Modulation - Explicit Dependency Management for Ruby

Modulation | mɒdjʊˈleɪʃ(ə)n | Music - a change from one key to another in a piece of music.

INSTALL | GUIDE | API | EXAMPLES | RDOC

Modulation provides an alternative way of organizing your Ruby code. Modulation lets you explicitly import and export declarations in order to better control dependencies in your codebase. Modulation helps you refrain from littering the global namespace with a myriad modules, or complex multi-level nested module hierarchies.

Using Modulation, you will always be able to tell where a class or module comes from, and you'll have full control over which parts of a module's code you wish to expose to the outside world. Modulation can also help you write Ruby code in a functional style, minimizing boilerplate code.

Note: Modulation is not a replacement for RubyGems. Rather, Modulation is intended for managing dependencies between source files inside your Ruby applications. Though it does support loading gems that were written using Modulation, it is not intended as a comprehensive solution for using third-party libraries.

Features

Rationale

You're probably asking yourself "what the ****?" , but when your Ruby app grows and is split into multiple files loaded using #require, you'll soon hit some issues:

  • Once a file is #required, any class, module or constant in it is available to any other file in your codebase. All "globals" (classes, modules, constants) are loaded, well, globally, in a single namespace. Name conflicts are easy in Ruby.
  • To avoid class name conflicts, classes need to be nested under a single hierarchical tree, sometime reaching 4 levels or more. Just look at Rails.
  • Since a #required class or module can be loaded in any file and then made available to all files, it's easy to lose track of where it was loaded, and where it is used.
  • There's no easy way to hide implementation-specific classes or methods. Yes, there's #private, #private_constant etc, but by default everything is #public!
  • Extracting functionality is harder when modules are namespaced and dependencies are implicit.
  • Writing reusable functional code requires wrapping it in modules using class << self, def self.foo ..., extend self or include Singleton (the pain of implementing singletons in Ruby has been discussed before.)

There's a recent discussion on the Ruby bug tracker regarding possible solutions to the problem of top-level name collision. Hopefully, the present gem could contribute to an eventual "official" API.

Personally, I have found that managing dependencies with #require in large codebases is... not as elegant or painfree as I would expect from a first-class development environment. I also wanted to have a better solution for writing in a functional style.

So I came up with Modulation, a small gem that takes a different approach to organizing Ruby code: any so-called global declarations are hidden unless explicitly exported, and the global namespace remains clutter-free. All dependencies between source files are explicit, visible, and easy to understand.

Installing Modulation

You can install the Modulation using gem install, or add it to your Gemfile:

gem 'modulation'

Organizing your code with Modulation

Modulation builds on the idea of a Ruby Module as a "collection of methods and constants". Using modulation, each Ruby source file becomes a module. Modules usually export method and constant declarations (usually an API for a specific, well-defined functionality) to be shared with other modules. Modules can also import declarations from other modules. Anything not exported remains hidden inside the module and normally cannot be accessed from the outside.

Each source file is evaluated in the context of a newly-created Module instance, with some additional methods for introspection and miscellaneous operations such as hot reloading.

Modulation provides an alternative APIs for loading modules. Instead of using require and require_relative, you use import, import_map and other APIs.

Exporting declarations

Any class, module or constant be exported using #export:

export :User, :Session

class User
...
end

class Session
...
end

A module may also expose a set of methods without using class << self, for example when writing in a functional style:

seq.rb

export :fib, :luc

def fib(n)
  (0..1).include?(n) ? n : (fib(n - 1) + fib(n - 2))
end

def luc(n)
  (0..1).include?(n) ? (2 - n) : (luc(n - 1) + luc(n - 2))
end

app.rb

require 'modulation'
Seq = import('./seq')
puts Seq.fib(10)

Another way to export methods and constants is by passing a hash to #export:

module.rb

export(
  foo: :bar,
  baz: -> { 'hello' },
  MY_CONST: 42
)

def bar
  :baz
end

app.rb

m = import('./module')
m.foo #=> :baz
m.baz #=> 'hello'
m::MY_CONST #=> 42

Any capitalized key will be interpreted as a const, otherwise it will be defined as a method. If the value is a symbol, Modulation will look for the corresponding method or const definition and will treat the key as an alias.

The export method can be called multiple times. Its behavior is additive:

# this:
export :foo, :bar

# is the same as this:
export :foo
export :bar

Importing declarations

Declarations from another module can be imported using #import:

require 'modulation'
Models = import('./models')
...

user = Models::User.new(...)

...

Alternatively, a module interested in a single declaration from another module can use the following technique:

require 'modulation'
User = import('./models')::User
...

user = User.new(...)

Using tags to designate common subdirectories

Normally, module paths are always relative to the file calling the #import method, just like #require_relative. This can become a problem once you start moving your source files around. In addition, in applications where your source files are arranged in multiple directories, it can quickly become tedious to do stuff like Post = import('../models/post').

Modulation provides an alternative to relative paths in the form of tagged sources. A tagged source is simply a path associated with a label. For example, an application may tag lib/models simply as @models. Once tags are defined, they can be used when importing files, e.g. import('@models/post').

Importing all source files in a directory

To load all source files in a directory you can use #import_all:

import_all('./ext') # will load ./ext/kernel.rb, ./ext/socket.rb etc 

Groups of modules providing a uniform interface can also be loaded using #import_map:

API = import_map('./math_api') #=> hash mapping filenames to modules
API.keys #=> ['add', 'mul', 'sub', 'div']
API['add'].(2, 2) #=> 4

The #import_map takes an optional block to transform hash keys:

API = import_map('./math_api') { |name, mod| name.to_sym }
API.keys #=> [:add, :mul, :sub, :div]
API[:add].(2, 2) #=> 4

Importing methods into classes and objects

Modulation provides the #extend_from and #include_from methods to include imported methods in classes and objects:

module Sequences
  extend_from('./seq.rb')
end

Sequences.fib(5)

# extend integers
require 'modulation'
class Integer
  include_from('./seq.rb')

  def seq(kind)
    send(kind, self)
  end
end

5.seq(:fib)

The #include_from method accepts an optional list of symbols to import:

class Integer
  include_from './seq.rb', :fib
end

5.fib

Default exports

A module may wish to expose just a single class or constant, in which case it can use #export_default:

user.rb

export_default :User

class User
  ...
end

app.rb

require 'modulation'
User = import('./user')
User.new(...)

The default exported value can also be defined directly thus:

config.rb

export_default(
  host: 'localhost',
  port: 1234,
  ...
)

app.rb

require 'modulation'
config = import('./config')
db.connect(config[:host], config[:port])

Accessing a module's root namespace from nested modules within itself

The special constant MODULE allows you to access the containing module from nested modules or classes. This lets you call methods defined in the module's root namespace, or otherwise introspect the module:

export :AsyncServer

# Await a promise-like callable
def await
  calling_fiber = Fiber.current
  p = ->(v = nil) {calling_fiber.resume v}
  yield p
  Fiber.yield
end

class AsyncServer < SomeTCPServer
  def async_read
    MODULE.await {|p| on_read {|data| p.(data)}}
  end
end

Accessing the global namespace

If you need to access the global namespace inside a module just prefix the class name with double colons:

class ::GlobalClass
  ...
end

::ENV = { ... }

what_is = ::THE_MEANING_OF_LIFE

Unit testing modules

Methods and constants that are not exported can be tested using the #__expose! method. Thus you can keep implementation details hidden, while being able to easily test them:

parser.rb

export :parse

def parse(inp)
  split(inp).map(&:to_sym)
end

# private method
def split(inp)
  inp.split(',').map(&:strip)
end

test_seq.rb

require 'modulation'
require 'minitest/autorun'

Parser = import('../lib/parser').__expose!

class FibTest < Minitest::Test
  def test_that_split_trims_split_parts
    assert_equal(%w[abc def ghi], Parser.split(' abc ,def , ghi  '))
  end
end

Mocking dependencies

Modules loaded by Modulation can be easily mocked when running tests or specs, using Modulation.mock:

require 'minitest/autorun'
require 'modulation'

module MockStorage
  extend self

  def get_user(user_id)
    {
      user_id: user_id,
      name: 'John Doe',
      email: '[email protected]'
    }
  end
end

class UserControllerTest < Minitest::Test
  def test_user_storage
    Modulation.mock('../lib/storage', MockStorage) do
      controller = UserController.new
      ...
    end
  end
end

Lazy Loading

Modulation allows the use of lazy-loaded modules - loading of modules only once they're needed by the application, in similar fashion to Module#auto_load. To lazy load modules use the #auto_import method, which takes a constant name and a path:

export :foo

auto_import :BAR, './bar'

def foo
  # the bar module will only be loaded once this method is called
  MODULE::BAR
end

Lazy-loaded constants must always be qualified. When referring to a lazy-loaded constant from the module's top namespace, use the MODULE namespace, as shown above.

The #auto_import method can also take a hash mapping constant names to paths. This is especially useful when multiple concerns are grouped under a single namespace:

export_default :SuperNet

module SuperNet
  auto_import(
    HTTP1:      './http1',
    HTTP2:      './http2',
    WebSockets: './websockets'
  )
end

SuperNet::HTTP1 #=> loads the http1 module

Reloading modules

Modules can be reloaded at run-time for easy hot code reloading:

require 'modulation'
SQL = import('./sql')
...
SQL.__reload!

Another way to reload modules is using Modulation.reload, which accepts a module or a filename:

require 'filewatcher'

FileWatcher.new(['lib']).watch do |fn, event|
  if(event == :changed)
    Modulation.reload(fn)
  end
end

When a module is reloaded, its entire content - constants and methods - will be replaced. That means that any code using that module could continue to use it without even being aware it was reloaded, providing its API has not changed.

Reloading of modules with default exports is also possible. Modulation will
extend the exported value with a #__reload! method. The value will need to be reassigned:

require 'modulation'
settings = import('settings')
...
settings = settings.__reload!

Retaining state between reloads

Before a module is reloaded, all of its methods and constants are removed. In some cases, a module might need to retain state across reloads. You can do this by simply using instance variables:

export :value, :inc

@counter ||= 0

def value
  @counter
end

def incr
  @counter += 1
end

Care must be taken not to reassign values outside of methods, as this will overwrite any value retained in the instance variable. To assign initial values, use the ||= operator as in the example above. See also the reload example.

Dependency introspection

Modulation allows runtime introspection of dependencies between modules. You can interrogate a module's dependencies (i.e. the modules it imports) by calling #__depedencies:

m1.rb

import ('./m2')

app.rb

m1 = import('./m1')
m1.__depedencies #=> [<Module m2>]

You can also iterate over a module's entire dependency tree by using #__traverse_dependencies:

m1 = import('./m1')
m1.__traverse_dependencies { |mod| ... }

To introspect reverse dependencies (modules using a particular module), use #__dependent_modules:

m1 = import('./m1')
m1.__depedencies #=> [<Module m2>]
m1.__dependencies.first.__dependent_modules #=> [<Module m1>]

Running Modulation-based applications

Modulation provides a binary script for running Modulation-based applications. mdl is a wrapper around Ruby that loads your application's main file as a module, and then runs your application's entry point method. Let's look at a sample application:

app.rb

def greet(name)
  puts "Hello, #{name}!"
end

def main
  print "Enter your name: "
  name = gets
  greet(name)
end

To run this application, execute mdl app.rb, or mdl run app.rb. mdl will automatically require the modulation gem and call the application's entry point, #main.

Packing applications with Modulation

Note: application packing is at the present time an experimental feature. There might be security concerns for packaging your app, such as leaking filenames from the developer's machine.

Modulation can also be used to package your entire application into a single portable file that can be copied to another machine and run as is. To package your app, use mdl pack. This command will perform a dynamic analysis of all the app's dependencies and will put them together into a single Ruby file.

For more information have a look at the app example.

Writing gems using Modulation

Modulation can be used to write gems, providing fine-grained control over your gem's public APIs and letting you hide any implementation details. In order to allow loading a gem using either #require or #import, code your gem's main file normally, but add require 'modulation/gem' at the top, and export your gem's main namespace as a default export, e.g.:

require 'modulation/gem'

export_default :MyGem

module MyGem
  ...
  MyClass = import('my_gem/my_class')
  ...
end

Importing gems using Modulation

Gems written using modulation can also be loaded using #import. If modulation does not find the module specified by the given relative path, it will attempt to load a gem by the same name. It is also possible to load specific files inside modules by specifying a sub-path:

require 'modulation'
MyFeature = import 'my_gem/my_feature'

Note: Since there's not much of a point in #importing gems that do not use Modulation to export symbols, Modulation will refuse to import any gem that does not depend on Modulation.

Writing modules that patch external classes or modules

It is generally recommended you refrain from causing side effects or patching external code in your modules. When you do have to patch external classes or modules (i.e. core, stdlib, or some third-party code) in your module, it's useful to remember that any module may be eventually reloaded by the application code. This means that any patching done during the loading of your module must be idempotent, i.e. have the same effect when performed multiple times. Take for example the following module code:

module ::Kernel
  # aliasing #sleep more than once will break your code
  alias_method :orig_sleep, :sleep

  def sleep(duration)
    STDERR.puts "Going to sleep..."
    orig_sleep(duration)
    STDERR.puts "Woke up!"
  end
end

Running the above code more than once would cause an infinite loop when calling Kernel#sleep. In order to prevent this situation, modulation provides the Module#alias_method_once method, which prevents aliasing the original method more than once:

module ::Kernel
  # alias_method_once is idempotent
  alias_method_once :orig_sleep, :sleep

  def sleep(duration)
    STDERR.puts "Going to sleep..."
    orig_sleep(duration)
    STDERR.puts "Woke up!"
  end
end

Coding style recommendations

  • Import modules into constants, not variables:
  Settings = import('./settings')
  • Place your exports at the top of your module, followed by #requires, followed by #imports:
  export :foo, :bar, :baz

  require 'json'

  Core = import('./core')

  ...

API Reference

This section will be expanded on in a future release.

__module_info

__reload!

alias_method_once()

auto_import()

auto_import_map()

export()

export_default()

export_from_receiver()

extend_from()

import()

import_all()

import_map()

include_from()

Modulation.full_backtrace!

Modulation.reload()

MODULE

MODULE.__module_info

Why you should not use Modulation

  • Modulation is not production-ready.
  • Modulation is not thread-safe.
  • Modulation doesn't play well with rdoc/yard.
  • Modulation (probably) doesn't play well with Marshal.
  • Modulation (probably) doesn't play well with code-analysis tools.