Function composition

From Rosetta Code
Task
Function composition
You are encouraged to solve this task according to the task description, using any language you may know.

Create a function, compose, whose two arguments f and g, are both functions with one argument. The result of compose is to be a function of one argument, (lets call the argument x), which works like applying function f to the result of applying function g to x, i.e,

compose(f, g) (x) = f(g(x))


Reference: Function composition

Hint: In some languages, implementing compose correctly requires creating a closure.

ActionScript[edit]

ActionScript supports closures, making function composition very straightforward.

function compose(f:Function, g:Function):Function {
return function(x:Object) {return f(g(x));};
}
function test() {
trace(compose(Math.atan, Math.tan)(0.5));
}

Ada[edit]

The interface of a generic functions package. The package can be instantiated with any type that has value semantics. Functions are composed using the operation '*'. The same operation applied to an argument evaluates it there: f * x. Functions can be composed with pointers to Ada functions. (In Ada functions are not first-class):

generic
type Argument is private;
package Functions is
type Primitive_Operation is not null
access function (Value : Argument) return Argument;
type Func (<>) is private;
function "*" (Left : Func; Right : Argument) return Argument;
function "*" (Left : Func; Right : Primitive_Operation) return Func;
function "*" (Left, Right : Primitive_Operation) return Func;
function "*" (Left, Right : Func) return Func;
private
type Func is array (Positive range <>) of Primitive_Operation;
end Functions;

Here is an implementation;

package body Functions is
function "*" (Left : Func; Right : Argument) return Argument is
Result : Argument := Right;
begin
for I in reverse Left'Range loop
Result := Left (I) (Result);
end loop;
return Result;
end "*";
 
function "*" (Left, Right : Func) return Func is
begin
return Left & Right;
end "*";
 
function "*" (Left : Func; Right : Primitive_Operation) return Func is
begin
return Left & (1 => Right);
end "*";
 
function "*" (Left, Right : Primitive_Operation) return Func is
begin
return (Left, Right);
end "*";
end Functions;

The following is an example of use:

with Ada.Numerics.Elementary_Functions;  use Ada.Numerics.Elementary_Functions;
with Ada.Text_IO; use Ada.Text_IO;
with Functions;
 
procedure Test_Compose is
package Float_Functions is new Functions (Float);
use Float_Functions;
 
Sin_Arcsin : Func := Sin'Access * Arcsin'Access;
begin
Put_Line (Float'Image (Sin_Arcsin * 0.5));
end Test_Compose;
Output:
 5.00000E-01

Aikido[edit]

 
import math
 
function compose (f, g) {
return function (x) { return f(g(x)) }
}
 
var func = compose(Math.sin, Math.asin)
println (func(0.5)) // 0.5
 
 

ALGOL 68[edit]

Translation of: Python
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

Note: Returning PROC (REAL x)REAL: f1(f2(x)) from a function apparently violates standard ALGOL 68's scoping rules. ALGOL 68G warns about this during parsing, and then rejects during runtime.

MODE F = PROC(REAL)REAL; # ALGOL 68 is strong typed #
 
# As a procedure for real to real functions #
PROC compose = (F f, g)F: (REAL x)REAL: f(g(x));
 
OP (F,F)F O = compose; # or an OPerator that can be overloaded #
 
# Example use: #
F sin arc sin = compose(sin, arc sin);
print((sin arc sin(0.5), (sin O arc sin)(0.5), new line))
Output:
+.500000000000000e +0 +.500000000000000e +0

ALGOL 68 is a stack based language, and the following apparently does not violate it's scoping rules.

Works with: ALGOL 68 version Standard - Jan 1975 Boston SC allowed Partial Parametrization.
Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386
MODE F = PROC(REAL)REAL; # ALGOL 68 is strong typed #
 
# As a procedure for real to real functions #
PROC compose = (F f, g)F: ((F f2, g2, REAL x)REAL: f2(g2(x)))(f, g, ); # Curry #
 
PRIO O = 7;
OP (F,F)F O = compose; # or an OPerator that can be overloaded #
 
# Example use: #
F sin arc sin = compose(sin, arc sin);
print((sin arc sin(0.5), (sin O arc sin)(0.5), new line))

AntLang[edit]

/Apply g to exactly one argument
compose1: {f: x; g: y; {f[g[x]]}}
/Extra: apply to multiple arguments
compose: {f: x; g: y; {f[g apply args]}}

AppleScript[edit]

-- Compose two functions where each function is
-- a script object with a call(x) handler.
on compose(f, g)
script
on call(x)
f's call(g's call(x))
end call
end script
end compose
 
script sqrt
on call(x)
x ^ 0.5
end call
end script
 
script twice
on call(x)
2 * x
end call
end script
 
compose(sqrt, twice)'s call(32)
-- Result: 8.0

A limitation of AppleScript's handlers (functions), which can be seen in the example above, is that they are not in themselves composable first class objects, and have to be lifted into script objects before they can be composed or passed as arguments.

We can generalise this lifting with an mReturn or mInject function, which injects a handler into a script for us. This allows use to write higher-order composition and pipelining functions which take a pair (or sequence of) ordinary handlers as arguments, and return a first class script object. (We can also use mReturn to equip AppleScript with map and fold functions which take a list and an ordinary handler as arguments).

-- EXAMPLE: run-time composition of half(succ(root(x))) 
-- from a list of lower-order handlers (not directly composable)
 
on run {}
 
compose([half, succ, root])'s lambda(5)
 
--> 1.61803398875
end run
 
 
-- SAMPLE HANDLER FUNCTIONS (not composable first-class objects in themselves)
 
on root(x)
x ^ 0.5
end root
 
on succ(x)
x + 1
end succ
 
on half(x)
x / 2
end half
 
 
-- LIFTING HANDLER FUNCTIONS INTO SCRIPT WRAPPERS AND COMPOSING THEM
-- (the 'AppleScript higher-order function monad')
 
-- Monadic functions: 1. return/unit, 2. bind
 
-- RETURN/UNIT
-- Lift a 2nd class handler function into a 1st class script wrapper
-- f --> mf
on mReturn(f)
script
property lambda : f
end script
end mReturn
 
-- BIND
-- Return a new script wrapper in which a function g is composed
-- with the existing lambda of the mf script
-- mf -> (f -> mg) -> mg
on mBind(mf, g)
script
on lambda(x)
mReturn(g)'s lambda(mf's lambda(x))
end lambda
end script
end mBind
 
-- Right-left composition of a list of 2 or more unadorned handlers
-- into a method of a new script object
-- e.g. [half, succ, root] -> a script with the lambda: half(succ(root(x)))
-- [f] -> g
on compose(fs) -- assuming that fs contains two or more handlers
foldr(mBind, mReturn(item -1 of fs), items 1 thru -2 of fs)
end compose
 
 
-- Right fold (reduce right)
-- foldr :: (a -> b -> a) -> a -> [b] -> a
on foldr(f, startValue, xs)
set mf to mReturn(f)
 
set v to startValue
set lng to length of xs
repeat with i from lng to 1 by -1
set v to mf's lambda(v, item i of xs, i, xs)
end repeat
return v
end foldr
 
Output:
1.61803398875

Applesoft BASIC[edit]

10 F$ = "SIN"
20 DEF FN A(P) = ATN(P/SQR(-P*P+1))
30 G$ = "FN A"
40 GOSUB 100"COMPOSE
50 SA$ = E$
 
60 X = .5 : E$ = SA$
70 GOSUB 200"EXEC
80 PRINT R
90 END
 
100 E$ = F$ + "(" + G$ + "(X))" : RETURN : REMCOMPOSE F$ G$
 
200 D$ = CHR$(4) : FI$ = "TEMPORARY.EX" : M$ = CHR$(13)
210 PRINT D$"OPEN"FI$M$D$"CLOSE"FI$M$D$"DELETE"FI$
220 PRINT D$"OPEN"FI$M$D$"WRITE"FI$
230 PRINT "CALL-998:CALL-958:R="E$":CONT"
240 PRINT D$"CLOSE"FI$M$D$"EXEC"FI$:CALL-998:END:RETURN

Argile[edit]

Only works for functions taking real and returning real (double precision, 64 bits)

Works with: Argile version 1.0.0
use std, math
 
let my_asin = new Function (.:<any,real x>:. -> real {asin x})
let my__sin = new Function (.:<any,real x>:. -> real { sin x})
let sinasin = my__sin o my_asin
print sin asin 0.5
print *my__sin 0.0
print *sinasin 0.5
~my_asin
~my__sin
~sinasin
 
=: <Function f> o <Function g> := -> Function {compose f g}
 
.:compose <Function f, Function g>:. -> Function
use array
let d = (new array of 2 Function)
(d[0]) = f ; (d[1]) = g
let c = new Function (.:<array of Function fg, real x>:. -> real {
*fg[0]( *fg[1](x) )
}) (d)
c.del = .:<any>:.{free any}
c
 
class Function
function(any)(real)->(real) func
any data
function(any) del
 
=: * <Function f> <real x> := -> real
Cgen "(*("(f.func)"))("(f.data)", "(x)")"
 
.: del Function <Function f> :.
unless f.del is nil
call f.del with f.data
free f
=: ~ <Function f> := {del Function f}
 
.: new Function <function(any)(real)-\>real func> (<any data>):. -> Function
let f = new Function
f.func = func
f.data = data
f


AutoHotkey[edit]

contributed by Laszlo on the ahk forum

MsgBox % compose("sin","cos",1.5)
 
compose(f,g,x) { ; function composition
Return %f%(%g%(x))
}

BBC BASIC[edit]

      REM Create some functions for testing:
DEF FNsqr(a) = SQR(a)
DEF FNabs(a) = ABS(a)
 
REM Create the function composition:
SqrAbs = FNcompose(FNsqr(), FNabs())
 
REM Test calling the composition:
x = -2 : PRINT ; x, FN(SqrAbs)(x)
END
 
DEF FNcompose(RETURN f%, RETURN g%)
LOCAL f$, p% : DIM p% 7 : p%!0 = f% : p%!4 = g%
f$ = "(x)=" + CHR$&A4 + "(&" + STR$~p% + ")(" + \
\ CHR$&A4 + "(&" + STR$~(p%+4) + ")(x))"
DIM p% LEN(f$) + 4 : $(p%+4) = f$ : !p% = p%+4
= p%
Output:
-2        1.41421356

Bori[edit]

double sin (double v)	{ return Math.sin(v); }
double asin (double v) { return Math.asin(v); }
Var compose (Func f, Func g, double d) { return f(g(d)); }
 
void button1_onClick (Widget widget)
{
double d = compose(sin, asin, 0.5);
label1.setText(d.toString(9));
}
Output:
on Android phone:
0.500000000

Brat[edit]

compose = { f, g | { x | f g x } }
 
#Test
add1 = { x | x + 1 }
double = { x | x * 2 }
b = compose(->double ->add1)
p b 1 #should print 4

C[edit]

Only works for functions taking a double and returning a double:

#include <stdlib.h>
 
/* generic interface for functors from double to double */
typedef struct double_to_double {
double (*fn)(struct double_to_double *, double);
} double_to_double;
 
#define CALL(f, x) f->fn(f, x)
 
 
/* functor returned by compose */
typedef struct compose_functor {
double (*fn)(struct compose_functor *, double);
double_to_double *f;
double_to_double *g;
} compose_functor;
/* function to be used in "fn" in preceding functor */
double compose_call(compose_functor *this, double x) {
return CALL(this->f, CALL(this->g, x));
}
/* returns functor that is the composition of functors
f & g. caller is responsible for deallocating memory */

double_to_double *compose(double_to_double *f,
double_to_double *g) {
compose_functor *result = malloc(sizeof(compose_functor));
result->fn = &compose_call;
result->f = f;
result->g = g;
return (double_to_double *)result;
}
 
 
 
#include <math.h>
 
/* we can make functors for sin and asin by using
the following as "fn" in a functor */

double sin_call(double_to_double *this, double x) {
return sin(x);
}
double asin_call(double_to_double *this, double x) {
return asin(x);
}
 
 
 
#include <stdio.h>
 
int main() {
double_to_double *my_sin = malloc(sizeof(double_to_double));
my_sin->fn = &sin_call;
double_to_double *my_asin = malloc(sizeof(double_to_double));
my_asin->fn = &asin_call;
 
double_to_double *sin_asin = compose(my_sin, my_asin);
 
printf("%f\n", CALL(sin_asin, 0.5)); /* prints "0.500000" */
 
free(sin_asin);
free(my_sin);
free(my_asin);
 
return 0;
}

C++[edit]

#include <functional>
#include <cmath>
#include <iostream>
 
// functor class to be returned by compose function
template <class Fun1, class Fun2>
class compose_functor :
public std::unary_function<typename Fun2::argument_type,
typename Fun1::result_type>
{
protected:
Fun1 f;
Fun2 g;
 
public:
compose_functor(const Fun1& _f, const Fun2& _g)
: f(_f), g(_g) { }
 
typename Fun1::result_type
operator()(const typename Fun2::argument_type& x) const
{ return f(g(x)); }
};
 
// we wrap it in a function so the compiler infers the template arguments
// whereas if we used the class directly we would have to specify them explicitly
template <class Fun1, class Fun2>
inline compose_functor<Fun1, Fun2>
compose(const Fun1& f, const Fun2& g)
{ return compose_functor<Fun1,Fun2>(f, g); }
 
int main() {
std::cout << compose(std::ptr_fun(::sin), std::ptr_fun(::asin))(0.5) << std::endl;
 
return 0;
}
Works with: C++11
composing std::function
#include <iostream>
#include <functional>
#include <cmath>
 
template <typename A, typename B, typename C>
std::function<C(A)> compose(std::function<C(B)> f, std::function<B(A)> g) {
return [f,g](A x) { return f(g(x)); };
}
 
int main() {
std::function<double(double)> f = sin;
std::function<double(double)> g = asin;
std::cout << compose(f, g)(0.5) << std::endl;
 
return 0;
}
Works with: C++14

This much simpler version uses decltype(auto).

 
#include <iostream>
#include <math.h>
 
template <class F, class G>
decltype(auto) compose(F&& f, G&& g)
{
return [=](auto x) { return f(g(x)); };
}
 
int main() {
std::cout << compose(sin, asin)(0.5) << "\n";
return 0;
}
 
Works with: GCC
GCC's C++ library has a built-in compose function
#include <iostream>
#include <cmath>
#include <ext/functional>
 
int main() {
std::cout << __gnu_cxx::compose1(std::ptr_fun(::sin), std::ptr_fun(::asin))(0.5) << std::endl;
 
return 0;
}

C#[edit]

using System;
class Program
{
static void Main(string[] args)
{
Func<int, int> outfunc = Composer<int, int, int>.Compose(functA, functB);
Console.WriteLine(outfunc(5)); //Prints 100
}
static int functA(int i) { return i * 10; }
static int functB(int i) { return i + 5; }
class Composer<A, B, C>
{
public static Func<C, A> Compose(Func<B, A> a, Func<C, B> b)
{
return delegate(C i) { return a(b(i)); };
}
}
}

Clojure[edit]

Function composition is built in to Clojure. Simply call the comp function.

A manual implementation could look like this:

(defn compose [f g]
(fn [x]
(f (g x))))
 
; Example
(def inc2 (compose inc inc))
(println (inc2 5)) ; prints 7

CoffeeScript[edit]

 
compose = ( f, g ) -> ( x ) -> f g x
 
# Example
add2 = ( x ) -> x + 2
mul2 = ( x ) -> x * 2
 
mulFirst = compose add2, mul2
addFirst = compose mul2, add2
multiple = compose mul2, compose add2, mul2
 
console.log "add2 2 #=> #{ add2 2 }"
console.log "mul2 2 #=> #{ mul2 2 }"
console.log "mulFirst 2 #=> #{ mulFirst 2 }"
console.log "addFirst 2 #=> #{ addFirst 2 }"
console.log "multiple 2 #=> #{ multiple 2 }"
 
Output:
add2 2 #=> 4
mul2 2 #=> 4
mulFirst 2 #=> 6
addFirst 2 #=> 8
multiple 2 #=> 12

Or, extending the Function prototype.

 
Function::of = (f) -> (args...) => @ f args...
 
# Example
add2 = (x) -> x + 2
mul2 = (x) -> x * 2
 
mulFirst = add2.of mul2
addFirst = mul2.of add2
multiple = mul2.of add2.of mul2
 
console.log "add2 2 #=> #{ add2 2 }"
console.log "mul2 2 #=> #{ mul2 2 }"
console.log "mulFirst 2 #=> #{ mulFirst 2 }"
console.log "addFirst 2 #=> #{ addFirst 2 }"
console.log "multiple 2 #=> #{ multiple 2 }"
 

Output is identical.

Common Lisp[edit]

compose returns a function that closes on the lexical variables f and g.

(defun compose (f g) (lambda (x) (funcall f (funcall g x))))

Example use:

>(defun compose (f g) (lambda (x) (funcall f (funcall g x))))
COMPOSE
>(let ((sin-asin (compose #'sin #'asin))))
(funcall sin-asin 0.5))
0.5

This alternate solution, more ugly and more difficult, never closes on any lexical variables. Instead, it uses runtime evaluation to insert the values of f and g into new code. This is just a different way to create a closure.

(defun compose (f g)
(eval `(lambda (x) (funcall ',f (funcall ',g x))))

In this last example, a macro is used to compose any number of single parameter functions.

CL-USER> (defmacro compose (fn-name &rest args)
(labels ((rec1 (args)
(if (= (length args) 1)
`(funcall ,@args x)
`(funcall ,(first args) ,(rec1 (rest args))))))
`(defun ,fn-name (x) ,(rec1 args))))

Because this macro expands into a defun form, the function returned by compose is in the function namespace and the use of funcall is not necessary.

CL-USER> (compose f #'ceiling #'sin #'sqrt)
F
CL-USER> (compose g #'1+ #'abs #'cos)
G
CL-USER> (compose h #'f #'g)
H
CL-USER> (values (f pi) (g pi) (h pi))
1
2.0L0
1
CL-USER> 

D[edit]

import std.stdio;
 
T delegate(S) compose(T, U, S)(in T delegate(U) f,
in U delegate(S) g) {
return s => f(g(s));
}
 
void main() {
writeln(compose((int x) => x + 15, (int x) => x ^^ 2)(10));
writeln(compose((int x) => x ^^ 2, (int x) => x + 15)(10));
}
Output:
115
625

Delphi[edit]

Anonymous methods were introduced in Delphi 2009, so next code works with Delphi 2009 and above:

program AnonCompose;
 
{$APPTYPE CONSOLE}
 
type
TFunc = reference to function(Value: Integer): Integer;
 
function Compose(F, G: TFunc): TFunc;
begin
Result:= function(Value: Integer): Integer
begin
Result:= F(G(Value));
end
end;
 
var
Func1, Func2, Func3: TFunc;
 
begin
Func1:=
function(Value: Integer): Integer
begin
Result:= Value * 2;
end;
 
Func2:=
function(Value: Integer): Integer
begin
Result:= Value * 3;
end;
 
Func3:= Compose(Func1, Func2);
 
Writeln(Func3(6)); // 36 = 6 * 3 * 2
Readln;
end.

Déjà Vu[edit]

It is already defined in the standard library as $.

compose f g:
labda:
f g

Dylan[edit]

define method compose(f,g)
method(x) f(g(x)) end
end;

Ela[edit]

It is already defined in standard prelude as (<<) operator.

let compose f g x = f (g x)

E[edit]

def compose(f, g) {
return fn x { return f(g(x)) }
}

EchoLisp[edit]

 
;; By decreasing order of performance
;; 1) user definition : lambda and closure
 
(define (ucompose f g ) (lambda (x) ( f ( g x))))
(ucompose sin cos)
(🔒 λ (_x) (f (g _x)))
 
;; 2) built-in compose : lambda
 
(compose sin cos)
(λ (_#:g1002) (#apply-compose (#list #cos #sin) _#:g1002))
 
;; 3) compiled composition
 
(define (sincos x) (sin (cos x)))
sincos → (λ (_x) (⭕️ #sin (#cos _x)))
 
Output:
 
((ucompose sin cos) 3) → -0.8360218615377305
((compose sin cos) 3) → -0.8360218615377305
(sincos 3) → -0.8360218615377305
 

Ela[edit]

It is already defined in standard prelude as (<<) operator.

compose f g x = f (g x)

Elixir[edit]

Translation of: Erlang
defmodule RC do
def compose(f, g), do: fn(x) -> f.(g.(x)) end
 
def multicompose(fs), do: List.foldl(fs, fn(x) -> x end, &compose/2)
end
 
sin_asin = RC.compose(&:math.sin/1, &:math.asin/1)
IO.puts sin_asin.(0.5)
 
IO.puts RC.multicompose([&:math.sin/1, &:math.asin/1, fn x->1/x end]).(0.5)
IO.puts RC.multicompose([&(&1*&1), &(1/&1), &(&1*&1)]).(0.5)
Output:
0.5
2.0
16.0

Emacs Lisp[edit]

A lambda form can be constructed with the desired f and g inserted. The result is simply a list. A list starting with lambda is a function.

(defun compose (f g)
`(lambda (x) (,f (,g x))))
 
(let ((func (compose '1+ '1+)))
(funcall func 5))
=>
7

A similar thing can be done with a macro like the following. It differs in that the arguments should be unquoted symbols, and if they're expressions then they're evaluated on every call to the resulting lambda.

(defmacro compose (f g)
`(lambda (x) (,f (,g x))))
 
(let ((func (compose 1+ 1+)))
(funcall func 5))
=>
7

Another possibility is the cl.el lexical-let to hold f and g for use in a new lambda.

(eval-when-compile (require 'cl)) ;; for `lexical-let' macro
(defun compose (f g)
(lexical-let ((f f)
(g g))
(lambda (x)
(funcall f (funcall g x)))))
 
(let ((func (compose '1+ '1+)))
(funcall func 5))
=>
7

Erlang[edit]

-module(fn).
-export([compose/1, multicompose/2]).
 
compose(F,G) -> fun(X) -> F(G(X)) end.
 
multicompose(Fs) ->
lists:foldl(fun compose/2, fun(X) -> X end, Fs).

Using them:

1> (fn:compose(fun math:sin/1, fun math:asin/1))(0.5).
0.5
2> Sin_asin_plus1 = fn:multicompose([fun math:sin/1, fun math:asin/1, fun(X) -> X + 1 end]).
#Fun<tests.0.59446746>
82> Sin_asin_plus1(0.5).
1.5

F#[edit]

The most-used composition operator in F# is >>. It implements forward composition, i.e. f >> g is a function which calls f first and then calls g on the result.

The reverse composition operator <<, on the other hand, exactly fulfills the requirements of the compose function described in this task.

We can implement composition manually like this (F# Interactive session):

> let compose f g x = f (g x);;
 
val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b

Usage:

> let sin_asin = compose sin asin;;
 
val sin_asin : (float -> float)
 
> sin_asin 0.5;;
val it : float = 0.5

Factor[edit]

When passing functions around and creating anonymous functions, Factor uses so called quotations. There is already a word (compose) that provides composition of quotations.

( scratchpad ) [ 2 * ] [ 1 + ] compose .
[ 2 * 1 + ]
( scratchpad ) 4 [ 2 * ] [ 1 + ] compose call .
9

Fantom[edit]

 
class Compose
{
static |Obj -> Obj| compose (|Obj -> Obj| fn1, |Obj -> Obj| fn2)
{
return |Obj x -> Obj| { fn2 (fn1 (x)) }
}
 
public static Void main ()
{
double := |Int x -> Int| { 2 * x }
|Int -> Int| quad := compose(double, double)
echo ("Double 3 = ${double(3)}")
echo ("Quadruple 3 = ${quad (3)}")
}
}
 

Forth[edit]

: compose ( xt1 xt2 -- xt3 )
>r >r :noname
r> compile,
r> compile,
postpone ;
;
 
' 2* ' 1+ compose ( xt )
3 swap execute . \ 7

Fortran[edit]

Modern Fortran standard has (limited) kind of higher-order functions (as result, argument, and with one level of nested functions) and optional arguments, and this enables to compose the following function (it is impure because Fortran has no closures). For simple cases function calls may be just nested to achieve the effect of function composition, because in fortran nested calls f(g(d(x))) generate a hierarchic set of function calls and the result of each function is transmitted to its calling function in a standard way for all functions.

 
module functions_module
implicit none
private ! all by default
public :: f,g
 
contains
 
pure function f(x)
implicit none
real, intent(in) :: x
real :: f
f = sin(x)
end function f
 
pure function g(x)
implicit none
real, intent(in) :: x
real :: g
g = cos(x)
end function g
 
end module functions_module
 
module compose_module
implicit none
private ! all by default
public :: compose
 
interface
pure function f(x)
implicit none
real, intent(in) :: x
real :: f
end function f
 
pure function g(x)
implicit none
real, intent(in) :: x
real :: g
end function g
end interface
 
contains
 
impure function compose(x, fi, gi)
implicit none
real, intent(in) :: x
procedure(f), optional :: fi
procedure(g), optional :: gi
real :: compose
 
procedure (f), pointer, save :: fpi => null()
procedure (g), pointer, save :: gpi => null()
 
if(present(fi) .and. present(gi))then
fpi => fi
gpi => gi
compose = 0
return
endif
 
if(.not. associated(fpi)) error stop "fpi"
if(.not. associated(gpi)) error stop "gpi"
 
compose = fpi(gpi(x))
 
contains
 
end function compose
 
end module compose_module
 
program test_compose
use functions_module
use compose_module
implicit none
write(*,*) "prepare compose:", compose(0.0, f,g)
write(*,*) "run compose:", compose(0.5)
end program test_compose
 

FunL[edit]

import math.{sin, asin}
 
def compose( f, g ) = x -> f( g(x) )
 
sin_asin = compose( sin, asin )
 
println( sin_asin(0.5) )
Output:
0.5

GAP[edit]

Composition := function(f, g)
return x -> f(g(x));
end;
 
h := Composition(x -> x+1, x -> x*x);
h(5);
# 26

Go[edit]

// Go doesn't have generics, but sometimes a type definition helps
// readability and maintainability. This example is written to
// the following function type, which uses float64.
type ffType func(float64) float64
 
// compose function requested by task
func compose(f, g ffType) ffType {
return func(x float64) float64 {
return f(g(x))
}
}

Example use:

package main
 
import "math"
import "fmt"
 
type ffType func(float64) float64
 
func compose(f, g ffType) ffType {
return func(x float64) float64 {
return f(g(x))
}
}
 
func main() {
sin_asin := compose(math.Sin, math.Asin)
fmt.Println(sin_asin(.5))
}
Output:
0.5

Groovy[edit]

Test program:

final times2 = { it * 2 }
final plus1 = { it + 1 }
 
final plus1_then_times2 = times2 << plus1
final times2_then_plus1 = times2 >> plus1
 
assert plus1_then_times2(3) == 8
assert times2_then_plus1(3) == 7

Haskell[edit]

This is already defined as the . (dot) operator in Haskell.

compose f g x = f (g x)

Example use:

Prelude> let compose f g x = f (g x)
Prelude> let sin_asin = compose sin asin
Prelude> sin_asin 0.5
0.5

Icon and Unicon[edit]

Icon and Unicon don't have a lambda function or native closure; however, they do have co-expressions which are extremely versatile and can be used to achieve the same effect. The list of functions to compose can be a 'procedure', 'co-expression", or an invocable string (i.e. procedure name or unary operator). It will correctly handle compose(compose(...),..).

There are a few limitations to be aware of:

  • type(compose(f,g)) returns a co-expression not a procedure
  • this construction only handles functions of 1 argument (a closure construct is better for the general case)


The solution below can be adapted to work in Icon by reverting to the old syntax for invoking co-expressions.

   x @ f                      # use this syntax in Icon instead of the Unicon f(x) to call co-expressions
every push(fL := [],!rfL) # use this instead of reverse(fL) as the Icon reverse applies only to strings

See Icon and Unicon Introduction:Minor Differences for more information

procedure main(arglist)
h := compose(sqrt,abs)
k := compose(integer,"sqrt",ord)
m := compose("-",k)
every write(i := -2 to 2, " h=(sqrt,abs)-> ", h(i))
every write(c := !"[email protected]", " k=(integer,\"sqrt\",ord)-> ", k(c))
write(c := "1"," m=(\"-\",k) -> ",m(c))
end
 
invocable all # permit string invocations
 
procedure compose(fL[]) #: compose(f1,f2,...) returns the functional composition of f1,f2,... as a co-expression
local x,f,saveSource
 
every case type(x := !fL) of {
"procedure"|"co-expression": &null # procedures and co-expressions are fine
"string" : if not proc(x,1) then runnerr(123,fL) # as are invocable strings (unary operators, and procedures)
default: runerr(123,fL)
}
 
fL := reverse(fL) # reverse and isolate from mutable side-effects
cf := create { saveSource := &source # don't forget where we came from
repeat {
x := (x@saveSource)[1] # return result and resume here
saveSource := &source # ...
every f := !fL do x := f(x) # apply the list of 'functions'
}
}
return (@cf, cf) # 'prime' the co-expr before returning it
 
end
Output:
-2 h=(sqrt,abs)-> 1.414213562373095
-1 h=(sqrt,abs)-> 1.0
0 h=(sqrt,abs)-> 0.0
1 h=(sqrt,abs)-> 1.0
2 h=(sqrt,abs)-> 1.414213562373095
1 k=(integer,"sqrt",ord)-> 7
@ k=(integer,"sqrt",ord)-> 8
Q k=(integer,"sqrt",ord)-> 9
1 m=("-",k) -> -7

J[edit]

Solution:

compose =: @

Example:

f compose g

Of course, given that @ is only one character long and is a built-in primitive, there is no need for the cover function compose. And @ is not the only composition primitive; composition is a very important concept in J. For more details, see the talk page.

Tentative new example:

f=: >.@(1&o.)@%:
g=: 1&+@|@(2&o.)
h=: [email protected]

Example use:

   (f, g, h) 1p1
1 2 1

Note: 1&o. is sine (mnemonic: sine is an odd circular function), 2&o. is cosine (cosine is an even circular function), %: is square root, >. is ceiling, | is absolute value and 1&+ adds 1.

Java[edit]

public class Compose {
 
// Java doesn't have function type so we define an interface
// of function objects instead
public interface Fun<A,B> {
B call(A x);
}
 
public static <A,B,C> Fun<A,C> compose(final Fun<B,C> f, final Fun<A,B> g) {
return new Fun<A,C>() {
public C call(A x) {
return f.call(g.call(x));
}
};
}
 
public static void main(String[] args) {
Fun<Double,Double> sin = new Fun<Double,Double>() {
public Double call(Double x) {
return Math.sin(x);
}
};
Fun<Double,Double> asin = new Fun<Double,Double>() {
public Double call(Double x) {
return Math.asin(x);
}
};
 
Fun<Double,Double> sin_asin = compose(sin, asin);
 
System.out.println(sin_asin.call(0.5)); // prints "0.5"
}
}

Java 8[edit]

Java 8's Function interface already has a .compose() default method:

Works with: Java version 8+
import java.util.function.Function;
 
public class Compose {
public static void main(String[] args) {
Function<Double,Double> sin_asin = ((Function<Double,Double>)Math::sin).compose(Math::asin);
 
System.out.println(sin_asin.apply(0.5)); // prints "0.5"
}
}

Implementing it yourself as a static method:

Works with: Java version 8+
import java.util.function.Function;
 
public class Compose {
public static <A,B,C> Function<A,C> compose(Function<B,C> f, Function<A,B> g) {
return x -> f.apply(g.apply(x));
}
 
public static void main(String[] args) {
Function<Double,Double> sin_asin = compose(Math::sin, Math::asin);
 
System.out.println(sin_asin.apply(0.5)); // prints "0.5"
}
}

JavaScript[edit]

Simple composition of two functions[edit]

function compose(f, g) {
return function(x) {
return f(g(x));
};
}

Example:

var id = compose(Math.sin, Math.asin);
print(id(0.5)); // 0.5


Multiple composition (ES5)[edit]

Recursion apart, multiple composition can be written in at least two general ways in JS:

  1. Iteratively (faster to run, perhaps more fiddly to write)
  2. With a fold / reduction (see http://rosettacode.org/wiki/Catamorphism). The fold is arguably simpler to write and reason about, though not quite as fast to execute.


(function () {
'use strict';
 
 
// iterativeComposed :: [f] -> f
function iterativeComposed(fs) {
 
return function (x) {
var i = fs.length,
e = x;
 
while (i--) e = fs[i](e);
return e;
}
}
 
// foldComposed :: [f] -> f
function foldComposed(fs) {
 
return function (x) {
return fs
.reduceRight(function (a, f) {
return f(a);
}, x);
};
}
 
 
var sqrt = Math.sqrt,
 
succ = function (x) {
return x + 1;
},
 
half = function (x) {
return x / 2;
};
 
 
// Testing two different multiple composition ([f] -> f) functions
 
return [iterativeComposed, foldComposed]
.map(function (compose) {
 
// both functions compose from right to left
return compose([half, succ, sqrt])(5);
 
});
})();
 
Output:
[1.618033988749895, 1.618033988749895]

Joy[edit]

Composition is the default operation in Joy. The composition of two functions is the concatenation of those functions, in the order in which they are to be applied.

g f

jq[edit]

The equivalent in jq of a function with one argument is a 0-arity filter. For example, in jq, exp is the exponential function and can be evaluated like so: 0.5 | exp.

We therefore illustrate here how a function that composes two 0-arity filters can be written:

 
# apply g first and then f
def compose(f; g): g | f;
 

Example: 0.5 | compose(asin; sin)

In practice, "compose" is rarely used since, given two 0-arity filters, f and g, the expression "g|f" can be passed as an argument to other functions.

Julia[edit]

compose(f::Function, g::Function) = x->f(g(x))
Output:
julia> compose(asin,sin)(0.5)
0.5

K[edit]

Functions are automatically curried in K if called with missing arguments.

compose: [email protected]@z}

Example:

  sin_asin: compose[_sin;_asin]
sin_asin 0.5
0.5

LFE[edit]

 
(defun compose (f g)
(lambda (x)
(funcall f
(funcall g x))))
 
(defun compose (funcs)
(lists:foldl #'compose/2
(lambda (x) x)
funcs))
 
(defun check ()
(let* ((sin-asin (compose #'math:sin/1 #'math:asin/1))
(expected (math:sin (math:asin 0.5)))
(compose-result (funcall sin-asin 0.5)))
(io:format '"Expected answer: ~p~n" (list expected))
(io:format '"Answer with compose: ~p~n" (list compose-result))))
 

If you pasted those into the LFE REPL, you can do the following:

 
> (funcall (compose #'math:sin/1 #'math:asin/1)
0.5)
0.49999999999999994
> (funcall (compose `(,#'math:sin/1
,#'math:asin/1
,(lambda (x) (+ x 1))))
0.5)
1.5
> (check)
Expected answer: 0.49999999999999994
Answer with compose: 0.49999999999999994
ok
>
 

LOLCODE[edit]

LOLCODE supports first-class functions only insofar as they may be stored in variables and returned from other functions. Alas, given the current lack of support for either lambdas or closures, function composition can only be reasonably simulated with the help of a few global variables.

HAI 1.3
 
I HAS A fx, I HAS A gx
 
HOW IZ I composin YR f AN YR g
fx R f, gx R g
HOW IZ I composed YR x
FOUND YR I IZ fx YR I IZ gx YR x MKAY MKAY
IF U SAY SO
FOUND YR composed
IF U SAY SO
 
HOW IZ I incin YR num
FOUND YR SUM OF num AN 1
IF U SAY SO
 
HOW IZ I sqrin YR num
FOUND YR PRODUKT OF num AN num
IF U SAY SO
 
I HAS A incsqrin ITZ I IZ composin YR incin AN YR sqrin MKAY
VISIBLE I IZ incsqrin YR 10 MKAY BTW, prints 101
 
I HAS A sqrincin ITZ I IZ composin YR sqrin AN YR incin MKAY
VISIBLE I IZ sqrincin YR 10 MKAY BTW, prints 121
 
KTHXBYE

Lua[edit]

function compose(f, g) return function(...) return f(g(...)) end end

Mathematica / Wolfram Language[edit]

Built-in function that takes any amount of function-arguments:

Composition[f, g][x]
Composition[f, g, h, i][x]

gives back:

f[g[x]]
f[g[h[i[x]]]]

Custom function:

compose[f_, g_][x_] := f[g[x]]
compose[Sin, Cos][r]

gives back:

Sin[Cos[r]]

Composition can be done in more than 1 way:

Composition[f,g,h][x]
[email protected]@[email protected]
x//h//g//f

all give back:

f[g[h[x]]]

The built-in function has a couple of automatic simplifications:

Composition[f, Identity, g]
Composition[f, InverseFunction[f], h][x]

becomes:

f[g[x]]
h[x]

Maxima[edit]

/* built-in */
load(to_poly_solver);
 
compose_functions([sin, cos]);
/* lambda([%g0],sin(cos(%g0)))*/
 
/* An implementation, to show a use of buildq */
compose(f, g) := buildq([f, g], lambda([x], f(g(x))));

Nemerle[edit]

using System;
using System.Console;
using System.Math;
 
module Composition
{
Compose[T](f : T -> T, g : T -> T, x : T) : T
{
f(g(x))
}
 
Main() : void
{
def SinAsin = Compose(Sin, Asin, _);
WriteLine(SinAsin(0.5));
}
}

NewLISP[edit]

> (define (compose f g) (expand (lambda (x) (f (g x))) 'f 'g))
(lambda (f g) (expand (lambda (x) (f (g x))) 'f 'g))
> ((compose sin asin) 0.5)
0.5
 

Nim[edit]

import future
 
proc compose[A,B,C](f: A -> B, g: B -> C): A -> C = (x: A) => f(g(x))
 
proc plustwo(x: int): int = x + 2
proc minustwo(x: int): int = x - 2
 
var plusminustwo = compose(plustwo, minustwo)
echo plusminustwo(10)

Objective-C[edit]

Works with: Mac OS X version 10.6+

We restrict ourselves to functions that take and return one object.

#include <Foundation/Foundation.h>
 
typedef id (^Function)(id);
 
// a commodity for "encapsulating" double f(double)
typedef double (*func_t)(double);
Function encapsulate(func_t f) {
return ^(id x) { return @(f([x doubleValue])); };
}
 
Function compose(Function a, Function b) {
return ^(id x) { return a(b(x)); };
}
 
// functions outside...
double my_f(double x)
{
return x+1.0;
}
 
double my_g(double x)
{
return x*x;
}
 
 
int main()
{
@autoreleasepool {
 
Function f = encapsulate(my_f);
Function g = encapsulate(my_g);
 
Function composed = compose(f, g);
 
printf("g(2.0) = %lf\n", [g(@2.0) doubleValue]);
printf("f(2.0) = %lf\n", [f(@2.0) doubleValue]);
printf("f(g(2.0)) = %lf\n", [composed(@2.0) doubleValue]);
 
}
return 0;
}

Objeck[edit]

 
bundle Default {
class Test {
@f : static : (Int) ~ Int;
@g : static : (Int) ~ Int;
 
function : Main(args : String[]) ~ Nil {
compose := Composer(F(Int) ~ Int, G(Int) ~ Int);
compose(13)->PrintLine();
}
 
function : F(a : Int) ~ Int {
return a + 14;
}
 
function : G(a : Int) ~ Int {
return a + 15;
}
 
function : Compose(x : Int) ~ Int {
return @f(@g(x));
}
 
function : Composer(f : (Int) ~ Int, g : (Int) ~ Int) ~ (Int) ~ Int {
@f := f;
@g := g;
return Compose(Int) ~ Int;
}
}
}
 

prints: 42

OCaml[edit]

let compose f g x = f (g x)

Example use:

# let compose f g x = f (g x);;
val compose : ('a -> 'b) -> ('c -> 'a) -> 'c -> 'b = <fun>
# let sin_asin = compose sin asin;;
val sin_asin : float -> float = <fun>
# sin_asin 0.5;;
- : float = 0.5

Octave[edit]

function r = compose(f, g)
r = @(x) f(g(x));
endfunction
 
r = compose(@exp, @sin);
r(pi/3)

Oforth[edit]

Oforth uses RPN notation. Function composition of f and g is just calling :

g f

If a block is needed, a compose function can be implemented :

: compose(f, g)  #[ g perform f perform ] ;

Usage :

1.2 compose(#asin, #sin) perform
[ 1, 2, 3, 4, 5 ] compose(#[ map(#sqrt) ], #[ filter(#isEven) ]) perform

The last line returns : [1.4142135623731, 2]

Order[edit]

Order supplies the built-in function 8compose for this purpose. However, a manual implementation might be:

#include <order/interpreter.h>
 
#define ORDER_PP_DEF_8comp ORDER_PP_FN( \
8fn(8F, 8G, 8fn(8X, 8ap(8F, 8ap(8G, 8X)))) )

Interpreter limitations mean that local variables containing functions must be called with the 8ap operator, but the functions themselves are still first-class values.

Oz[edit]

declare
fun {Compose F G}
fun {$ X}
{F {G X}}
end
end
 
SinAsin = {Compose Float.sin Float.asin}
in
{Show {SinAsin 0.5}}

PARI/GP[edit]

Works with: PARI/GP version 2.4.2 and above
compose(f, g)={
x -> f(g(x))
};
 
compose(x->sin(x),x->cos(x)(1)

Usage note: In Pari/GP 2.4.3, this can be expressed more succinctly:

compose(sin,cos)(1)

Pascal[edit]

See Delphi

Perl[edit]

sub compose {
my ($f, $g) = @_;
 
sub {
$f -> ($g -> (@_))
};
}
 
use Math::Trig;
print compose(sub {sin $_[0]}, \&asin)->(0.5), "\n";

Perl 6[edit]

Works with: rakudo version 2015-09-30

The function composition operator is , U+2218 RING OPERATOR (with a "Texas" version o for the Unicode challenged). Here we compose a routine, an operator, and a lambda:

sub triple($n) { 3 * $n }
my &f = &triple&prefix:<->{ $^n + 2 };
say &f(5); # Prints "-21".

PHP[edit]

Works with: PHP version 5.3+
<?php
function compose($f, $g) {
return function($x) use ($f, $g) { return $f($g($x)); };
}
 
$trim_strlen = compose('strlen', 'trim');
echo $result = $trim_strlen(' Test '), "\n"; // prints 4
?>
Works with: PHP version pre-5.3 and 5.3+

works with regular functions as well as functions created by create_function()

<?php
function compose($f, $g) {
return create_function('$x', 'return '.var_export($f,true).'('.var_export($g,true).'($x));');
}
 
$trim_strlen = compose('strlen', 'trim');
echo $result = $trim_strlen(' Test '), "\n"; // prints 4
?>

PicoLisp[edit]

(de compose (F G)
(curry (F G) (X)
(F (G X)) ) )
(def 'a (compose inc dec))
(def 'b (compose 'inc 'dec))
(def 'c (compose '((A) (inc A)) '((B) (dec B))))
: (a 7)
-> 7
 
: (b 7)
-> 7
 
: (c 7)
-> 7

PostScript[edit]

PostScript functions typically pops operand stack for argument, so calling two functions one after another naturally makes a compound function, and making a compound requires just defing them:
/square { dup mul } def
/plus1 { 1 add } def
/sqPlus1{ square plus1 } def
 
% if the task really demands we make a function called "compose", well
/compose { def } def  % so now we can say:
/sqPlus1 { square plus1 } compose

Prolog[edit]

Works with SWI-Prolog and module lambda, written by Ulrich Neumerkel found there http://www.complang.tuwien.ac.at/ulrich/Prolog-inedit/lambda.pl

:- use_module(lambda).
 
compose(F,G, FG) :-
FG = \X^Z^(call(G,X,Y), call(F,Y,Z)).
 
Output:
 ?- compose(sin, asin, F), call(F, 0.5, Y).
F = \_G4586^_G4589^ (call(asin,_G4586,_G4597),call(sin,_G4597,_G4589)),
Y = 0.5.

PowerShell[edit]

You can simply call g inside f like this:

 
function g ($x) {
$x + $x
}
function f ($x) {
$x*$x*$x
}
f (g 1)
 

Or g and f can become paramaters of a new function fg

 
function fg (${function:f}, ${function:g}, $x) {
f (g $x)
}
fg f g 1
 

In both cases the answer is:

 8 

PureBasic[edit]

;Declare how our function looks like
Prototype.i Func(Arg.i)
 
; Make a procedure that composes any functions of type "Func"
Procedure Compose(*a.Func,*b.Func, x)
ProcedureReturn *a(*b(x))
EndProcedure
 
; Just a procedure fitting "Func"
Procedure f(n)
ProcedureReturn 2*n
EndProcedure
 
; Yet another procedure fitting "Func"
Procedure g(n)
ProcedureReturn n+1
EndProcedure
 
;- Test it
X=Random(100)
Title$="With x="+Str(x)
Body$="Compose(f(),g(), x) ="+Str(Compose(@f(),@g(),X))
MessageRequester(Title$,Body$)

Purity[edit]

 
data compose = f => g => $f . $g
 

Python[edit]

compose = lambda f, g: lambda x: f( g(x) )

Example use:

>>> compose = lambda f, g: lambda x: f( g(x) )
>>> from math import sin, asin
>>> sin_asin = compose(sin, asin)
>>> sin_asin(0.5)
0.5
>>>

Qi[edit]

Qi supports partial applications, but only when calling a function with one argument.

 
(define compose
F G -> (/. X
(F (G X))))
 
((compose (+ 1) (+ 2)) 3) \ (Outputs 6) \
 

Alternatively, it can be done like this:

 
(define compose F G X -> (F (G X)))
 
(((compose (+ 1)) (+ 2)) 3) \ (Outputs 6) \
 

R[edit]

compose <- function(f,g) function(x) { f(g(x)) }
r <- compose(sin, cos)
print(r(.5))

Racket[edit]

 
(define (compose f g)
(lambda (x) (f (g x))))
 

Also available as a compose1 builtin, and a more general compose where one function can produce multiple arguments that are sent the the next function in the chain. (Note however that this is rarely desired.)

REBOL[edit]

rebol [
Title: "Functional Composition"
Author: oofoe
Date: 2009-12-06
URL: http://rosettacode.org/wiki/Functional_Composition
]

 
; "compose" means something else in REBOL, therefore I use a 'compose-functions name.
 
compose-functions: func [
{compose the given functions F and G}
f [any-function!]
g [any-function!]
] [
func [x] compose [(:f) (:g) x]
]

Functions "foo" and "bar" are used to prove that composition actually took place by attaching their signatures to the result.

foo: func [x] [reform ["foo:" x]]
bar: func [x] [reform ["bar:" x]]
 
foo-bar: compose-functions :foo :bar
print ["Composition of foo and bar:" mold foo-bar "test"]
 
sin-asin: compose-functions :sine :arcsine
print [crlf "Composition of sine and arcsine:" sin-asin 0.5]
Output:
Composition of foo and bar: "foo: bar: test"

Composition of sine and arcsine: 0.5

REXX[edit]

compose: procedure;  parse arg f,g,x;  interpret 'return' f"(" g'(' x "))"
 
exit /*control never gets here, but this was added just in case.*/

Ruby[edit]

This compose method gets passed two Method objects or Proc objects

def compose(f,g)
lambda {|x| f[g[x]]}
end
s = compose(Math.method(:sin), Math.method(:cos))
p s[0.5] # => 0.769196354841008
 
# verify
p Math.sin(Math.cos(0.5)) # => 0.769196354841008

Scala[edit]

def compose[A](f: A => A, g: A => A) = { x: A => f(g(x)) }
 
def add1(x: Int) = x+1
val add2 = compose(add1, add1)

We can achieve a more natural style by creating a container class for composable functions, which provides the compose method 'o':

class Composable[A](f: A => A) {
def o (g: A => A) = compose(f, g)
}
 
implicit def toComposable[A](f: A => A) = new Composable(f)
 
val add3 = (add1 _) o add2
> (add2 o add3)(37)
res0: Int = 42

Scheme[edit]

(define (compose f g) (lambda (x) (f (g x))))
 
;; or:
 
(define ((compose f g) x) (f (g x)))
 

Example:

 
(display ((compose sin asin) 0.5))
(newline)
Output:
0.5

Sidef[edit]

func compose(f, g) {
func(x) { f(g(x)) };
};
var fg = compose(func(x){Math.sin(x)}, func(x){Math.cos(x)});
say fg(0.5); # => 0.7691963548410084218525147580510688880995

Slate[edit]

Function (method) composition is standard:

[| :x | x + 1] ** [| :x | x squared] applyTo: {3}

Smalltalk[edit]

| composer fg |
composer := [ :f :g | [ :x | f value: (g value: x) ] ].
fg := composer value: [ :x | x + 1 ]
value: [ :x | x * x ].
 
(fg value:3) displayNl.

Standard ML[edit]

This is already defined as the o operator in Standard ML.

fun compose (f, g) x = f (g x)

Example use:

- fun compose (f, g) x = f (g x);
val compose = fn : ('a -> 'b) * ('c -> 'a) -> 'c -> 'b
- val sin_asin = compose (Math.sin, Math.asin);
val sin_asin = fn : real -> real
- sin_asin 0.5;
val it = 0.5 : real

Swift[edit]

func compose<A,B,C>(f: (B) -> C, g: (A) -> B) -> (A) -> C {
return { f(g($0)) }
}
 
let sin_asin = compose(sin, asin)
println(sin_asin(0.5))
Output:
0.5

Tcl[edit]

Works with: Tcl version 8.5

This creates a compose procedure that returns an anonymous function term that should be expanded as part of application to its argument.

package require Tcl 8.5
namespace path {::tcl::mathfunc}
 
proc compose {f g} {
list apply [list {f g x} {{*}$f [{*}$g $x]}] $f $g]
}
 
set sin_asin [compose sin asin]
{*}$sin_asin 0.5 ;# ==> 0.5
{*}[compose abs int] -3.14 ;# ==> 3

UNIX Shell[edit]

Each function takes its argument from standard input, and puts its result to standard output. Then the composition of f and g is a shell pipeline, c() { g | f; }.

Works with: Bourne Shell
compose() {
eval "$1() { $3 | $2; }"
}
 
downvowel() { tr AEIOU aeiou; }
upcase() { tr a-z A-Z; }
compose c downvowel upcase
echo 'Cozy lummox gives smart squid who asks for job pen.' | c
# => CoZY LuMMoX GiVeS SMaRT SQuiD WHo aSKS FoR JoB PeN.
Works with: Bourne Again SHell

This solution uses no external tools, just Bash itself.

 
#compose a new function consisting of the application of 2 unary functions
 
compose () { f="$1"; g="$2"; x="$3"; "$f" "$("$g" "$x")";}
 
 
chartolowervowel()
# Usage: chartolowervowel "A" --> "a"
 
#Based on a to_upper script in Chris F. A. Johnson's book Pro Bash Programming Ch7. String Manipulation
#(with minor tweaks to use local variables and return the value of the converted character
#http://cfajohnson.com/books/cfajohnson/pbp/
#highly recommended I have a copy and have bought another for a friend
{
 
local LWR="";
 
case $1 in
A*) _LWR=a ;;
# B*) _LWR=b ;;
# C*) _LWR=c ;;
# D*) _LWR=d ;;
E*) _LWR=e ;;
# F*) _LWR=f ;;
# G*) _LWR=g ;;
# H*) _LWR=h ;;
I*) _LWR=i ;;
# J*) _LWR=j ;;
# K*) _LWR=k ;;
# L*) _LWR=L ;;
# M*) _LWR=m ;;
# N*) _LWR=n ;;
O*) _LWR=o ;;
# P*) _LWR=p ;;
# Q*) _LWR=q ;;
# R*) _LWR=r ;;
# S*) _LWR=s ;;
# T*) _LWR=t ;;
U*) _LWR=u ;;
# V*) _LWR=v ;;
# W*) _LWR=w ;;
# X*) _LWR=x ;;
# Y*) _LWR=y ;;
# Z*) _LWR=z ;;
*) _LWR=${1%${1#?}} ;;
esac;
echo "$_LWR";
}
 
strdownvowel()
# Usage: strdownvowel "STRING" --> "STRiNG"
 
#Based on an upword script in Chris F. A. Johnson's book Pro Bash Programming Ch7. String Manipulation
#(with minor tweaks to use local variables and return the value of the converted string
#http://cfajohnson.com/books/cfajohnson/pbp/
#highly recommended I have a copy and have bought another for a friend
 
{
local _DWNWORD=""
local word="$1"
while [ -n "$word" ] ## loop until nothing is left in $word
do
chartolowervowel "$word" >> /dev/null
_DWNWORD=$_DWNWORD$_LWR
word=${word#?} ## remove the first character from $word
 
done
Echo "$_DWNWORD"
}
 
 
 
 
chartoupper()
# Usage: chartoupper "s" --> "S"
 
#From Chris F. A. Johnson's book Pro Bash Programming Ch7. String Manipulation
#(with minor tweaks to use local variables and return the value of the converted character
#http://cfajohnson.com/books/cfajohnson/pbp/
#highly recommended I have a copy and have bought another for a friend
{
local UPR="";
 
case $1 in
a*) _UPR=A ;;
b*) _UPR=B ;;
c*) _UPR=C ;;
d*) _UPR=D ;;
e*) _UPR=E ;;
f*) _UPR=F ;;
g*) _UPR=G ;;
h*) _UPR=H ;;
i*) _UPR=I ;;
j*) _UPR=J ;;
k*) _UPR=K ;;
l*) _UPR=L ;;
m*) _UPR=M ;;
n*) _UPR=N ;;
o*) _UPR=O ;;
p*) _UPR=P ;;
q*) _UPR=Q ;;
r*) _UPR=R ;;
s*) _UPR=S ;;
t*) _UPR=T ;;
u*) _UPR=U ;;
v*) _UPR=V ;;
w*) _UPR=W ;;
x*) _UPR=X ;;
y*) _UPR=Y ;;
z*) _UPR=Z ;;
*) _UPR=${1%${1#?}} ;;
esac;
echo "$_UPR";
}
 
strupcase()
# Usage: strupcase "string" --> "STRING"
 
#Based on an upword script in Chris F. A. Johnson's book Pro Bash Programming Ch7. String Manipulation
#(with minor tweaks to use local variables and return the value of the converted string
#http://cfajohnson.com/books/cfajohnson/pbp/
#highly recommended I have a copy and have bought another for a friend
 
{
local _UPWORD=""
local word="$1"
while [ -n "$word" ] ## loop until nothing is left in $word
do
chartoupper "$word" >> /dev/null
_UPWORD=$_UPWORD$_UPR
word=${word#?} ## remove the first character from $word
 
done
Echo "$_UPWORD"
}
 
compose strdownvowel strupcase "Cozy lummox gives smart squid who asks for job pen."
# --> CoZY LuMMoX GiVeS SMaRT SQuiD WHo aSKS FoR JoB PeN.


es[edit]

With shell pipelines:

fn compose f g {
result @ {$g | $f}
}
 
fn downvowel {tr AEIOU aeiou}
fn upcase {tr a-z A-Z}
fn-c = <={compose $fn-downvowel $fn-upcase}
echo 'Cozy lummox gives smart squid who asks for job pen.' | c
# => CoZY LuMMoX GiVeS SMaRT SQuiD WHo aSKS FoR JoB PeN.

With function arguments:

fn compose f g {
result @ x {result <={$f <={$g $x}}}
}
 
fn downvowel x {result `` '' {tr AEIOU aeiou <<< $x}}
fn upcase x {result `` '' {tr a-z A-Z <<< $x}}
fn-c = <={compose $fn-downvowel $fn-upcase}
echo <={c 'Cozy lummox gives smart squid who asks for job pen.'}
# => CoZY LuMMoX GiVeS SMaRT SQuiD WHo aSKS FoR JoB PeN.

Unlambda[edit]

``s`ksk

Ursala[edit]

Functional composition is a built in operation expressible as f+g for functions f and g, hence hardly worth defining. However, it could be defined without using the operator like this.

compose("f","g") "x" = "f" "g" "x"

test program:

#import nat
#cast %n
 
test = compose(successor,double) 3
Output:
7

VBScript[edit]

I'm not convinced that this is really a 'closure'. It looks to me more like a cute trick with Eval().

Implementation

 
option explicit
class closure
 
private composition
 
sub compose( f1, f2 )
composition = f2 & "(" & f1 & "(p1))"
end sub
 
public default function apply( p1 )
apply = eval( composition )
end function
 
public property get formula
formula = composition
end property
 
end class
 

Invocation

 
dim c
set c = new closure
 
c.compose "ucase", "lcase"
wscript.echo c.formula
wscript.echo c("dog")
 
c.compose "log", "exp"
wscript.echo c.formula
wscript.echo c(12.3)
 
function inc( n )
inc = n + 1
end function
 
c.compose "inc", "inc"
wscript.echo c.formula
wscript.echo c(12.3)
 
function twice( n )
twice = n * 2
end function
 
c.compose "twice", "inc"
wscript.echo c.formula
wscript.echo c(12.3)
 
Output:
lcase(ucase(p1))
dog
exp(log(p1))
12.3
inc(inc(p1))
14.3
inc(twice(p1))
25.6

Wortel[edit]

The @ operator applied to a array literal will compose the functions in the array and ^ with a group literal will do the same, but also quotes operators.

! @[f g] x ; f(g(x))
! ^(f g) x ; f(g(x))

Defining the compose function

@var compose &[f g] &x !f!g x

zkl[edit]

Utils.Helpers.fcomp('+(1),'*(2))(5) //-->11
Which is implemented with a closure (.fp), T is a read only list and L(x).apply(L(g,f)) --> L(g(f(x)):
fcn fcomp(f,g,h,etc){
fcn(x,hgf){ T(x).apply(hgf)[0] }.fp1(vm.arglist.reverse()); }