Question

考虑以下代码

{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Test () where

import Data.Proxy
import Data.Void
import GHC.TypeLits

data N = S N | Z
type family Index e l where
  Index e (Either e x) = 'Z
  Index e (Either e' y) = S (Index e y)
  Index e Void = TypeError ('Text " not found")

在GHCi上加载这个我运行跟随

λ> :set -XRankNTypes
λ> :kind! forall a . Index a (Either a Void)
forall a . Index a (Either a Void) :: N
= 'Z
λ> :kind! forall a . Index a (Either a (Either Int Void))
forall a . Index a (Either a (Either Int Void)) :: N
= 'Z
λ> :kind! forall a . Index a (Either Int (Either a Void))
forall a . Index a (Either Int (Either a Void)) :: N
= Index a (Either Int (Either a Void))
λ> :kind! Index Char (Either Int (Either Char Void))
Index Char (Either Int (Either Char Void)) :: N
= 'S 'Z

现在很明显GHC没有评估Index e (Either Int y) = S (Index e y)，因为e可能是Int所以它就会停止。有没有普遍量化e的方法，以便e ~ Int不起作用 forall e，然后继续？理想情况下，我想得到：

λ> :kind! forall a . Index a (Either Int (Either Char Void))
forall a . Index a (Either Int (Either Char Void)) :: N
= 'S ('S (TypeError ...))

输入家庭来检查有效性而不是可满足性

0 个答案: