Raise ArgumentErrors with help on incorrect GPIO.open call

This addresses some confusion when `:write` was passed rather than
`:output` for the second parameter. Raising ArgumentError with the
allowed values saves a look at the docs.

This also does something similar with the gpio spec.

Author: fhunleth

lib/gpio.ex

@@ -239,8 +239,9 @@ defmodule Circuits.GPIO do
     open(gpio_spec, direction, options)
   end
 
-  def open(gpio_spec, direction, options)
-      when is_gpio_spec(gpio_spec) and direction in [:input, :output] do
+  def open(gpio_spec, direction, options) do
+    check_gpio_spec!(gpio_spec)
+    check_direction!(direction)
     check_options!(options)
 
     {backend, backend_defaults} = default_backend()
@@ -254,14 +255,27 @@ defmodule Circuits.GPIO do
     backend.open(gpio_spec, direction, all_options)
   end
 
+  defp check_gpio_spec!(gpio_spec) do
+    if not gpio_spec?(gpio_spec) do
+      raise ArgumentError, "Invalid GPIO spec: #{inspect(gpio_spec)}"
+    end
+  end
+
+  defp check_direction!(direction) do
+    if direction not in [:input, :output] do
+      raise ArgumentError,
+            "Invalid direction: #{inspect(direction)}. Options are :input or :output"
+    end
+  end
+
   defp check_options!([]), do: :ok
 
   defp check_options!([{:initial_value, value} | rest]) do
     case value do
       0 -> :ok
       1 -> :ok
       :not_set -> Logger.warning("Circuits.GPIO no longer supports :not_set for :initial_value")
-      _ -> raise(ArgumentError, ":initial_value should be :not_set, 0, or 1")
+      _ -> raise ArgumentError, ":initial_value should be :not_set, 0, or 1"
     end
 
     check_options!(rest)

test/circuits_gpio_test.exs

@@ -233,6 +233,14 @@ defmodule Circuits.GPIOTest do
     GPIO.close(gpio)
   end
 
+  test "raises argument error on invalid gpio_spec" do
+    assert_raise ArgumentError, fn -> GPIO.open(:invalid_gpio_spec, :output) end
+  end
+
+  test "raises argument error on invalid direction" do
+    assert_raise ArgumentError, fn -> GPIO.open({@gpiochip, 0}, :bogus) end
+  end
+
   test "ignores unknown open options" do
     {:ok, gpio} = GPIO.open({@gpiochip, 1}, :input, bogus: true)
     GPIO.close(gpio)