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通过独立派生

deriving haskell

3

Julian Birch · 技术社区 · 7 年前

我不太确定我在这里做错了什么:

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,一切都是正常的(加法在代数包中定义,但很容易解释)。

不过,现在我想巧妙地使用派生VIA和Standalonederiving,甚至无法编译下一行

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

但这让我

    * 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)'

有人能告诉我我做错了什么吗?我在运行GHC 8.6.2

1 回复 | 直到 7 年前

1

9

HTNW 7 年前

它是

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)

这会产生一种失配误差,比如 Additive (Square a) :: Constraint 已经饱和,但您已经将其应用于另外两个参数。

这与速记形式相反:

data T' = ...
  deriving Class via T