module Fetchable

# File lib/fetchable.rb, line 4
def fetch(key, not_found_value = NO_DEFAULT_GIVEN)
  if not_found_value != NO_DEFAULT_GIVEN && block_given?
    raise ArgumentError.new("Cannot provide both a default arg and block to #fetch")
  end

  result = public_send(:[], key)

  if result.nil?
    return yield(key) if block_given?
    return not_found_value unless not_found_value == NO_DEFAULT_GIVEN

    raise KeyError.new("key not found #{key}")
  else
    result
  end
end