class Union (left :: # Type) (right :: # Type) (union :: # Type) 
  | left right -> union, right union -> left, union left -> right

こっちの定義の方がやや見やすい

class Union (left :: Row k) (right :: Row k) (union :: Row k) | left right -> union, right union -> left, union left -> right

union :: forall r1 r2 r3. Union r1 r2 r3 => { | r1 } -> { | r2 } -> { | r3 }

union { x: 1, y: "y" } { y: 2, z: true }
 :: { x :: Int, y :: String, y :: Int, z :: Boolean }

rowが重複していても、全部mergeしている