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

@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

@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

# 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 increment(number) do
  number + 1
end

count = 0

# Return the updated value from the function, and use it to
# reaassign to our variable.
count = increment(count)

TylerPachal

# Assign the result of the if/else statement to a variable
flag = 
  if foo == bar do
    # If the condition is met, change the flag
    false
  else
    # If the condition is not met, return flag to keep the value the same
    flag
  end

TylerPachal

# Do not do this in Elixir

def increment(number) do
  # Because of lexical scoping, this will have no impact to anything 
  # outside the scope of this function.
  number = number + 1
end

count = 0
increment(count)

TylerPachal

# Do not do this in Elixir

flag = true

if foo == bar do

  # Anything that happens inside this if block will have no impact 
  # on the outside of this if block.
  flag = false
  IO.inspect(flag) # false