An opinionated way of dealing with behaviours.

Opinionated means that it offers an easy way of defining a "facade" for a behaviour. This facade then delegates calls to the real implementation, which is either given directly to Knigge or fetched from the configuration.

Knigge can be used directly in a behaviour, or in a separate module by passing the behaviour which should be "facaded" as an option.

Table of Contents

• Table of Contents
• Installation
• Motivation
• Examples
  • Without Knigge
  • Using Knigge to reduce boilerplate
    • Specifying a default implementation
    • The behaviour key is optional
    • Specifying the implementation directly
  • defdefault - Fallback implementations for optional callbacks
• Options
• Verifying your implementations - mix knigge.verify
• Knigge and the :test environment
  • Compiler Warnings

Installation

Simply add knigge to your list of dependencies in your mix.exs:

def deps do
  [
    {:knigge, "~> 1.4"}
  ]
end

Differences between the versions are explained in the Changelog.

Motivation

Knigge was born out of a desire to standardize dealing with behaviours and their implementations.

As great fans of mox we longed for an easy way to swap out implementations from the configuration which lead us to introduce a facade pattern, where a module's sole responsibility was loading the correct implementation and delegating calls.

This pattern turned out to be very flexible and useful but required a fair bit of boilerplate code. Knigge was born out of an attempt to reduce this boilerplate to the absolute minimum.

You can read about our motivation in depth in our devblog.

Examples

Without Knigge

Imagine a behaviour looking like this:

defmodule MyGreatBehaviour do
  @callback my_great_callback(my_argument :: any()) :: any()
end

Now imagine you want to delegate calls to this behaviour like this:

defmodule MyGreatBehaviourFacade do
  @behaviour MyGreatBehaviour

  @implementation Application.fetch_env!(:my_application, __MODULE__)

  defdelegate my_great_callback, to: @implementation
end

With this in place you can reference the "real implementation" by calling functions on your facade:

MyGreatBehaviourFacade.my_great_callback(:with_some_argument)

Using Knigge to reduce boilerplate

Knigge allows you to reduce this boilerplate to the absolute minimum:

defmodule MyGreatBehaviourFacade do
  use Knigge,
    behaviour: MyGreatBehaviour,
    otp_app: :my_application
end

Specifying a default implementation

It's also possible to provide a default implementation:

defmodule MyGreatBehaviourFacade do
  use Knigge,
    behaviour: MyGreatBehaviour,
    otp_app: :my_application,
    default: MyDefaultImplementation
end

Compared to the "boilerplate" version above, it's as if you'd written:

  @implementation Application.get_env(:my_application, __MODULE__, MyDefaultImplementation)

instead of:

  @implementation Application.fetch_env!(:my_application, __MODULE__)

The behaviour key is optional

Technically even passing the behaviour is optional, it defaults to the current __MODULE__. This means that the example from above could be shortened even more to:

defmodule MyGreatBehaviour do
  use Knigge, otp_app: :my_application

  @callback my_great_callback(my_argument :: any()) :: any()
end

Under the hood this compiles down to the explicit delegation visible on the top.

Specifying the implementation directly

In case you don't want to fetch your implementation from the configuration, Knigge also allows you to explicitly pass the implementation of the behaviour with the aptly named key implementation:

defmodule MyGreatBehaviourFacade do
  use Knigge,
    behaviour: MyGreatBehaviour,
    implementation: MyGreatImplementation
end

defdefault - Fallback implementations for optional callbacks

Now imagine you have a more sophisticated behaviour with some optional callbacks:

defmodule MySophisticatedBehaviour do
  @callback an_optional_callback() :: any()
  @callback a_required_callback() :: any()

  @optional_callbacks an_optional_callback: 0
end

As you would expect Knigge delegates calls to this callback as usual. But since it's optional this delegation might fail. A common pattern is to check if the implementation exports the function in question:

if function_exported?(MyImplementation, :an_optional_callback, 0) do
  MyImplementation.an_optional_callback()
else
  :my_fallback_implementation
end

Knigge offers an easy way to specify these fallback implementations with defdefault:

defmodule MySophisticatedFacade do
  use Knigge,
    behaviour: MySophisticatedBehaviour,
    otp_app: :my_application

  defdefault an_optional_callback do
    :my_fallback_implementation
  end
end

Knigge tries to determine at compile-time if the implementation exports the function in question and only uses the default if this is not the case. As such defdefault incurs no runtime overhead and compiles to a simple def.

Of course defdefaults can accept arguments as any usual function:

defdefault my_optional_callback_with_arguments(first_argument, another_argument) do
  case first_argument do
    # ...
  end
end

Options

Knigge expects either the otp_app key or the implementation key. If neither is provided an error will be raised at compile time.

When using the otp_app configuration you can also pass config_key, which results in a call looking like this: Application.fetch_env!(otp_app, config_key). config_key defaults to __MODULE__.

By default Knigge does as much work as possible at compile time. This will be fine most of the time. In case you want to swap out the implementation at runtime - by calling Application.put_env/2 - you can force Knigge to do all delegation at runtime. As you might expect this incurs runtime overhead, since the implementing module will have to be loaded for each call.

If you want to do delegation at runtime simply pass delegate_at_runtime?: true as option - by default Knigge delegates at runtime in your :tests.

For further information about options check the Knigge.Options module.

Verifying your implementations - mix knigge.verify

Before version 1.2.0 Knigge tried to check at compile time if the implementation of your facade existed. Due to the way the Elixir compiler goes about compiling your modules this didn't work as expected - checkout this page if you're interested in the details.

As an alternative Knigge now offers the mix knigge.verify task which verifies that the implementation modules of your facades actually exist. The task returns with an error code when an implementation is missing, which allows you to plug it into your CI pipeline - for example as MIX_ENV=prod mix knigge.verify.

For details check the documentation of mix knigge.verify by running mix help knigge.verify.

Knigge and the :test environment

To give the maximum amount of flexibility Knigge delegates at runtime in your :test environment and at compile time everywhere else.

This allows you to easily swap out your behaviour implementation - for example by calling Application.put_env/3 - and it also avoids a bunch of compiler warnings.

Compiler Warnings

With the default configuration Knigge does not generate any compiler warnings.

In case you change the delegate_at_runtime? configuration to anything which excludes the :test environment you will - most likely - encounter compiler warnings like this:

warning: function MyMock.my_great_callback/1 is undefined (module MyMock is not available)
  lib/my_facade.ex:1

warning: function MyMock.another_callback/0 is undefined (module MyMock is not available)
  lib/my_facade.ex:1

This can quickly become quite unnerving. Luckily you can explicitly tell the compiler to ignore this module in your mix.exs file.

To disable the check simply add a single line to your mix.exs' project/0 function:

def project do
  [
    # ...
    xref: [exclude: [MyMock]]
  ]
end

Where MyMock is the name of your configured module in question.