class OptionClass def or_nil end def ==(that) case that when OptionClass then or_nil == that.or_nil else or_nil == that end end private def assert_option(result) case result when OptionClass then return result else raise TypeError, "Must be an Option" end end end class SomeClass < OptionClass def initialize(value) @value = value end def to_a [get] end def get @value end def get_or_else(&blk) get end def each(&blk) blk.call(get) nil end def or_nil get end def present? true end def empty? false end def map(&blk) Option(blk.call(get)) end def flat_map(&blk) assert_option(blk.call(get)) end def fold(if_empty, &blk) blk.call(get) end def exists?(&blk) !! blk.call(get) end def filter(&blk) exists?(&blk) ? self : None end def inside(&blk) blk.call(get) self end def or_else(&blk) self end end class NoneClass < OptionClass def to_a [] end def get raise IndexError, "None.get" end def get_or_else(&blk) blk.call end def each(&blk) nil end def or_nil nil end def present? false end def empty? true end def map(&blk) flat_map(&blk) end def flat_map(&blk) self end def fold(if_empty, &blk) if_empty.call end def exists?(&blk) false end def filter(&blk) self end def inside(&blk) self end def or_else(&blk) assert_option(blk.call) end end None = NoneClass.new Some = SomeClass def Some(value) SomeClass.new(value) end def Option(value) value.nil? ? None : Some(value) end