haskell - () 作为空约束

Question

如何表示空约束？

对于以下文件

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE KindSignatures #-}

import Data.Kind(Type, Constraint)

type Empty = (() :: Type -> Constraint)

main :: IO ()
main = return ()

ghc 8.2.2 答案

constraint.hs:6:15: error:
    • Expected kind ‘* -> Constraint’, but ‘()’ has kind ‘*’
    • In the type ‘(() :: Type -> Constraint)’
      In the type declaration for ‘Empty’
  |
6 | type Empty = (() :: Type -> Constraint)
  |  

我想念什么？

我知道以下解决方案

{-# LANGUAGE FlexibleInstances #-}

class Empty x
instance Empty x

但我想知道为什么 () 不起作用

Answer 1

() has kind * or Constraint, depending on context, never a -> Constraint. Similarly (,) has kind * -> * -> * or Constraint -> Constraint -> Constraint, depending on context.

— Simon Peyton-Jones

只是 () 仅针对类型或约束进行重载。也就是说，您应该编写 () => a，而不是 (() a) => a。所以我认为:

class Empty x
instance Empty x

这里是正确的解决方案。 (也许类似的东西应该在 base 中。)