MikaCredoRules.NoNilComparison (MikaCredoRules v0.1.0)

Comparing against nil must use is_nil/1, not an equality operator.

value == nil and value != nil obscure intent and invite ==/=== inconsistency. is_nil/1 says exactly what is being asked, works in guards, and is the required spelling in Ecto queries.

# BAD — equality operator against nil
defmodule MyApp.Worker do
  def missing?(value), do: value == nil
  def present?(value), do: value != nil
  def fallback(value) when value === nil, do: :default
end

# GOOD — is_nil/1, in bodies and guards alike
defmodule MyApp.Worker do
  def missing?(value), do: is_nil(value)
  def present?(value), do: not is_nil(value)
  def fallback(value) when is_nil(value), do: :default
end

nil on either side is caught:

value == nil    # caught
nil == value    # caught
value !== nil   # caught

Pattern matches on nil (case value do nil -> ...) and nil passed as an argument are not comparisons and are never flagged.