测试两个异构值之间的相等性

Question

我正在使用-XExistentialQuantificationGHC扩展为特定类型类(Shape)的值创建异构容器:

-- Container type
data Object = forall a. Shape a => Object a

-- 'Shape' class. Methods not important
class Eq s => Shape s where
    doStuff :: s -> s

鉴于所有实例Shape都是实例Eq,是否有办法制作Object实例Eq？

Answer 1

如果添加Typeable约束,则可以:

import Data.Typeable

data Object = forall a. (Shape a, Typeable a) => Object a

instance Eq Object where
  Object x == Object y =
    case cast y of
      Just y' -> x == y'
      Nothing -> False 

这里,如果所需的类型(从使用with 推断)与实际类型匹配,cast y则返回,否则. Just y'y'==xyNothing