haskell - 比较 Either 类型的内容

Question

有没有办法对类型构造函数使用模式通配符来使这个丑陋的代码更短:

eitherCompare (Left  a) (Left  b) = compare a b
eitherCompare (Left  a) (Right b) = compare a b
eitherCompare (Right a) (Left  b) = compare a b
eitherCompare (Right a) (Right b) = compare a b

类似的东西(不会编译)

eitherCompare :: Ord a => Either a a -> Either a a -> Ordering
eitherCompare (_  a) (_  b) = compare a b

或者其他方法？

Answer 1

您无法通过模式匹配来做到这一点，但您仍然可以通过使用辅助函数来简化代码:

eitherCompare x y = compare (fromEither x) (fromEither y)
 where fromEither (Left a) = a
       fromEither (Right a) = a