Question

我是Haskell的新手，所以如果我忽略了一些基本的东西，我会道歉，但我正在创建一个有三个参数的Coord类型：Position，Vector和Scalar。 Coord必须是Num的实例，并且必须具有特定的实现方法。我相信我已经拥有了我所需要的东西，但我假设我错过了一些基本的东西，因为我收到了基于Num实例的错误。

我的代码：

data Position x y = Position (x, y)
data Vector x y = Vector (x, y)
data Scalar n = Scalar n
data Coord x y n = Coord (Position x y, Vector x y, Scalar n)

instance Num Coord where
    negate (Position (x, y)) = Position (-x, -y)
    negate (Vector (x, y)) = Vector (-x, -y)
    (+) (Vector (a, b) Position (x, y)) = Position (x+a, y+b)
    (+) (Vector (a, b) Vector (c, d)) = Vector (c+a, d+b)
    (+) (Scalar x Scalar y) = Scalar (x+y)
    (*) (Vector (a, b) Scalar c) = Vector (a*c, b*c)
    (*) (Position (x1, y1) Position (x2, y2)) = (x1*x2) + (y1*y2)
    (*) (Scalar x Scalar y) = Scalar (x*y)
    (-) (Vector (a, b) Position (x, y)) = Position (x-a, y-b)
    (-) (Position (x1, y1) Position (x2, y2)) = Vector (x2-x1, y2-y1)
    (-) (Scalar x Scalar y) = Scalar (x-y)
    abs (Position (x, y)) = Scalar sqrt((x*x)+(y*y))
    abs (Vector (x, y)) = Scalar sqrt((x*x)+(y*y))
    signum _ = error "undefined"
    fromInteger _ = error "undefined"

我得到的错误：

Expecting three more arguments to ‘Coord’
The first argument of ‘Num’ should have kind ‘*’,
  but ‘Coord’ has kind ‘* -> * -> * -> *’
In the instance declaration for ‘Num Coord’

任何有关如何使用Num的说明都将非常感激（我假设这是导致错误的原因）

谢谢。

Answer 1

在查看建议后，我终于找到了一个有效的解决方案。

data Coord = Position (Double, Double) | Vector (Double, Double) | Scalar Double deriving (Show)

instance Num Coord where
    negate (Position (x, y)) = Position (-x, -y)
    negate (Vector (x, y)) = Vector (-x, -y)
    (+) (Vector (a, b)) (Position (x, y)) = Position (x+a, y+b)
    (+) (Position (x, y)) (Vector (a, b)) = Position (x+a, y+b)
    (+) (Vector (a, b)) (Vector (c, d)) = Vector (c+a, d+b)
    (+) (Scalar x) (Scalar y) = Scalar (x+y)
    (*) (Vector (a, b)) (Scalar c) = Vector (a*c, b*c)
    (*) (Scalar c) (Vector (a, b)) = Vector (a*c, b*c)
    (*) (Position (x1, y1)) (Position (x2, y2)) = Scalar((x1*x2) + (y1*y2))
    (*) (Scalar x) (Scalar y) = Scalar (x*y)
    (-) (Vector (a, b)) (Position (x, y)) = Position (x-a, y-b)
    (-) (Position (x, y)) (Vector (a, b)) = Position (a-x, b-y)
    (-) (Position (x1, y1)) (Position (x2, y2)) = Vector (x2-x1, y2-y1)
    (-) (Scalar x) (Scalar y) = Scalar (x-y)
    abs (Position (x, y)) = Scalar (sqrt((x*x)+(y*y)))
    abs (Vector (x, y)) = Scalar (sqrt((x*x)+(y*y)))
    signum _ = error "undefined"
    fromInteger _ = error "undefined"

希望这可以帮助将来犯同样错误的人。

Answer 2

你的问题：括号。

而不是

negate Position (x, y) = Position (-x, -y)

DO

negate (Position (x, y)) = Position (-x, -y)

前者隐含地暗示negate是一个带有两个参数的函数 - Position值构造函数和(x,y)，而不是类型为Position的单个参数，因为它将其解析为

((negate Position) (x,y)) = ...

您的其他功能定义也有同样的问题。

Haskell Num实例

2 个答案: