3
{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-}
import Data.Typeable;

data EnumBox = forall s. (Enum s, Show s) => EB s
           deriving Typeable

instance Show EnumBox where
  show (EB s) = "EB " ++ show s

这行得通。但是,如果我想为 EnumBox 添加一个 Class Enum 的实例,喜欢:

instance Enum EnumBox where
  succ (EB s) = succ s

它失败并显示以下消息:

Could not deduce (s ~ EnumBox)
from the context (Enum s, Show s)
  bound by a pattern with constructor
             EB :: forall s. (Enum s, Show s) => s -> EnumBox,
           in an equation for `succ'
  at typeclass.hs:11:9-12
  `s' is a rigid type variable bound by
      a pattern with constructor
        EB :: forall s. (Enum s, Show s) => s -> EnumBox,
      in an equation for `succ'
      at typeclass.hs:11:9
In the first argument of `succ', namely `s'
In the expression: succ s
In an equation for `succ': succ (EB s) = succ s

为什么第一个节目可以推断出第二个成功却不能?

4

1 回答 1

2

你唯一的问题是succ有类型

succ :: Enum a => a -> a

所以你需要

succ (EB s) = EB . succ $ s

只是再次装箱。

你也可能想要

instance Enum EnumBox where
    toEnum = EB
    fromEnum (EB i) = fromEnum i

因为这是完整性的最低定义,因为

succ = toEnum . succ . fromEnum
于 2013-09-01T16:25:22.637 回答