如何建模一个点? [英] How to model a point?

问题描述

我正在尝试使用 x 和 y 坐标对一个点进行建模.我想以这样一种方式来做到这一点，即不可能混淆 x 和 y 值.

I'm trying to model a point with x and y coordinates. I'd like to do it in such a way as to make it impossible to mix up the x and y values.

所以，我想出了这个:

package com.craigtreptow.scrayz

package object types {
  type X = Double
  type Y = Double
  type Point = (X,Y)
}

然后我像这样使用它:

package com.craigtreptow.scrayz.types

object Main extends App {
  val x: X = 1.1
  val y: Y = 2.2

  val p: Point = (x, x)

  println(p)
}

我预计这会产生错误，但它奏效了:

I expected that to produce an error, but it worked:

~/c/scrayz(master|✚2…)> sbt run                      
[info] Loading project definition from /Users/Ctreptow/code/scrayz/project
[info] Loading settings for project scrayz from build.sbt ...
[info] Set current project to Scrayz (in build file:/Users/Ctreptow/code/scrayz/)
[info] running com.craigtreptow.scrayz.types.Main 
(1.1,1.1)
[success] Total time: 1 s, completed Mar 9, 2020 6:08:49 PM

是否可以像这样对 Point 进行建模，使得上述代码会产生编译器错误?

Is it possible to model a Point like this, such that the above code would produce a compiler error?

推荐答案

注意给定

type X = Double
type Y = Double

我们有X = Y，即它们是等价类型.也许将每个坐标包装成自己的类型

we have X = Y, that is, they are equivalent types. Perhaps wrap each coordinate in its own type

final case class X(v: Double) extends AnyVal
object X {
  implicit def doubleToX(x: Double): X = X(x)
}
final case class Y(v: Double) extends AnyVal
object Y {
  implicit def doubleToY(y: Double): Y = Y(y)
}
final case class Point(x: X, y: Y)

val x: X = 1.1
val y: Y = 2.2

val p: Point = Point(x, x) // Error: type mismatch; found: X required: Y

注意 case class X(v: Double) 如何将 double 包装到 specialised 语义上下文中，我们知道这个 double 表示 X 轴坐标，而不是 general 纯双，可以有多种含义.类似的推理适用于将点建模为案例类而不是纯元组.

Note how case class X(v: Double) wraps the double into specialised semantic context where we know this double means X-axis coordinate, as opposed to general pure double which could take any number of meanings. Similar reasoning holds for modelling point as case class as opposed to pure tuple.

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