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,我的数据类型不再是Coercible
自动的。有没有办法告诉 GHC 我可以Flow
自动强制类型值?我可以coerce
手动输入每个字段,但我想coerce
一次输入整个数据类型(这是DerivingVia
工作所必需的)。
我尝试使用这样的RoleAnnotations
扩展:
type role Flow representational representational
但我看到一个错误:
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
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^