Yeah, it's one of those rare Ruby gotchas. If you assign some kind of a data structure to a constant, you'll get a warning if you try to reassign the constant,

Yeah, it's one of those rare Ruby gotchas. If you assign some kind of a data structure to a constant, you'll get a warning if you try to reassign the constant, but the data structure itself will silently mutable. To disallow this behavior, simply freeze the data structure. If you have nested data structures, you must freeze deeply. ENUM = [0, 1, 2].freeze LOOKUP = { :A => [1, 2, 3].freeze, :B => [4, 5, 6].freeze }.freeze # or: LOOKUP = { :A => [1, 2, 3], :B => [4, 5, 6] }.tap { |s| s.values.each(&:freeze) }.freeze