Implement this protocol to provide groups.

Group gates are similar to actor gates, but they apply to a category of entities rather than specific ones. They can be toggled on or off for the name of the group (as an atom) instead of a specific term.

Group gates take precedence over boolean gates but are overridden by actor gates.

The semantics to determine which entities belong to which groups are application specific. Entities could have an explicit list of groups they belong to, or the groups could be abstract and inferred from some other attribute. For example, an :employee group could comprise all %User{} structs with an email address matching the company domain, or an :admin group could be made of all users with %User{admin: true}.

In order to be affected by a group gate, an entity should implement the FunWithFlags.Group protocol. The protocol automatically falls back to a default Any implementation, which states that any entity belongs to no group at all. This makes it possible to safely use "normal" actors when querying group gates, and to implement the protocol only for structs and types for which it matters.

The protocol can be implemented for custom structs or literally any other type.

defmodule MyApp.User do
  defstruct [:email, admin: false, groups: []]
end

defimpl FunWithFlags.Group, for: MyApp.User do
  def in?(%{email: email}, :employee),  do: Regex.match?(~r/@mycompany.com$/, email)
  def in?(%{admin: is_admin}, :admin),  do: !!is_admin
  def in?(%{groups: list}, group_name), do: group_name in list
end

elisabeth = %User{email: "elisabeth@mycompany.com", admin: true, groups: [:engineering, :product]}
FunWithFlags.Group.in?(elisabeth, :employee)
true
FunWithFlags.Group.in?(elisabeth, :admin)
true
FunWithFlags.Group.in?(elisabeth, :engineering)
true
FunWithFlags.Group.in?(elisabeth, :marketing)
false

defimpl FunWithFlags.Group, for: Map do
  def in?(%{group: group_name}, group_name), do: true
  def in?(_, _), do: false
end

FunWithFlags.Group.in?(%{group: :dumb_tests}, :dumb_tests)
true

With the protocol implemented, actors can be used with the library functions:

FunWithFlags.disable(:database_access)
FunWithFlags.enable(:database_access, for_group: :engineering)