D is a complex multi-paradigm programming language. At the same time, if you know C programming language and you want to start using D then you just need to look through some examples.

Hellow Wolrd!

C programs can be easily translated to D. The following program prints “Hello, World!” to the standard output.

#include <stdio.h>

const char* const nullTerminatedStrPtr = "Hello, World!";

int main(void)
    return 0;

D doesn’t have a preprocessor. Use import core.stdc.MODULE; construction to import MODULE from the C Standard library.

import core.stdc.stdio;

// terminates with a null character
immutable char[] nullTerminatedStr = "Hello, World!\0"; 

int main()
    // calls external C function
    return 0;

Module core.stdc.stdio contains the puts prototype:

extern(C) @system nothrow @nogc int puts(in char* s);

Common D “Hello, World!” program which is based on Phobos looks more simple:

Deduces the type of a declared variable from its initialization 
immutable str = "Hello, World!";

void main()
    // Scoped and selective imports can be used.
    import std.stdio : writeln;

Phobos is the standard runtime library that comes with the D language compiler.

See also

To find a collection of common C techniques, and to find out how to do the corresponding task in D click here. However most of them can be implemented in C style.

Simple project with dub

DUB is a build tool for D projects with support for automatically retrieving dependencies and integrating them in the build process. The design emphasis is on maximum simplicity for simple projects, while providing the opportunity to customize things when needed.

Plotting with matplotlib (python)

These are two projects that can be used with the D programming language:

  • Plotcli is a command line application written in D that can create plots from text/csv files and from piped data, making it useful during data analysis.
  • PLplot is a cross-platform software package written in C for creating scientific plots. It includes low-level D bindings.

But these two are not so convenient to use, in comparison with matplotlib.

matplotlib is a python 2D plotting library which produces publication quality figures in a variety of hardcopy formats and interactive environments across platforms. matplotlib can be used in python scripts, the python and ipython shell, web application servers, and different graphical user interface toolkits. To integrate with python the PyD package can be used.

PyD is a library that provides seamless interoperability between the D programming language and Python. The minimal configuration file for this example is

    "name": "plotting_example",
    "dependencies": {
        "pyd": "~>0.9.4",
    "subConfigurations": {
        "pyd": "python34",


The python should be installed. PyD searches the version of the python that is noted in the sub-configuration ("pyd": "python34" in this example). For more information, see the PyD’s dub configuration file.

The following program reads data from a file and shows the histogram.

import pyd.pyd;
import pyd.embedded;
import pyd.extra;

`d_to_python_numpy_ndarray` converts a D array to numpy.ndarray.
`toNumpyArray` is only an alias.
alias toNumpyArray = d_to_python_numpy_ndarray;

A static constructor is a function that performs initializations of 
thread local data before the `main()` function gets control for the 
main thread. 

Shared static constructors are executed before any static 
constructors, and are intended for initializing any shared global
shared static this() {
    //initializes PyD package.

void main()
    auto pythonContext = new InterpContext();
    Uniform Function Call Syntax (UFCS) 
    is used in the following line of code.
    Equivalent code would be just:
    pythonContext.sample = toNumpyArray(readData("view/data.txt"));
    pythonContext.sample = "data/data.txt".readData.toNumpyArray;

double[] readData(string file)
    import std.algorithm.iteration : map, splitter;
    import std.array : array;
    import std.conv : to;
    import std.file : readText;

    return file
        .readText   //Reads the contents of a text file into a string.
        .splitter        //Lazily splits words.
        .map!(to!double) //Lazily converts words to doubles.
        .array;          //Creates an array.

immutable script = `
import numpy as np
import matplotlib
import matplotlib.pyplot as plt

num_bins = 50
ax = plt.subplot()
n, bins, patches = ax.hist(sample, num_bins, normed=1)
Histogram image

Web Application

Web application is a pretty good example of the last chapters of the book, where the reader is suggested to make use of the means of expression in the language. As a rule, web application is a complex product, both in terms of knowledge of the language and in terms of complexity used in this library.

And this example is no exception. Then why do people who want to learn this language very quickly have a need in it? Many of them have a reason and it is that they need to integrate quickly programs written in D with other services, programming languages and databases.

The article “Creating a simple restful web app with node.js, Express, and MongoDB” by Christopher Buecheler is taken as a basis for this example.


To create a skeleton web application, run:

$ dub vibed-mongo vibe.d

This will make in directory vibed-mongo with a minimal HTTP server based on vibe.d.

The configuration file dub.json will look something like this:

    "name": "vibed-mongo",
    "dependencies": {
        "vibe-d": "~>0.7.23"
    "versions": ["VibeDefaultMain"]

The version "VibeDefaultMain" includes the main function defined by default.

The project has the following structure:

After installing MongoDB run Mongo servers

$ mongod

In another console run Mongo console

$ mongo
> use vibed
switched to db vibed
> db.createCollection("userlist", {autoIndexID : true})
{ "ok" : 1 }
> db.userlist.insert({
    'username' : 'test1',
    'email' : 'test1@test.com',
    'fullname' : 'Bob Smith',
    'age' : 27,
    'location' : 'San Francisco',
    'gender' : 'male'
WriteResult({ "nInserted" : 1 })
> exit

The script above will create a vibed database with a userlist collection, which will contain one record.


Comparing with the original article global.js was slightly changed:

            type: 'POST',
            data: newUser,
            url: '/users/adduser',
            success: function(data){
                $('#addUser fieldset input').val('');
            error: function(xhr, textStatus, error){
            type: 'DELETE',
            url: '/users/deleteuser/' + $(this).attr('rel'),
            success: function(data){
            error: function(xhr, textStatus, error){


vibe.d is a good example of the use of declarative programming with D. Service performs an insert, select and remove operations for user entries at a MongoDB.

module service;

import std.conv;
import vibe.d;

class MongoService
    private MongoCollection collection;
    const string title;

    this(MongoCollection collection, string title = null)
        this.collection = collection;
        this.title = title;

    void index()
        logInfo("MongoService: GET /");
        render!("index.dt", title);

    void postAdduser(
        string username,
        string email,
        string fullname,
        int age,
        string location,
        string gender,
        HTTPServerResponse res,
        logInfo(text("MongoService: POST /adduser : ", username));
        import vibe.utils.validation;
        auto bson = Json.emptyObject;
        bson.username = validateUserName(username);
        bson.email = email.validateEmail;
        bson.fullname = fullname;
        enforce(age < 200 && age >= 0, "wrong age");
        bson.age = age;
        bson.location = location;
        bson.gender = gender.toLower;

    Json getUserlist()
        logInfo("MongoService: GET /userlist");
        return Json(collection.find!Json.array);

    void pullOutUser(
        BsonObjectID _id,
        HTTPServerResponse res,
        logInfo(text("MongoService: GET /deleteuser/", _id));
        collection.remove(["_id": _id]);


Following static constructor connects vebid MongoDB, creates vibe.d server and implements an error handler.

import vibe.d;
import service;

shared static this()
    immutable string title = "vibe.d web app";

    logInfo("Connecting to DB...");
    auto db = connectMongoDB("localhost").getDatabase("vibed");
    auto collection = db["userlist"];

    logInfo("Querying DB...");
    //Bson query = Bson(["name" : Bson("hans")]);
    auto result = collection.find();

    logInfo("Iterating results...");
    foreach (i, doc; result)
        logInfo("Item %d: %s", i, doc.toJson().toString());

    auto mongoService = new MongoService(collection, title);

    auto mongoServiceSettings = new WebInterfaceSettings;
    mongoServiceSettings.urlPrefix = "/users";

    auto router = new URLRouter;
    router.registerWebInterface(mongoService, mongoServiceSettings);
        .get("/", (req, res)  
            { res.redirect("/users"); } )
        .get("*", serveStaticFiles("public/"));

    auto settings = new HTTPServerSettings;
        bindAddresses = [""];
        port = 8080;
        errorPageHandler = 
            (req, res, error)
                with(error) res.writeBody(
                    format("Code: %s\n Message: %s\n Exception: %s",
                        error.exception ? error.exception.msg : ""));

    listenHTTP(settings, router);