TylerPachal

@doc """
Asserts that the given changeset is invalid, and that when the
assertion_expression is applied to the error_message it results in a truthy
value.
"""
defmacro assert_invalid(changeset, field, assertion_expression) when is_atom(field) do
  expr = Macro.to_string(assertion_expression)
  quote do
    c = unquote(changeset)

TylerPachal

Custom Compile-Time Warnings in Elixir Using IO.warn/1

When you are writing macros which run at compile-time, it can be useful to emit custom compile-time warnings.

Skip down to The Solution

Example Scenario

Say you are using a text file to dynamically define function clauses. Our function will be called official_language/1; its only argument is the name of a country, and it returns an :ok tuple with a list of the countries official languages.

TylerPachal

def join_game(user_id, game_id) do
  with {:ok, user} <- Users.get(user_id), 
       {:ok, game} <- Games.get(game_id),
       false       <- Game.is_full?(game),
       false       <- Game.is_started?(game),
       true        <- User.has_permission?(user, game)
do
  Game.add_user(game, user)
else
  # Don't care what specific thing failed

TylerPachal

# You don't always need pattern matching
is_admin = Map.get(user, "is_admin") || false

TylerPachal

@doc """
Assert that a Kafka contains a message on a given topic.

Example usage:

  assert_kafka_contains "my_topic", fn message_body ->
    Jason.decode!(message_body)["name"] == "tyler"
  end
"""
defmacro assert_kafka_contains(topic, opts \\ [], assert_body_fn) do

TylerPachal

@doc """
Asserts that the given changeset is invalid, and that the given field is
invalid and contains an error message that contains the asserted_message
substring.

Example usage:

  assert_invalid changeset, :people, error_message =~ "tyler is not allowed"
"""
defmacro assert_invalid(changeset, field, assertion_expression) when is_atom(field) do

TylerPachal

# Without specifying a child_spec, you get the default one:
# default = %{
#   id: __MODULE__,
#   start: {__MODULE__, :start_link, [init_arg]}
# }

# A: calls start_link/1 with empty list of args
# children = [
#   DevWorker
# ]

TylerPachal

# An experiment to render a flat-list of nested errors.
#
# The default translate_errors() function would render (nested) errors like this:
#   %{
#     "pets" => [
#       %{},
#      %{"type" => ["is invalid"]},
#       %{},
#       %{"type" => ["is invalid"]}
#     ]

TylerPachal

# When using Ecto Schemas for validating, I wanted to see what error information was present in the
# Ecto.Changeset, and then compare that to what Phoenix would render by default.
#
# Q: Can you get the index of nested objects' errors?
# A: Yes, for nested lists anything valid will have an empty entry in the list, while invalid 
# objects will have some error information.  Either way you could use Enum.with_index() to figure
# out a specific index.

## lib/phoenix_playground/accounts/user.ex
defmodule PhoenixPlayground.Accounts.User do

TylerPachal

def init(:ok) do
  children =
    [
      kafka_producer_spec(),
      kafka_consumer_spec(),
      other_thing_spec(),
      Api # Start the Api from the child_spec() function defined in that module
    ]
    end