Adapter

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Bridge between effectful and plain Elixir code — both directions.

Skuld.Adapter

Wraps an effectful implementation as a plain module. Plain code (Phoenix controllers, GenServers, CLI) calls the adapter's functions like regular Elixir functions — the adapter runs the effect stack internally:

defmodule MyApp.UserService.Adapter do
  use Skuld.Adapter,
    contract: MyApp.UserService,         # DoubleDown contract (plain behaviour)
    impl: MyApp.UserService.EffectfulImpl,  # effectful implementation
    stack: fn comp ->
      comp
      |> Port.with_handler(%{MyApp.Inventory => MyApp.InventoryService})
      |> Throw.with_handler()
    end
end

# Called like a plain function:
MyApp.UserService.Adapter.find_user("123")
# => {:ok, %User{...}}

For each defcallback in the contract, the adapter generates a function that calls the effectful impl, pipes through the stack, and runs with Comp.run!().

Skuld.Adapter.EffectfulContract

Generates an effectful @behaviour from a plain DoubleDown contract:

defmodule MyApp.Todos.Contract do
  use DoubleDown.Contract
  defcallback get_todo(id :: String.t()) :: {:ok, Todo.t()} | {:error, term()}
end

defmodule MyApp.Todos.Effectful do
  use Skuld.Adapter.EffectfulContract,
    double_down_contract: MyApp.Todos.Contract
end

# Implement:
defmodule MyApp.Todos.EffectfulImpl do
  @behaviour MyApp.Todos.Effectful
  def get_todo(id) do
    {:ok, %Todo{id: id}}
  end
end

The generated @callback declarations wrap return types in computation(). The module also gets __port_effectful__?/0 for auto-detection by the Port system.

Four directions

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