(local Either {})
(local Left {})
(local Right {})
(setmetatable Right Either)
(setmetatable Left Either)

(fn Either.new [self x]
  (local obj { :value (or x {}) })
  (tset self "__index" self)
  (setmetatable obj self))
(fn Either.of [x] (Right:new x))

(fn Right.map [self f] (Either.of (f self.value)))
(fn Left.map [self f] self)

{
 : Either
 : Left
 : Right
}