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double_down

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Builds on the Mox pattern — generates behaviours and dispatch facades from `defcallback` declarations — and adds stateful test doubles powerful enough to test Ecto.Repo operations without a database.

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double_down lib double_down dispatch stub_handler.ex
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lib/double_down/dispatch/stub_handler.ex 

defmodule DoubleDown.Dispatch.StubHandler do

  @moduledoc """

  Behaviour for stateless stub handler modules.



  Implement this behaviour to make a stateless stub usable by module

  name in `DoubleDown.Double.stub/2..4`:



      # Instead of:

      Double.stub(Repo, Repo.Test.new())



      # Write:

      Double.stub(Repo, Repo.Test)



      # With a fallback function:

      Double.stub(Repo, Repo.Test, fn :all, [User] -> [] end)



  ## Callbacks



    * `new/2` — build a 2-arity dispatch function from a fallback

      function and options. The returned function has the signature

      `fn operation, args -> result end`.



  ## Example



      defmodule MyApp.TestStore do

        @behaviour DoubleDown.Dispatch.StubHandler



        @impl true

        def new(fallback_fn, _opts) do

          fn operation, args ->

            case {operation, args} do

              {:get, [id]} -> %{id: id}

              _ when is_function(fallback_fn) -> fallback_fn.(operation, args)

              _ -> raise "unhandled"

            end

          end

        end

      end

  """



  @doc """

  Build a 2-arity dispatch function from a fallback function and options.



    * `fallback_fn` — an optional 2-arity function `(operation, args) -> result`

      for operations the stub doesn't handle directly. `nil` if not provided.

    * `opts` — additional options for configuring the stub.



  Returns a 2-arity function `fn operation, args -> result end` suitable

  for use as a `Double.stub` function fallback.

  """

  @callback new(

              fallback_fn :: (atom(), [term()] -> term()) | nil,

              opts :: keyword()

            ) :: (atom(), [term()] -> term())

end