在类型级别未定义 [英] Undefined at the type level

问题描述

通常当我使用Haskell代码时，我会用类型注释来存储，而 undefined 。

Often when I'm playing with Haskell code, I stub things out with a type annotation and undefined.

foo :: String -> Int
foo = undefined

是否有可以使用的类型级别undefined以类似的方式吗？

Is there a type-level "undefined" that I could use in a similar way?

（理想情况下，结合一种注释）

type Foo :: * -> *
type Foo = Undefined

进一步思考同一个线程：有没有办法我为这种方式创建类型的类型类实例？比以下理论方法更简单的方法？

Further thought on the same thread: is there a way for me to stub out typeclass instances for types created this way? An even easier way than the following theoretical way?

instance Monad Foo where
  return = undefined
  (>>=) = undefined

推荐答案

可以使用 EmptyDataDecls 存根出一个类型，并与 KindSignatures 你可以给它一种：

You can use EmptyDataDecls to stub out a type, and with KindSignatures you can give it a kind:

{-# LANGUAGE EmptyDataDecls, KindSignatures #-}

data Foo :: * -> *

您也可以将 Monad 没有警告的实例使用GHC的这个选项。

You can also stub out the Monad instance without warnings with this option to GHC.

{-# OPTIONS_GHC -fno-warn-missing-methods #-}

instance Monad Foo

然后您不需要任何实现 return 和>> = 。

And then you don't need to leave any implementation for return and >>=.

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