A file to define environment dependencies for applications

Different programming communities have ways to document dependencies required for a program or application to function.

Ruby has the Gemfile, Python has requirements.txt, and Node has package.json.

The .dependencies file expands on those good ideas in order to provide system-level dependency documentation.

In a file named .dependencies at the root of your project:

ruby: "~> 2.3.0", file: "Gemfile"
postgres: "~> 9.3.0"
python: "~> 2.7.0", file: "requirements.txt"
node: "~> 5.5.0", file: "package.json"

Let's break it down, by focusing on first line:

1. ruby is the name of the system-level software being dependend on by your program
2. "~> 2.3.0" defines the version constraint for the ruby dependency, ~> means "loose" which is equivalent to saying "I want any version including 2.3.0 and above but below 2.4".
3. file is an optional key that allows you to specify where the system-level dependency can find its own dependencies, and in Ruby programs usually document their dependencies using a Bundler Gemfile.

The issue is that there are system-level dependencies that are often omitted from these dependency documentation systems. For instance it's rare for my Ruby applications to specify that they require PostgreSQL, Redis and ffmpeg in order to function.

There are OS-specific solutions like the Brewfile but they're only partially solving the problem because not everyone uses the same operating system (OS X in this case), and it's equally important to specify which version of the software you depend on.

The file name makes its purpose obvious. It starts with a . and will show up early in the file tree because the letter "d" is early in the alphabet.

Now let's talk about the contents of the .dependencies file.

Eventually this file will be read by machine more often than by humans but considering how crucial it is to developers, it needs to be as human-friendly as possible.

Parsing the file needs to be easy for both humans and machines. This is why it uses a simple key/value format. It does look like YAML, but it doesn't need to be. Not every machine out there can parse YAML.

  postgresql: "~> 9.3.0"

Why a quoted string for the version? Well, because Bundler's format is the best version specification format I know of. The string allows for complex modifications on the version number. It allows strict versions specifications ("9.1.3"), open-ended ("> 0.1.0"), ranges ([">= 4.1.0", "< 5.1"]), and even loose ones ("~> 9.3.0" meaning equal or greater than 9.3.0 but less than 9.4).

Finally there's a little bonus, a second value called file which hints at where each system-level dependency should look for its own dependencies. This feels important to me because I don't intend this dependency specification file to replace all existing language-specific ones. I think there's a gap in our dependency documentations and I just want to fill it, not revolutionize everything.