Behaviour for custom equality logic in the Eq DSL.

Implement this behaviour to define reusable Eq comparators that can be used in the DSL without implementing the Eq protocol.

This is useful for teams that want to avoid teaching developers about protocols, or want struct-specific equality without global protocol implementations.

Basic Example

defmodule UserById do
  @behaviour Funx.Eq.Dsl.Behaviour

  @impl true
  def eq(_opts) do
    Funx.Eq.contramap(&(&1.id))
  end
end

# In DSL - bare usage (preferred)
use Funx.Eq

eq do
  UserById  # Compares by id
end

# Or with `on` directive
eq do
  on UserById
end

With Options

defmodule UserByName do
  @behaviour Funx.Eq.Dsl.Behaviour

  @impl true
  def eq(opts) do
    case_sensitive = Keyword.get(opts, :case_sensitive, true)

    if case_sensitive do
      Funx.Eq.contramap(&(&1.name))
    else
      Funx.Eq.contramap(fn u -> String.downcase(u.name) end)
    end
  end
end

# In DSL - bare with options
eq do
  {UserByName, case_sensitive: false}
end

# Or with `on` directive
eq do
  on UserByName, case_sensitive: false
end

Composing Multiple Behaviours

Behaviour modules can be composed with other eq expressions:

eq do
  UserById                              # bare behaviour
  {UserByName, case_sensitive: false}   # bare with options
  on :email                             # projection
end

Why Use This Instead of Protocols?

• Simpler: Just one function returning an Eq map
• No protocol knowledge required: Easier for team onboarding
• Module-specific: Override struct equality without global protocol
• Options support: Built-in support for configuration

The returned Eq map typically uses Funx.Eq.contramap/2 to build projection-based equality, but can implement any custom comparison logic.