小编sup*_*tux的帖子

以下是我到目前为止的尝试:

module Main where

data FooT = One | Two deriving (Show, Read)
{-
That is what I want
foo :: (Show a, Read a) => a
foo = One
-}

--class Footable (Show a, Read a) => a where
class Footable a where
  --foo2 :: (Show a, Read a) => a
  foo2 :: a

instance Footable FooT where
  foo2 = One

-- test = print foo2

我想测试编译.我认为问题不在于普遍量化.ghc说a是一个'严格的类型变量' 编辑(刚性类型变量),但我真的不理解这是什么.这个问题似乎与此有关

编辑

正如我在@ sepp2k的评论中写的那样,它可能与存在主义类型有关,但我偶然发现了一个奇怪的行为:

这确实编译: …