Flatten Ruby implementation

flatten.rb

# frozen_string_literal: true

def flatten(arg)
  return "arg should be an array, but was #{arg.inspect} instead" if arg.nil? || !arg.is_a?(Array)

  @result = []
  analyze(arg)
  @result
end

def analyze(member)
  member.each do |m|
    m.is_a?(Array) ? analyze(m) : @result << m
  end
end

arg = [[1, 2, [3]], 4]
puts flatten(arg).inspect