不同类型的列表？ [英] List of different types?

问题描述

data Plane = Plane { point :: Point, normal :: Vector Double }
data Sphere = Sphere { center :: Point, radius :: Double }

class Shape s where
    intersect :: s -> Ray -> Maybe Point
    surfaceNormal :: s -> Point -> Vector Double

我也制作了 Plane 和 Sphere 形状。

试图将球体和飞机存储在同一个列表中，但它不起作用。我知道它不应该工作，因为 Sphere 和 Plane 是两种不同的类型，但它们都是 Shape ，所以它不应该工作？如何将形状和平面存储在列表中？

I'm trying to store spheres and planes in the same list, but it doesn't work. I understand that it shouldn't work because Sphere and Plane are two different types, but they are both instances of Shape, so shouldn't it work? How would I store shapes and planes in a list?

shapes :: (Shape t) => [t]
shapes = [ Sphere { center = Point [0, 0, 0], radius = 2.0 },
         Plane { point = Point [1, 2, 1], normal = 3 |> [0.5, 0.6, 0.2] }
         ]

推荐答案

<这个问题代表了面向对象和功能思维之间的转折点。有时候，即使是复杂的Haskellers也仍然处于这种心理转变之中，他们的设计经常会落入 存在型类型化模式。

这个问题的一个功能性解决方案涉及将类型类型转换为数据类型（通常一旦完成，需要类型类型消失）：

A functional solution to this problem involves reifying the typeclass into a data type (usually once this is done, the need for the typeclass vanishes):

data Shape = Shape {
    intersect :: Ray -> Maybe Point,
    surfaceNormal :: Point -> Vector Double
}

现在您可以轻松构建 Shape s，因为它是一个单形类型。由于Haskell不支持向下转换，因此通过消除 Plane s和 Sphere s之间的表示区别，不会丢失任何信息。具体的数据类型变成构造 Shape s的函数：

Now you can easily construct a list of Shapes, because it is a monomorphic type. Because Haskell does not support downcasting, no information is lost by removing the representational distinction between Planes and Spheres. The specific data types become functions that construct Shapes:

plane :: Point -> Vector Double -> Shape
sphere :: Point -> Double -> Shape

如果您无法捕获所有关于中的形状的知识，按照托马斯的说法，你可以用代数数据类型枚举这些情况。但如果可能的话，我会建议不要这样做;相反，尝试找到您需要的形状的基本特征，而不是仅列举示例。

If you cannot capture everything you need to know about a shape in the Shape data type, you can enumerate the cases with an algebraic data type, as Thomas suggested. But I would recommend against that if possible; instead, try to find the essential characteristics of a shape that you need rather than just listing off examples.

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