为 Linear.V 派生包装器时出现类型检查错误

Question

我正在尝试为 Linear.V 类型制作一个新类型包装器并派生有用的实例。我在尝试这个：

{-# LANGUAGE DataKinds, PolyKinds, ScopedTypeVariables, 
StandaloneDeriving, FlexibleContexts, UndecidableInstances, 
GeneralizedNewtypeDeriving, PartialTypeSignatures, TypeFamilies #-}

import Linear.V 
import Control.Lens.At

data Foo = Foo1 | Foo2 deriving (Show, Eq)

尝试 1 - 我认为 GeneralizedNewtypeDeriving 可以，但不行：

newtype Bar n = Bar {
  getBar :: V n Foo
} deriving (Show, Eq, Ixed)

我收到这个错误：

    • Couldn't match representation of type ‘f (V n Foo)’
                               with that of ‘f (Bar n)’
        arising from the coercion of the method ‘ix’
          from type ‘Index (V n Foo)
                     -> Control.Lens.Type.Traversal' (V n Foo) (IxValue (V n Foo))’
            to type ‘Index (Bar n)
                     -> Control.Lens.Type.Traversal' (Bar n) (IxValue (Bar n))’
      NB: We cannot know what roles the parameters to ‘f’ have;
        we must assume that the role is nominal
    • When deriving the instance for (Ixed (Bar n))

我已经尝试 2 使用这样的独立推导：

newtype Bar n = Bar {
  getBar :: V n Foo
} deriving (Show, Eq)
type instance Index (Bar n) = Int
type instance IxValue (Bar n) = Foo

deriving instance Ixed (V n Foo) => Ixed (Bar n)

但后来我得到了一个不同的错误：

• Couldn't match representation of type ‘f1 (V n Foo)’
                           with that of ‘f1 (Bar n)’
    arising from a use of ‘GHC.Prim.coerce’
  NB: We cannot know what roles the parameters to ‘f1’ have;
    we must assume that the role is nominal
• In the expression:
    GHC.Prim.coerce
      @(Index (V n Foo)
        -> Control.Lens.Type.Traversal' (V n Foo) (IxValue (V n Foo)))
      @(Index (Bar n)
        -> Control.Lens.Type.Traversal' (Bar n) (IxValue (Bar n)))
      ix
  In an equation for ‘ix’:
      ix
        = GHC.Prim.coerce
            @(Index (V n Foo)
              -> Control.Lens.Type.Traversal' (V n Foo) (IxValue (V n Foo)))
            @(Index (Bar n)
              -> Control.Lens.Type.Traversal' (Bar n) (IxValue (Bar n)))
            ix
  When typechecking the code for ‘ix’
    in a derived instance for ‘Ixed (Bar n)’:
    To see the code I am typechecking, use -ddump-deriv
  In the instance declaration for ‘Ixed (Bar n)’
• Relevant bindings include
    ix :: Index (Bar n)
          -> Control.Lens.Type.Traversal' (Bar n) (IxValue (Bar n))
      (bound at a.hs:12:1)

我不确定为什么会出现这两个错误。这能以某种方式完成吗？我对高级类型级别的功能不太熟悉，到目前为止我也无法真正手动编写这个特定的实例定义，所以我也认为这是一个解决方案。但我更愿意以某种方式使用 deriving 机制，因为它似乎更可重用。

编辑：我试过这个手动实例声明：

type instance Index (Bar n) = Int
type instance IxValue (Bar n) = Foo

instance Ixed (Bar n) where
  ix i f (Bar v) = ix i f v

但这会产生以下错误：

• Couldn't match type ‘V n Foo’ with ‘Bar n’
  Expected type: f (Bar n)
    Actual type: f (V n Foo)
• In the expression: ix i f v
  In an equation for ‘ix’: ix i f (Bar v) = ix i f v
  In the instance declaration for ‘Ixed (Bar n)’
• Relevant bindings include
    v :: V n Foo (bound at a.hs:14:15)
    f :: IxValue (Bar n) -> f (IxValue (Bar n)) (bound at a.hs:14:8)
    i :: Index (Bar n) (bound at a.hs:14:6)
    ix :: Index (Bar n)
          -> Control.Lens.Type.Traversal' (Bar n) (IxValue (Bar n))
      (bound at a.hs:14:3)

在我看来，编译器无法弄清楚 V n Foo 和 Bar n 的 Index 是 Int。但我不确定。

Answer 1

你快到了。剩下的问题就是改造 ix traversal for the underlying V n Foo, which ultimately returns a function V n Foo -> f (V n Foo), into an ix traversal for the wrapper type Bar n, which should ultimately return a function Bar n -> f (Bar n). We have to "unpack" the definition of Traversal' 就知道了。

在您的代码中，ix i f v 的类型为 f (V n Foo)，因此使用 Bar 构造函数 fmap 就足够了：

type instance Index (Bar n) = Int
type instance IxValue (Bar n) = Foo

instance Ixed (Bar n) where
  ix i f (Bar v) = fmap Bar (ix i f v)

为 Linear.V 派生包装器时出现类型检查错误

A typecheck errors in deriving wrapper for Linear.V

haskell

typeclass

haskell-lens

newtype

deriving