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Question

我不太确定我做错了什么：

data Vector2D u = Vector2D { 
    _x :: u, 
    _y :: u 
} deriving stock (Show, Eq, Functor, Foldable, Traversable)

{-# INLINE addVector2 #-}
addVector2 :: (Additive a) => Vector2D a -> Vector2D a -> Vector2D a 
addVector2 (Vector2D { _x = x1, _y = y1 }) (Vector2D { _x = x2, _y = y2 }) = 
    Vector2D { _x = x1 + x2, _y = y1 + y2 }

instance (Additive a) => Additive (Vector2D a) where
    (+) = addVector2

newtype Square a = Square {
    unpackSquare :: Vector2D a
} deriving stock (Show, Eq)

到目前为止，一切正常（Additive 在 algebra 包中定义，但非常不言自明）。

但是，现在我想聪明地使用 DerivingVia 和 StandaloneDeriving，但我什至无法编译下一行

deriving instance (Additive a) => Additive (Square a) via (Vector2D a)

但这让我感动

    * Expected kind `k0 -> * -> Constraint',
        but `Additive (Square a)' has kind `Constraint'
    * In the stand-alone deriving instance for
        `(Additive a) => Additive (Square a) via (Vector2D a)'

谁能告诉我我做错了什么？我是运行 GHC 8.6.2

Answer 1

是

deriving via (Vector2D a) instance (Additive a) => Additive (Square a)

你写的方式，via看起来像一个类型变量

deriving instance (Additive a) => Additive (Square a) via (Vector2D a)
-- <==>
deriving instance forall a via. (Additive a) => Additive (Square a) (via) (Vector2D a)

这会产生种类不匹配错误，因为 Additive (Square a) :: Constraint 已经饱和，但您已将其应用于另外两个参数。

这是 shorthand 形式的倒退：

data T' = ...
  deriving Class via T

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Deriving Via With Standalone Deriving

haskell

deriving

derivingvia