Anonymous functions in D

We can use anonymous functions in D.

delegate keyword

Example with argument:

import std.stdio;

void evenNumbers(int[] numbers, void delegate(int) callback) {
 foreach (int number; numbers) {
 if (number % 2 == 0) callback(number);
 }
}

void main() {
 auto numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 evenNumbers(numbers, (num) => writeln(num));
 // or numbers.evenNumbers((num) => writeln(num));
 // or numbers.evenNumbers((num) { writeln(num); });
}

In this case (num) => writeln(num) is an argument, callable function.

Output:

% rdmd delegate.d
2
4
6
8
10

Alternative syntax, ruby's blocks style:

import std.stdio, std.string, std.file, std.path;

void withingDir(string dir, void delegate() callback) {
 auto cwd = absolutePath(".").chomp(".");
 writeln("Current directory is: ", cwd);
 chdir(dir);
 callback();
 chdir(cwd);
}

void main() {
 withingDir("/var", {
 writefln("I'm in %s", absolutePath(".").chomp("."));
 });
}

Output:

% rdmd delegate.d
Current directory is: /Users/pavel/d-try/
I'm in /private/var/

If I want to have more rubish syntax I can make like this:

alias void delegate() Block;

void withingDir(string dir, Block callback) {
 ...
}

function keyword.

I don't know what is a difference between delegate and function, they doing same job.

Example one:

import std.stdio;

void evenNumbers(int[] numbers, void function(int) callback) {
 foreach (int number; numbers) {
 if (number % 2 == 0) callback(number);
 }
}

void main() {
 auto numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 numbers.evenNumbers(function (num) {
 writeln(num);
 });
}

Example two:

import std.stdio, std.string, std.file, std.path;

void withingDir(string dir, void function() callback) {
 auto cwd = absolutePath(".").chomp(".");
 writeln("Current directory is: ", cwd);
 chdir(dir);
 callback();
 chdir(cwd);
}

void main() {
 withingDir("/var", function () {
 writefln("I'm in %s", absolutePath(".").chomp("."));
 });
 // or withingDir("/var", { ... });
}

Read more:

• http://ddili.org/ders/d.en/lambda.html