjunctions. 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
2&% 1 2 3 NB. divide 2 by
2 1 0.666667
%&2 (1 2 3) NB. divide by 2 (need parenthesis to break up list formation)
0.5 1 1.5
-: 1 2 3 NB. but divide by 2 happens a lot so it's a primitive
0.5 1 1.5
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>
From Java 8, lambda expressions may be used. Example (from Oracle): <lang java>public class ListenerTest {
public static void main(String[] args) {
JButton testButton = new JButton("Test Button");
testButton.addActionListener(new ActionListener(){
@Override public void actionPerformed(ActionEvent ae){
System.out.println("Click Detected by Anon Class");
}
});
testButton.addActionListener(e -> System.out.println("Click Detected by Lambda Listner"));
// Swing stuff
JFrame frame = new JFrame("Listener Test");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(testButton, BorderLayout.CENTER);
frame.pack();
frame.setVisible(true);
}
}</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.
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>
Klingphix
<lang Klingphix>:+2 + 2 + ;
- 2 * 2 * ;
- apply exec ;
23 45 @+2 apply print nl 8 4 @*2 apply print nl
" " input</lang>
Kotlin
Kotlin is a functional language. Example showing how the builtin map function is used to get the average value of a transformed list of numbers: <lang scala>fun main(args: Array<String>) {
val list = listOf(1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0)
val a = list.map({ x -> x + 2 }).average()
val h = list.map({ x -> x * x }).average()
val g = list.map({ x -> x * x * x }).average()
println("A = %f G = %f H = %f".format(a, g, h))
}</lang>
Another example showing the syntactic sugar available to Kotlin developers which allows them to put the lambda expression out of the parenthesis whenever the function is the last argument of the higher order function. Notice the usage of the inline modifier, which inlines the bytecode of the argument function on the callsite, reducing the object creation overhead (an optimization for pre Java 8 JVM environments, like Android) (translation from Scala example): <lang scala>inline fun higherOrderFunction(x: Int, y: Int, function: (Int, Int) -> Int) = function(x, y)
fun main(args: Array<String>) {
val result = higherOrderFunction(3, 5) { x, y -> x + y }
println(result)
}</lang>
- Output:
8
Lambdatalk
<lang Scheme> {def add
{lambda {:f :g :x}
{+ {:f :x} {:g :x}}}}
{add sin cos 10} -> -1.383092639965822 // {+ {sin 10} {cos 10}}
{S.map sqrt {S.serie 1 5}} -> 1 1.4142135623730951 1.7320508075688772 2 2.23606797749979
{S.reduce + {S.serie 1 10}} -> 55 </lang>
Lily
<lang lily>define square(x: Integer): Integer {
return x * x
}
var l = [1, 2, 3] # Inferred type: List[Integer].
- Transform using a user-defined function.
print(l.map(square)) # [1, 4, 9]
- Using a built-in method this time.
print(l.map(Integer.to_s)) # ["1", "2", "3"]
- Using a lambda (with the type of 'x' properly inferred).
print(l.map{|x| (x + 1).to_s()}) # ["2", "3", "4"]
- In reverse order using F#-styled pipes.
Boolean.to_i |> [true, false].map |> print
define apply[A, B](value: A, f: Function(A => B)): B {
return f(value)
}
- Calling user-defined transformation.
print(apply("123", String.parse_i)) # Some(123)</lang>
Lingo
Lingo doesn't support first-class functions. But functions can be passed as "symbols", and then be called via Lingo's 'call' command. Global functions - i.e. either built-in functions or user-defined functions in movie scripts - are always methods of the core '_movie' object, for other object functions (methods) also the object has to be specified. Here as example an implementation of a generic "map" function:
<lang lingo>-- in some movie script
-- Runs provided function (of some object) on all elements of the provided list, returns results as new list -- @param {list} aList -- @param {symbol} cbFunc -- @param {object} [cbObj=_movie] -- @return {list}
on map (aList, cbFunc, cbObj)
if voidP(cbObj) then cbObj = _movie res = [] cnt = aList.count repeat with i = 1 to cnt res[i] = call(cbFunc, cbObj, aList[i]) end repeat return res
end</lang>
<lang lingo>l = [1, 2, 3]
-- passes the built-in function 'sin' (which is a method of the _movie object) as argument to map res = map(l, #sin)
put res -- [0.8415, 0.9093, 0.1411]</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>
Luck
Higher-order functions can be used to implement conditional expressions: <lang luck>function lambda_true(x: 'a)(y: 'a): 'a = x;; function lambda_false(x: 'a)(y: 'a): 'a = y;; function lambda_if(c:'a -> 'a -> 'a )(t: 'a)(f: 'a): 'a = c(t)(f);;
print( lambda_if(lambda_true)("condition was true")("condition was false") );;</lang>
M2000 Interpreter
We can pass by reference a standard function, or we can pass by value a lambda function (also we can pass by reference as reference to lambda function) <lang M2000 Interpreter> Function Foo (x) {
=x**2
} Function Bar(&f(), k) {
=f(k)
} Print Bar(&foo(), 20)=400 Group K {
Z=10
Function MulZ(x) {
=.Z*x
.Z++
}
} Print Bar(&K.MulZ(), 20)=200 Print K.Z=11 </lang>
Example using lambda function
<lang M2000 Interpreter> Foo = Lambda k=1 (x)-> {
k+=2
=x**2+K
} \\ by ref1 Function Bar1(&f(), k) {
=f(k)
} Print Bar1(&Foo(), 20)=403 \\ by ref2 Function Bar2(&f, k) {
=f(k)
} Print Bar2(&Foo, 20)=405 \\ by value Function Bar(f, k) {
=f(k)
} \\ we sent a copy of lambda, and any value type closure copied too Print Bar(Foo, 20)=407 Print Bar1(&Foo(), 20)=407 \\ we can get a copy of Foo to NewFoo (also we get a copy of closure too) NewFoo=Foo Print Bar1(&Foo(), 20)=409 Print Bar2(&Foo, 20)=411 Print Bar2(&NewFoo, 20)=409
</lang>
Mathematica / Wolfram Language
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 referred 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
<lang>6 ПП 04 П7 КПП7 В/О 1 В/О</lang>
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>
Morfa
<lang morfa> func g(a: int, b: int, f: func(int,int): int): int {
return f(a, b);
}
import morfa.base;
func main(): void {
println("Add: ", g(2, 3, func(a: int, b: int) { return a + b; }));
println("Multiply: ", g(2, 3, func(a: int, b: int) { return a * b; }));
} </lang>
Nanoquery
<lang nanoquery>def first(function)
return function()
end
def second()
return "second"
end
result = first(second) println result</lang>
- Output:
second
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>
Nim
<lang nim>proc first(fn: proc): auto =
return fn()
proc second(): string =
return "second"
echo first(second)</lang>
Oberon-2
Works with oo2c version 2 <lang oberon2> MODULE HOFuns; IMPORT
NPCT:Tools, Out;
TYPE
Formatter = PROCEDURE (s: STRING; len: LONGINT): STRING;
VAR
words: ARRAY 8 OF STRING;
PROCEDURE PrintWords(w: ARRAY OF STRING; format: Formatter);
VAR
i: INTEGER;
BEGIN
i := 0;
WHILE (i < LEN(words)) DO
Out.Object(format(words[i],16));
INC(i)
END;
Out.Ln
END PrintWords;
BEGIN
words[0] := "Al-Andalus"; words[1] := "contributed"; words[2] := "significantly"; words[3] := "to"; words[4] := "the"; words[5] := "field"; words[6] := "of"; words[7] := "medicine"; PrintWords(words,Tools.AdjustLeft); PrintWords(words,Tools.AdjustCenter); PrintWords(words,Tools.AdjustRight)
END HOFuns. </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>
Oforth
If you add # before a function or method name you push the function object on the stack (instead of performing the function). This allows to pass functions to other functions, as for any other object. Here we pass #1+ to map : <lang Oforth>[1, 2, 3, 4, 5 ] map(#1+)</lang>
Ol
<lang scheme>
- typical use
(for-each display '(1 2 "ss" '(3 4) 8))
- ==> 12ss(quote (3 4))8'()
- manual implementation in details
(define (do f x)
(f x))
(do print 12345)
- ==> 12345
</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>
- Output:
55 720 169 27 127 3 20
Order
Functions in Order can accept any other named function, local variable, or anonymous function as arguments:
<lang C>
- 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
<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>
using FreePascal : Higher-order function MAP / REDUCE ( FOLDL / FOLDR ) / FILTER
<lang Pascal> PROGRAM HigherOrderMapReduce;
{$mode Delphi} {$H+} {$J-} {$R+} (*) https://www.freepascal.org/docs-html/prog/progch1.html (*)
(*)
Free Pascal Compiler version 3.2.0 [2020/06/14] for x86_64
The free and readable alternative at C/C++ speeds
compiles natively to almost any platform, including raspberry PI *
Can run indepENDently from DELPHI / Lazarus
For debian Linux: apt -y install fpc
It contains a text IDE called fp
(*)
USES
Math,
SysUtils,
Variants;
{$WARN 6058 off : Call to subroutine "$1" marked as inline is not inlined} // Use for variants
TYPE
Varyray = array of variant ;
FunA = FUNCTION ( x : variant ) : variant ;
FunB = PROCEDURE ( x : variant ) ;
FunC = FUNCTION ( x,y : variant ) : variant ;
FunD = FUNCTION ( x,y : longint ) : longint ;
PROCEDURE Show ( x: variant ) ;
BEGIN write( x, ' ' ) ; END ;
FUNCTION Map ( f: FunA ; x: Varyray ) : Varyray ; overload ;
VAR
Ar : array of variant ;
k : integer ;
BEGIN
SetLength ( Ar, length ( x ) ) ;
result := Ar ;
FOR k := Low ( x ) TO High ( x ) DO
Ar [ k ] := f ( x [ k ] ) ;
result := Ar ;
END;
PROCEDURE Map ( f: FunB ; x: Varyray ) ; overload ;
VAR
k : integer ;
BEGIN
FOR k := Low ( x ) TO High ( x ) DO f ( x [ k ] ) ;
END;
FUNCTION Map ( f: FunC ; x, y: Varyray ) : Varyray ; overload ;
VAR
Ar : array of variant ;
k : integer ;
BEGIN
SetLength ( Ar, length ( x ) ) ;
result := Ar ;
FOR k := Low ( x ) TO High ( x ) DO
Ar [ k ] := f ( x [ k ] , y [ k ] ) ;
result := Ar ;
END;
FUNCTION Map ( f: FunD ; x, y: Varyray ) : Varyray ; overload ;
VAR
Ar : array of variant ;
k : integer ;
BEGIN
SetLength ( Ar, length ( x ) ) ;
result := Ar ;
FOR k := Low ( x ) TO High ( x ) DO
Ar [ k ] := f ( x [ k ] , y [ k ] ) ;
result := Ar ;
END;
FUNCTION Filter ( f: FunA ; x: Varyray ) : Varyray ;
VAR
Ar : array of variant ;
__ : variant ;
k : integer ;
len : integer ;
BEGIN
SetLength ( Ar, 0 ) ;
result := Ar ;
FOR k := Low ( x ) TO High ( x ) DO
BEGIN
__ := f ( x [ k ] ) ;
IF __ <> False THEN
BEGIN
len := Length ( Ar ) ;
SetLength ( Ar, len + 1 ) ;
Ar [ len ] := __ ;
END ;
END ;
result := Ar ;
END;
FUNCTION FoldL ( f: FunC ; x: Varyray ) : variant ; overload ;
VAR
k : integer ;
BEGIN
result := ord ( f ( 0, x [ 0 ] ) = 0 ) ;
FOR k := Low ( x ) TO High ( x ) DO
result := f ( result , x [ k ] ) ;
END ;
FUNCTION FoldL ( f: FunD ; x: Varyray ) : variant ; overload ;
VAR
k : integer ;
BEGIN
result := ord ( f ( 0, x [ 0 ] ) = 0 ) ;
FOR k := Low ( x ) TO High ( x ) DO
result := f ( result , x [ k ] ) ;
END ;
FUNCTION Reduce ( f: FunC ; x: Varyray ) : variant ; overload ;
BEGIN result := FoldL ( f , x ) ; END ;
FUNCTION Reduce ( f: FunD ; x: Varyray ) : variant ; overload ;
BEGIN result := FoldL ( f , x ) ; END ;
FUNCTION FoldR ( f: FunC ; x: Varyray ) : variant ; overload ;
VAR
k : integer ;
BEGIN
result := ord ( f ( 0, x [ 0 ] ) = 0 ) ;
FOR k := High ( x ) DOWNTO Low ( x ) DO
result := f ( result , x [ k ] ) ;
END ;
FUNCTION FoldR ( f: FunD ; x: Varyray ) : variant ; overload ;
VAR
k : integer ;
BEGIN
result := ord ( f ( 0, x [ 0 ] ) = 0 ) ;
FOR k := High ( x ) DOWNTO Low ( x ) DO
result := f ( result , x [ k ] ) ;
END ;
(*) TEST Functions (*)
(*)
Special thanks to PascalDragon & winni ( FreePascal.org ),
who explained the specifics of the compiler.
(*)
FUNCTION Add ( x, y: variant ) : variant ;
BEGIN result := x + y ; END ;
FUNCTION Add1 ( x: variant ) : variant ;
BEGIN result := x + 1 ; END ;
FUNCTION AND_xy ( x, y: variant ) : variant ;
BEGIN result := ( x and y ) = True ; END ;
FUNCTION AVG ( x: Varyray ) : variant ;
VAR
k : integer ;
BEGIN
result := 0.0 ;
FOR k := Low ( x ) TO High ( x ) DO
result := result + ( x [ k ] - result ) / ( k + 1 );
END ;
FUNCTION cosine ( x: variant ) : variant ;
BEGIN result := cos ( x ); END ;
FUNCTION Is_Even ( x: variant ) : variant ;
BEGIN
IF ( ( x mod 2 ) = 0 ) THEN
result := x
ELSE
result := False
END;
FUNCTION Mult( x, y: variant ) : variant ;
BEGIN result := x * y ; END ;
FUNCTION Is_Odd ( x: variant ) : variant ;
BEGIN
IF ( ( x mod 2 ) <> 0 ) THEN
result := x
ELSE
result := False
END;
FUNCTION OR_xy ( x, y: variant ) : variant ;
BEGIN result := ( x or y ) = True; END ;
FUNCTION RandFillInt ( x: variant ) : variant ;
BEGIN result := Random ( 100 ) ; END ;
FUNCTION RandFillFloat ( x: variant ) : variant ;
VAR
tmp : float = 100.0 ;
BEGIN result := ( Random ( ) ) * tmp ; END ;
FUNCTION sine ( x: variant ) : variant ;
BEGIN result := sin ( x ); END ;
FUNCTION ForAll ( f: FunA ; x: Varyray ) : variant ;
VAR
k : integer ;
BEGIN
result := True ;
FOR k := Low ( x ) TO High ( x ) DO
result := AND_xy ( result , f ( x [ k ] ) ) ;
END ;
FUNCTION AnyOf ( f: FunA ; x: Varyray ) : variant ;
VAR
k : integer ;
BEGIN
result := False ;
FOR k := Low ( x ) TO High ( x ) DO
result := OR_xy ( result , f ( x [ k ] ) ) ;
END ;
VAR
a,b,c : array of variant ;
Accu : variant ;
BEGIN
Randomize ;
setlength ( a, 7 ) ;
setlength ( b, 7 ) ;
setlength ( c, 7 ) ;
a := Map ( RandFillInt , a ) ;
Map ( show , a ) ;
writeln ;
writeln ;
Accu := FoldR ( add , a ) ;
WriteLn ( 'Sum = ' , Accu ) ;
writeln ;
writeln ;
Accu := Reduce ( mult , a ) ;
WriteLn ( 'Product = ' , Accu ) ;
writeln ;
writeln ;
b := Map ( RandFillInt , b ) ;
Map ( show , b ) ;
writeln ;
writeln ;
Accu := avg ( a ) ;
WriteLn ( 'avg = ' ,Accu );
writeln ;
writeln ;
c := Filter ( Is_Odd , a ) ;
Map ( show , c ) ;
writeln ;
writeln ;
Map ( show , b ) ;
Accu := ForAll ( Is_Odd , b ) ;
writeln ;
writeln ;
WriteLn ( 'ForAll Is_Odd = ' , Accu ) ;
writeln ;
writeln ;
Map ( show , b ) ;
Accu := AnyOf ( Is_Even , b ) ;
writeln ;
writeln ;
WriteLn ( 'AnyOf Is_Even = ' , Accu ) ;
writeln ;
writeln ;
setlength ( c, 0 ) ; setlength ( b, 0 ) ; setlength ( a, 0 ) ;
END.
</lang>JPD 2021/07/07
Output:
Random ( Like me :)
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 $sub->() for @ret; # it allows for simple inline application of the s/// and tr/// constructs @ret
}
print join ", " => apply {tr/aeiou/AEIOU/} qw/one two three four/;
- OnE, twO, thrEE, fOUr</lang>
<lang perl>sub first {shift->()}
sub second {'second'}
print first \&second;
print first sub{'sub'};</lang>
Phix
<lang Phix>procedure use(integer fi, a, b)
?fi(a,b)
end procedure
function add(integer a, b)
return a + b
end function
use(add,23,45)</lang>
- Output:
68
Phixmonti
<lang Phixmonti>def suma + enddef
def apply exec enddef
23 45 getid suma apply print </lang>
PHP
<lang php>function first($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();
}
$result = first(function() { return 'second'; });</lang>
PicoLisp
<lang PicoLisp>: (de first (Fun)
(Fun) )
-> first
- (de second ()
"second" )
-> second
- (first second)
-> "second"
- (de add (A B)
(+ A B) )
-> add
- (add 1 2)
-> 3
- (de call-it (Fun X Y)
(Fun X Y) )
-> call-it
- (call-it add 1 2)
-> 3
- (mapcar inc (1 2 3 4 5))
-> (2 3 4 5 6)
- (mapcar + (1 2 3) (4 5 6))
-> (5 7 9)
- (mapcar add (1 2 3) (4 5 6))
-> (5 7 9)</lang>
PL/I
<lang PL/I> f: procedure (g) returns (float);
declare g entry (float);
get (x); put (g(x));
end f;
x = f(p); /* where "p" is the name of a function. */
</lang>
Pop11
<lang pop11>;;; Define a function define x_times_three_minus_1(x);
return(3*x-1);
enddefine;
- Pass it as argument to built-in function map and print the result
mapdata({0 1 2 3 4}, x_times_three_minus_1) =></lang>
PostScript
Postscript functions are either built-in operators or executable arrays (procedures). Both can take either as arguments. <lang> % operator example % 'ifelse' is passed a boolean and two procedures /a 5 def a 0 gt { (Hello!) } { (World?) } ifelse ==
% procedure example % 'bar' is loaded onto the stack and passed to 'foo' /foo { exec } def /bar { (Hello, world!) } def /bar load foo == </lang>
PowerShell
<lang PowerShell> function f ($y) {
$y*$y
} function g (${function:f}, $y) {
(f $y)
} </lang>
You can implement a function inside a function.
<lang PowerShell> function g2($y) {
function f2($y) {
$y*$y
}
(f2 $y)
} </lang> Calling: <lang PowerShell> g f 5 g2 9 </lang> Output:
25 81
Prolog
<lang prolog> first(Predicate) :- call(Predicate). second(Argument) :- write(Argument).
- -first(second('Hello World!')).
</lang>
PureBasic
<lang PureBasic>Prototype.d func(*text$)
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.
Q
Its helpful to remember that in Q, when parameters aren't named in the function declaration, x is assumed to be the first parameter.
<lang Q> q)sayHi:{-1"Hello ",x;} q)callFuncWithParam:{x["Peter"]} q)callFuncWithParam sayHi Hello Peter q)callFuncWithParam[sayHi] Hello Peter</lang>
Quackery
As a dialogue in the Quackery shell…
/O> [ say "Doing the word: " ... dup echo ... do ] is task ( x --> ) ... Stack empty. /O> 10 11 ' + task ... Doing the word: + Stack: 21 /O> 2 ' * task ... Doing the word: * Stack: 42 /O> ' drop task ... Doing the word: drop Stack empty.
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>
#lang racket/base (define (add f g x) (+ (f x) (g x))) (add sin cos 10)
</lang>
Raku
(formerly 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 parametrized block introduced with ->, which serves as a "lambda":
<lang perl6>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>
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 $v2 call
23.54 "doit" callit</lang>
- Output:
callit called with doit doit called with 23.54
REBOL
<lang REBOL>REBOL [ Title: "Function Argument" URL: http://rosettacode.org/wiki/Function_as_an_Argument ]
map: func [ "Apply function to contents of list, return new list." f [function!] "Function to apply to list." data [block! list!] "List to transform." /local result i ][ result: copy [] repeat i data [append result f i] result]
square: func [ "Calculate x^2." x [number!] ][x * x]
cube: func [ "Calculate x^3." x [number!] ][x * x * x]
- Testing
x: [1 2 3 4 5] print ["Data: " mold x] print ["Squared:" mold map :square x] print ["Cubed: " mold map :cube x] print ["Unnamed:" mold map func [i][i * 2 + 1] x]</lang>
Output:
Data: [1 2 3 4 5] Squared: [1 4 9 16 25] Cubed: [1 8 27 64 125] Unnamed: [3 5 7 9 11]
Retro
<lang Retro>:disp (nq-) call n:put ;
- 31 [ (n-n) #100 * ] disp
</lang>
REXX
<lang rexx>/*REXX program demonstrates the passing of a name of a function to another function. */ call function 'fact' , 6; say right( 'fact{'$"} = ", 30) result call function 'square' , 13; say right( 'square{'$"} = ", 30) result call function 'cube' , 3; say right( 'cube{'$"} = ", 30) result call function 'reverse', 721; say right( 'reverse{'$"} = ", 30) result exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ cube: return $**3 fact: procedure expose $; !=1; do j=2 to $; !=!*j; end; return ! function: arg ?.; parse arg ,$; signal value (?.) reverse: return 'REVERSE'($) square: return $**2</lang>
- output when using the default input:
fact{6} = 720
square{13} = 169
cube{3} = 27
reverse{721} = 127
Ring
<lang ring>
- Project : Higher-order functions
docalcs(1,10,"squares",:square) docalcs(1,10,"cubes",:cube)
func square(n)
return n * n
func cube(n)
return n * n * n
func docalcs(from2,upto,title,func2)
see title + " -> " + nl
for i = from2 to upto
x = call func2(i)
see x + nl
next
see nl
</lang> Output:
squares -> 1 4 9 16 25 36 49 64 81 100 cubes -> 1 8 27 64 125 216 343 512 729 1000
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>
Rust
Functions are first class values and identified in the type system by implementing the FnOnce, FnMut or the Fn trait which happens implicitly for functions and closures. <lang rust>fn execute_with_10<F: Fn(u64) -> u64> (f: F) -> u64 { f(10) }
fn square(n: u64) -> u64 { n*n }
fn main() { println!("{}", execute_with_10(|n| n*n )); // closure println!("{}", execute_with_10(square)); // function }</lang>
- Output:
100 100
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.)
SenseTalk
<lang sensetalk>function Map oldlist, func put () into newlist repeat with each item of oldlist insert (func)(it) after newlist end repeat return newlist end Map</lang>
<lang sensetalk>put ("tomato", "aubergine", "courgette") into fruits put Map(fruits, Uppercase) </lang>
Sidef
<lang ruby>func first(f) {
return f();
}
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.)
SuperCollider
<lang SuperCollider>f = { |x, y| x.(y) }; // a function that takes a function and calls it with an argument f.({ |x| x + 1 }, 5); // returns 5</lang>
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
Ursa
Functions are first-class objects in Ursa. <lang ursa>def first (function f) return (f) end
def second () return "second" end
out (first second) endl console
- "second" is output to the console</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
VBA
Based on the Pascal solution <lang pascal>Sub HigherOrder()
Dim result As Single
result = first("second")
MsgBox result
End Sub Function first(f As String) As Single
first = Application.Run(f, 1) + 2
End Function Function second(x As Single) As Single
second = x / 2
End Function</lang>
Visual Basic .NET
Each example below performs the same task and utilizes .NET delegates, which are objects that refer to a static method or to an instance method of a particular object instance.
c.f. C#
- Output (for each example):
f=Add, f(6, 2) = 8 f=Mul, f(6, 2) = 12 f=Div, f(6, 2) = 3
Named methods
<lang vbnet>' Delegate declaration is similar to C#. Delegate Function Func2(a As Integer, b As Integer) As Integer
Module Program
Function Add(a As Integer, b As Integer) As Integer
Return a + b
End Function
Function Mul(a As Integer, b As Integer) As Integer
Return a * b
End Function
Function Div(a As Integer, b As Integer) As Integer
Return a \ b
End Function
' Call is a keyword and must be escaped using brackets.
Function [Call](f As Func2, a As Integer, b As Integer) As Integer
Return f(a, b)
End Function
Sub Main()
Dim a = 6
Dim b = 2
' Delegates in VB.NET could be created without a New expression from the start. Both syntaxes are shown here.
Dim add As Func2 = New Func2(AddressOf Program.Add)
' The "As New" idiom applies to delegate creation.
Dim div As New Func2(AddressOf Program.Div)
' Directly coercing the AddressOf expression:
Dim mul As Func2 = AddressOf Program.Mul
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))
End Sub
End Module</lang>
Lambda expressions
Lambda expressions in VB.NET are similar to those in C#, except they can also explicitly specify a return type and exist as standalone "anonymous delegates". An anonymous delegate is created when a lambda expression is assigned to an implicitly typed variable (in which case the variable receives the type of the anonymous delegate) or when the target type given by context (at compile-time) is MulticastDelegate, Delegate, or Object. Anonymous delegates are derived from MulticastDelegate and are implicitly convertible to all compatible delegate types. A formal definition of delegate compatibility can be found in the language specification. <lang vbnet>Module Program
' Uses the System generic delegate; see C# entry for details.
Function [Call](f As Func(Of Integer, Integer, Integer), a As Integer, b As Integer) As Integer
Return f(a, b)
End Function
Sub Main()
Dim a = 6
Dim b = 2
Console.WriteLine("f=Add, f({0}, {1}) = {2}", a, b, [Call](Function(x As Integer, y As Integer) x + y, a, b))
' With inferred parameter types:
Console.WriteLine("f=Mul, f({0}, {1}) = {2}", a, b, [Call](Function(x, y) x * y, a, b))
' The block syntax must be used in order to specify a return type. As there is no target type in this case, the parameter types must be explicitly specified. anon has an anonymous, compiler-generated type.
Dim anon = Function(x As Integer, y As Integer) As Integer
Return x \ y
End Function
' Parameters are contravariant and the return type is covariant. Note that this conversion is not valid CLR variance (which disallows boxing conversions) and so is compiled as an additional anonymous delegate that explicitly boxes the return value.
Dim example As Func(Of Integer, Integer, Object) = anon
' Dropped-return-type conversion.
Dim example2 As Action(Of Integer, Integer) = anon
Console.WriteLine("f=Div, f({0}, {1}) = {2}", a, b, [Call](anon, a, b))
End Sub
End Module</lang>
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>
Wren
<lang ecmascript>var first = Fn.new { |f|
System.print("first function called")
f.call()
}
var second = Fn.new { System.print("second function called") }
first.call(second)</lang>
- Output:
first function called second function called
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() } fcn(g){ g() }(fcn{ "Hello World!".println() } )</lang>
ZX Spectrum Basic
Input "FN " token first, then enclose it in double quotation marks. <lang zxbasic>10 DEF FN f(f$,x,y)=VAL ("FN "+f$+"("+STR$ (x)+","+STR$ (y)+")") 20 DEF FN n(x,y)=(x+y)^2 30 PRINT FN f("n",10,11)</lang>
- Programming Tasks
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