dyaml/docs/tutorials/getting_started.md

Getting started

Welcome to D:YAML! D:YAML is a YAML parser library for the D programming language. This tutorial will explain how to set D:YAML up and use it in your projects.

This is meant to be the simplest possible introduction to D:YAML. Some of this information might already be known to you. Only basic usage is covered.

Setting up

Install the DMD compiler

Digital Mars D compiler, or DMD, is the most commonly used D compiler. You can find its newest version here. Download the version of DMD for your operating system and install it.

Note: Other D compilers exist, such as GDC and LDC.

Install dub

dub is a build system and package manager for D. It is the standard way to manage D projects and their dependencies, compilation and so on.

DMD may include DUB in future releases, but at this point we need to install it separately. See installation instructions.

Your first D:YAML project

Create a directory for your project and in that directory, create a new file named input.yaml and paste this code into the file:

Hello World : [Hello, World]
Answer: 42

This will serve as input for our example.

Now we need to parse it. Create a new file with name main.d. Paste following code into the file:

import std.stdio;
import yaml;

void main()
{
    //Read the input.
    Node root = Loader("input.yaml").load();

    //Display the data read.
    foreach(string word; root["Hello World"])
    {
        writeln(word);
    }
    writeln("The answer is ", root["Answer"].as!int);

    //Dump the loaded document to output.yaml.
    Dumper("output.yaml").dump(root);
}

Explanation of the code

First, we import the dyaml.all module. This is the only D:YAML module you need to import - it automatically imports all needed modules.

Next we load the file using the Loader.load() method. Loader is a struct used for parsing YAML documents. The load() method loads the file as one YAML document, or throws YAMLException, D:YAML exception type, if the file could not be parsed or contains more than one document. Note that we don't do any error checking here in order to keep the example as simple as possible.

Node represents a node in a YAML document. It can be a sequence (array), mapping (associative array) or a scalar (value). Here the root node is a mapping, and we use the index operator to get subnodes with keys "Hello World" and "Answer". We iterate over the former, as it is a sequence, and use the Node.as() method on the latter to read its value as an integer.

You can iterate over a mapping or sequence as if it was an associative or normal array, respectively. If you try to iterate over a scalar, it will throw a YAMLException.

You can iterate using Node as the iterated type, or specify the type iterated nodes are expected to have. D:YAML will automatically convert to that type if possible. Here we specify the string type, so we iterate over the "Hello World" sequence as an array of strings. If it is not possible to convert to iterated type, a YAMLException is thrown. For instance, if we specified int here, we would get an error, as "Hello" cannot be converted to an integer.

The Node.as() method is used to read value of a scalar node as specified type. If the scalar does not have the specified type, D:YAML will try to convert it, throwing YAMLException if not possible.

Finally we dump the document we just read to output.yaml with the Dumper.dump() method. Dumper is a struct used to dump YAML documents. The dump() method writes one or more documents to a file, throwing YAMLException if the file could not be written to.

D:YAML tries to preserve style information in documents so e.g. [Hello, World] is not turned into:

- Hello
- World

However, comments are not preserved and neither are any extra formatting whitespace that doesn't affect the meaning of YAML contents.

Compiling

We're going to use dub, which we installed at the beginning, to compile our project.

Create a file called dub.json with the following contents:

{
    "name": "getting-started",
    "targetType": "executable",
    "sourceFiles": ["main.d"],
    "mainSourceFile": "main.d",
    "dependencies":
    {
        "dyaml": { "version" : "~>0.5.0" },
    },
}

This file tells dub that we're building an executable called getting-started from a D source file main.d, and that our project depends on D:YAML 0.5.0 or any newer, bugfix release of D:YAML 0.5 . DUB will automatically find and download the correct version of D:YAML when the project is built.

Now run the following command in your project's directory:

dub build

dub will automatically download D:YAML and compile it, and then then it will compile our program. This will generate an executable called getting-started or getting-started.exe in your directory. When you run it, it should produce the following output:

Hello
World
The answer is 42

Conclusion

You should now have a basic idea about how to use D:YAML. To learn more, look at the API documentation and other tutorials. You can find code for this example in the example/getting_started directory in the package.