Configy: Simple, efficient YAML -> struct converter

Why (would I use this) ?

Any decently-sized application needs to eventually be configured. When an application's complexity is past the threshold where its options can be managed by command line arguments alone, one need to add a configuration file.

Enter a myriad of questions:

• What format do I use ?
• How do I document it ?
• What are sensible default values ?
• How do I accurately report errors ?

And so on. This framework provide an opiniated answer to all those questions.

How (do I use this) ?

Let's start with an hello world:

module helloworld;

import configy.read;

import std.stdio;
import std.typecons;
import core.thread;

struct Config
{
    /// The name to greet
    string name;

    /// Other people to greet
    SetInfo!(string[]) extra_names;

    /// How often to greet the user
    Duration frequency = 1.seconds;

    /// How many times should we repeat the greeting (0: no limit)
    @Optional size_t limit;
}

int main (string[] args)
{
    // `parseConfigSimple` will print to `stderr` if an error happened
    Nullable!Config configN = parseConfigFileSimple!Config("config.yaml");
    if (configN.isNull())
        return 1;
    auto config = configN.get();

    // Your program goes here
    for (size_t i = 0; config.limit == 0 || i < config.limit; ++i)
    {
        writeln("Hello World to you ", config.name);
        if (config.extra_names.set)
            writefln("And to you too, %-(%s, %)", config.extra_names.value);
        Thread.sleep(config.frequency);
    }
    return 0;
}

This code can be found here, along with the associated configuration:

name: "Lord Commander Tenant"
extra_names:
  - The Gary
  - Mooncake

This demonstrate a few important properties of the config parser:

• Any field with an initializer (different from .init) is considered optional;
• Fields can be set to optional through to Optional UDA if their init value should be their default;
• Using SetInfo allows on to have optional fields that carry whether they were set or not;
• Duration is natively parsed;

Full documentation

The full, up-to-date API documentation can be found here. A list of example for common use cases can be found here.

Rich error reporting

Because humans make mistakes, and configuration are aimed towards humans, special care is given to errors. Using the helloworld example, here are some of the errors that would be reported:

# Missing required option `name`
limit: 1

# Wrong key: Should be `name`, not `username`
username: "Lord Commander Tenant"

This error is optional, but enabled by default. See strict mode in documentation.