Provides a mechanism to turn features on and off within your application.

Declare a module where you'll define the available FFs and just declare them all:

defmodule Flags do
  use FeatureFlippers, otp_app: :my_app

  feature_flipper :foo?
  feature_flipper :bar?, expires: "2000-01-01"
  feature_flipper :baz?, disabled: true
end

Each feature_flipper/2 declaration defines a function returning boolean in the containing module. So, in your code, use them as follows:

if Flags.foo? do
  # new feature
else
  # original code
end

It means that if you remove a feature flipper declaration, your code won't compile and mix will also indicate where the missing feature flippers are being used. This may be useful when decommissioning feature flippers.

You can turn them on/off in compile time setting them in your config/config.exs (or in runtime through config/runtime.exs):

config :my_app, Flags,
  foo?: true
  bar?: false
  baz: true

You can also set them later by updating the application environment:

Application.get_env(:my_app, Flags)
|> Keyword.update(:bar?, true, fn _ -> true end)
|> (&Application.put_env(:my_app, Flags, &1)).()

While running unit tests, you might want to enable them all:

Flags.all()
|> Enum.map(fn key -> {key, true} end)
|> (&Application.put_env(:my_app, Flags, &1)).()

At any moment, you can discover which feature flippers have expired calling expired/0.

If you want to use a different storage for your flags instead of Application environment, you can specify the :callback option, such as:

defmodule Flags do
  use FeatureFlippers, callback: &Flags.on?/1

  def on?(flag) do
    # access `flag` or return `nil` if not found.
  end
end