不能将'Reader'作为字段来'强制'数据类型

Question

我有以下Haskell代码，可以完美编译：

import Control.Monad.Reader (Reader (..))
import Data.Coerce (Coercible, coerce)

data Flow i o = Flow (i -> o) (o -> i)

coerceFlow
    :: (Coercible i i', Coercible o o')
    => Flow i o
    -> Flow i' o'
coerceFlow = coerce

但是，如果我将Flow类型的定义更改为以下内容：

data Flow i o = Flow (i -> Reader Int o) (o -> i)

我开始看到一个奇怪的错误：

Coerce.hs:10:14: error:
    • Couldn't match type ‘o’ with ‘o'’ arising from a use of ‘coerce’
      ‘o’ is a rigid type variable bound by
        the type signature for:
          coerceFlow :: forall i i' o o'.
                        (Coercible i i', Coercible o o') =>
                        Flow i o -> Flow i' o'
        at Coerce.hs:(6,1)-(9,17)
      ‘o'’ is a rigid type variable bound by
        the type signature for:
          coerceFlow :: forall i i' o o'.
                        (Coercible i i', Coercible o o') =>
                        Flow i o -> Flow i' o'
        at Coerce.hs:(6,1)-(9,17)
    • In the expression: coerce
      In an equation for ‘coerceFlow’: coerceFlow = coerce
    • Relevant bindings include
        coerceFlow :: Flow i o -> Flow i' o' (bound at Coerce.hs:10:1)
   |
10 | coerceFlow = coerce
   |              ^^^^^^

据我了解，我的数据类型不再自动为Coercible。有没有办法告诉GHC我可以自动强制Flow类型的值？我可以手动coerce每个字段，但我想一次coerce整个数据类型（这对于DerivingVia起作用是必需的。）

我尝试使用RoleAnnotations扩展名，如下所示：

type role Flow representational representational
但是我看到一个错误：

Coerce.hs:6:1: error:
    • Role mismatch on variable o:
        Annotation says representational but role nominal is required
    • while checking a role annotation for ‘Flow’
  |
6 | type role Flow representational representational
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

我具有以下经过完美编译的Haskell代码：import Control.Monad.Reader（Reader（..））import Data.Coerce（Coercible，coerce）data Flow io = Flow（i-> o）（o-> i）coerceFlow ...

Answer 1

让我们调查：