Move release/1 from Conn to it's own protocol.

lib/wafer/conn.ex

@@ -23,12 +23,18 @@ defmodule Wafer.Conn do
       with {:ok, conn} <- Driver.acquire(bus_name: bus, address: address),
           do: {:ok, %HTS221{conn: conn}}
     end
-
-    def release(%HTS221{conn: conn}), do: Driver.release(conn)
   end
   ```
   """
 
+  defmacro __using__(_env) do
+    quote do
+      @behaviour Wafer.Conn
+    end
+
+    Protocol.assert_impl!(Wafer.Release, __MODULE__)
+  end
+
   @type t :: any
   @type options :: [option]
   @type option :: {atom, any}
@@ -37,9 +43,4 @@ defmodule Wafer.Conn do
   Acquire a connection to a peripheral using the provided driver.
   """
   @callback acquire(options) :: {:ok, t} | {:error, reason :: any}
-
-  @doc """
-  Release all resources associated with this connection.
-  """
-  @callback release(module) :: :ok | {:error, reason :: any}
 end

lib/wafer/drivers/circuits/gpio.ex

@@ -36,14 +36,19 @@ defmodule Wafer.Driver.Circuits.GPIO do
       {:error, reason} -> {:error, reason}
     end
   end
+end
+
+defimpl Wafer.Release, for: Wafer.Driver.Circuits.GPIO do
+  alias Wafer.Driver.Circuits.GPIO.Wrapper
+  alias Wafer.Driver.Circuits.GPIO
 
   @doc """
   Release all resources related to this GPIO pin connection.
 
   Note that other connections may still be using the pin.
   """
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{ref: ref} = _conn) when is_reference(ref), do: Wrapper.close(ref)
+  @spec release(GPIO.t()) :: :ok | {:error, reason :: any}
+  def release(%GPIO{ref: ref} = _conn) when is_reference(ref), do: Wrapper.close(ref)
 end
 
 defimpl Wafer.GPIO, for: Wafer.Driver.Circuits.GPIO do

lib/wafer/drivers/circuits/i2c.ex

@@ -38,9 +38,17 @@ defmodule Wafer.Driver.Circuits.I2C do
         {:error, reason}
     end
   end
+end
 
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{ref: ref} = _conn), do: Wrapper.close(ref)
+defimpl Wafer.Release, for: Wafer.Driver.Circuits.I2C do
+  alias Wafer.Driver.Circuits.I2C.Wrapper
+  alias Wafer.Driver.Circuits.I2C
+
+  @doc """
+  Release all resources associated with this device.
+  """
+  @spec release(I2C.t()) :: :ok | {:error, reason :: any}
+  def release(%I2C{ref: ref} = _conn), do: Wrapper.close(ref)
 end
 
 defimpl Wafer.Chip, for: Wafer.Driver.Circuits.I2C do

lib/wafer/drivers/circuits/spi.ex

@@ -34,12 +34,17 @@ defmodule Wafer.Driver.Circuits.SPI do
       {:error, reason} -> {:error, reason}
     end
   end
+end
+
+defimpl Wafer.Release, for: Wafer.Driver.Circuits.SPI do
+  alias Wafer.Driver.Circuits.SPI.Wrapper
+  alias Wafer.Driver.Circuits.SPI
 
   @doc """
   Close the SPI bus connection.
   """
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{ref: ref} = _conn) when is_reference(ref), do: Wrapper.close(ref)
+  @spec release(SPI.t()) :: :ok | {:error, reason :: any}
+  def release(%SPI{ref: ref} = _conn) when is_reference(ref), do: Wrapper.close(ref)
 end
 
 defimpl Wafer.SPI, for: Wafer.Driver.Circuits.SPI do

lib/wafer/drivers/elixir_ale/gpio.ex

@@ -35,14 +35,19 @@ defmodule Wafer.Driver.ElixirALE.GPIO do
       {:error, reason} -> {:error, reason}
     end
   end
+end
+
+defimpl Wafer.Release, for: Wafer.Driver.ElixirALE.GPIO do
+  alias Wafer.Driver.ElixirALE.GPIO.Wrapper
+  alias Wafer.Driver.ElixirALE.GPIO
 
   @doc """
   Release all resources related to this GPIO pin connection.
 
   Note that other connections may still be using the pin.
   """
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{pid: pid} = _conn), do: Wrapper.release(pid)
+  @spec release(GPIO.t()) :: :ok | {:error, reason :: any}
+  def release(%GPIO{pid: pid} = _conn), do: Wrapper.release(pid)
 end
 
 defimpl Wafer.GPIO, for: Wafer.Driver.ElixirALE.GPIO do

lib/wafer/drivers/elixir_ale/i2c.ex

@@ -38,9 +38,17 @@ defmodule Wafer.Driver.ElixirALE.I2C do
         {:error, reason}
     end
   end
+end
 
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{pid: pid} = _conn) when is_pid(pid), do: Wrapper.release(pid)
+defimpl Wafer.Release, for: Wafer.Driver.ElixirALE.I2C do
+  alias Wafer.Driver.ElixirALE.I2C.Wrapper
+  alias Wafer.Driver.ElixirALE.I2C
+
+  @doc """
+  Release all resources associated with this I2C device.
+  """
+  @spec release(I2C.t()) :: :ok | {:error, reason :: any}
+  def release(%I2C{pid: pid} = _conn) when is_pid(pid), do: Wrapper.release(pid)
 end
 
 defimpl Wafer.Chip, for: Wafer.Driver.ElixirALE.I2C do

lib/wafer/drivers/elixir_ale/spi.ex

@@ -35,12 +35,17 @@ defmodule Wafer.Driver.ElixirALE.SPI do
       {:error, reason} -> {:error, reason}
     end
   end
+end
+
+defimpl Wafer.Release, for: Wafer.Driver.ElixirALE.SPI do
+  alias Wafer.Driver.ElixirALE.SPI.Wrapper
+  alias Wafer.Driver.ElixirALE.SPI
 
   @doc """
   Close the SPI bus connection.
   """
-  @spec release(t) :: :ok | {:error, reason :: any}
-  def release(%__MODULE__{pid: pid} = _conn) when is_pid(pid), do: Wrapper.release(pid)
+  @spec release(SPI.t()) :: :ok | {:error, reason :: any}
+  def release(%SPI{pid: pid} = _conn) when is_pid(pid), do: Wrapper.release(pid)
 end
 
 defimpl Wafer.SPI, for: Wafer.Driver.ElixirALE.SPI do

lib/wafer/drivers/fake.ex

@@ -22,9 +22,6 @@ defmodule Wafer.Driver.Fake do
     {:ok, %__MODULE__{opts: opts}}
   end
 
-  @impl Wafer.Conn
-  def release(%__MODULE__{}), do: :ok
-
   defp emit_warning do
     :wafer
     |> Application.get_env(__MODULE__)
@@ -32,6 +29,10 @@ defmodule Wafer.Driver.Fake do
   end
 end
 
+defimpl Wafer.Release, for: Wafer.Driver.Fake do
+  def release(%Wafer.Driver.Fake{}), do: :ok
+end
+
 defimpl Wafer.Chip, for: Wafer.Driver.Fake do
   import Wafer.Guards
 

lib/wafer/release.ex

@@ -0,0 +1,55 @@
+defprotocol Wafer.Release do
+  alias Wafer.Conn
+
+  @moduledoc """
+  A protocol for releasing connections.  The opposite of `Conn`'s `acquire/1`.
+
+  ## Deriving
+
+  If you're implementing your own `Conn` type that simply delegates to one of
+  the lower level drivers that you can derive this protocol automatically:
+
+  ```elixir
+  defstruct MyConn do
+    @derive Wafer.Release
+    defstruct [:conn]
+  end
+  ```
+
+  If your type uses a key other than `conn` for the inner connection you can specify it while deriving:
+
+  ```elixir
+  defstruct MyConn do
+    @derive {Wafer.Release, key: :pin_conn}
+    defstruct [:pin_conn]
+  end
+  ```
+  """
+
+  @doc """
+  Release all resources associated with the connection.  Usually in preparation
+  for shutdown.
+  """
+  @spec release(Conn.t()) :: :ok
+  def release(conn)
+end
+
+defimpl Wafer.Release, for: Any do
+  defmacro __deriving__(module, struct, options) do
+    key = Keyword.get(options, :key, :conn)
+
+    unless Map.has_key?(struct, key) do
+      raise(
+        "Unable to derive `Wafer.Release` for `#{module}`: key `#{inspect(key)}` not present in struct."
+      )
+    end
+
+    quote do
+      defimpl Wafer.Release, for: unquote(module) do
+        def release(%{unquote(key) => inner_conn}), do: Wafer.Release.release(inner_conn)
+      end
+    end
+  end
+
+  def release(unknown), do: {:error, "`Wafer.Release` not implemented for #{inspect(unknown)}"}
+end

test/drivers/circuits_gpio_test.exs

@@ -5,6 +5,7 @@ defmodule WaferDriverCircuits.GPIOTest do
   alias Wafer.Driver.Circuits.GPIO.Dispatcher, as: Dispatcher
   alias Wafer.Driver.Circuits.GPIO.Wrapper
   alias Wafer.GPIO, as: GPIO
+  alias Wafer.Release, as: Release
   @moduledoc false
 
   describe "acquire/1" do
@@ -25,7 +26,7 @@ defmodule WaferDriverCircuits.GPIOTest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the pin" do
       conn = conn()
 
@@ -35,7 +36,7 @@ defmodule WaferDriverCircuits.GPIOTest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end
 

test/drivers/circuits_i2c_test.exs

@@ -4,7 +4,7 @@ defmodule WaferCircuits.I2CTest do
   alias Wafer.Chip
   alias Wafer.Driver.Circuits.I2C, as: Subject
   alias Wafer.Driver.Circuits.I2C.Wrapper
-  alias Wafer.I2C
+  alias Wafer.{Release, I2C}
   @moduledoc false
 
   describe "acquire/1" do
@@ -72,7 +72,7 @@ defmodule WaferCircuits.I2CTest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the bus connection" do
       conn = conn()
 
@@ -82,7 +82,7 @@ defmodule WaferCircuits.I2CTest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end
 

test/drivers/circuits_spi_test.exs

@@ -3,7 +3,7 @@ defmodule WaferCircuits.SPITest do
   use Mimic
   alias Wafer.Driver.Circuits.SPI, as: Subject
   alias Wafer.Driver.Circuits.SPI.Wrapper
-  alias Wafer.SPI
+  alias Wafer.{Release, SPI}
   @moduledoc false
 
   describe "acquire/1" do
@@ -23,7 +23,7 @@ defmodule WaferCircuits.SPITest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the bus connection" do
       conn = conn()
 
@@ -33,7 +33,7 @@ defmodule WaferCircuits.SPITest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end
 

test/drivers/elixir_ale_gpio_test.exs

@@ -5,6 +5,7 @@ defmodule WaferDriverElixirALE.GPIOTest do
   alias Wafer.Driver.ElixirALE.GPIO.Dispatcher, as: Dispatcher
   alias Wafer.Driver.ElixirALE.GPIO.Wrapper
   alias Wafer.GPIO, as: GPIO
+  alias Wafer.Release, as: Release
   @moduledoc false
 
   describe "acquire/1" do
@@ -25,7 +26,7 @@ defmodule WaferDriverElixirALE.GPIOTest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the pin" do
       conn = conn()
 
@@ -35,7 +36,7 @@ defmodule WaferDriverElixirALE.GPIOTest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end
 

test/drivers/elixir_ale_i2c_test.exs

@@ -4,7 +4,7 @@ defmodule WaferElixirALE.I2CTest do
   alias Wafer.Chip
   alias Wafer.Driver.ElixirALE.I2C, as: Subject
   alias Wafer.Driver.ElixirALE.I2C.Wrapper
-  alias Wafer.I2C
+  alias Wafer.{Release, I2C}
   @moduledoc false
 
   describe "acquire/1" do
@@ -75,7 +75,7 @@ defmodule WaferElixirALE.I2CTest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the bus connection" do
       conn = conn()
 
@@ -85,7 +85,7 @@ defmodule WaferElixirALE.I2CTest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end
 

test/drivers/elixir_ale_spi_test.exs

@@ -3,7 +3,7 @@ defmodule WaferElixirALE.SPITest do
   use Mimic
   alias Wafer.Driver.ElixirALE.SPI, as: Subject
   alias Wafer.Driver.ElixirALE.SPI.Wrapper
-  alias Wafer.SPI
+  alias Wafer.{Release, SPI}
   @moduledoc false
 
   describe "acquire/1" do
@@ -24,7 +24,7 @@ defmodule WaferElixirALE.SPITest do
     end
   end
 
-  describe "release/1" do
+  describe "Release.release/1" do
     test "closes the bus connection" do
       conn = conn()
 
@@ -34,7 +34,7 @@ defmodule WaferElixirALE.SPITest do
         :ok
       end)
 
-      assert :ok = Subject.release(conn)
+      assert :ok = Release.release(conn)
     end
   end