编译时检查一些属性

Question

给定以下 Scala 代码：

sealed trait Color
case object Red extends Color
case object Blue extends Color

sealed trait Car {
  def isBroken: Boolean
  def color: Color
}

如何定义这样的方法：

def fixBrokenRedCar(c: A): B

也就是说，A和B应该是什么？该方法应该只接受同时为 Red 和 isBroken = true 的 Car。否则它应该发出一个编译错误。此外，输出 B 应包含有关其类型的信息，这样如果我创建另一个方法 destroyRedCar(c: B) 并将其应用于输出，它应该相应地进行编译。

Answer 1

您应该将数据移动到类型级别：

trait Bool
trait T extends Bool
trait F extends Bool

trait Color
trait Red extends Color
trait Blue extends Color

trait Car[Clr <: Color, Brkn <: Bool]

def fixBrokenCar[Cr <: Car[Red, T]](c: Cr) = new Car[Red, F]{}

scala> fixBrokenCar(new Car[Blue, T]{})
<console>:16: error: inferred type arguments [Car[Blue,T]] do not conform to method fixBrokenCar's type parameter bounds [Cr <: Car[Red,T]]
              fixBrokenCar(new Car[Blue, T]{})
              ^
<console>:16: error: type mismatch;
 found   : Car[Blue,T]
 required: Cr
              fixBrokenCar(new Car[Blue, T]{})
                           ^

scala> fixBrokenCar(new Car[Red, T]{})
res3: Car[Red,F] = $anon@67d9a642

给"destroy"吧：

def destroyRedCar(c: Car[Red, _]) = true

scala> destroyRedCar(fixBrokenCar(new Car[Red, T]{}))
res10: Boolean = true

scala> destroyRedCar(new Car[Red, T]{})
res11: Boolean = true

scala> destroyRedCar(new Car[Blue, T]{})
<console>:15: error: type mismatch;
 found   : Car[Blue,T]
 required: Car[Red, ?]
              destroyRedCar(new Car[Blue, T]{})
                            ^

如果您需要 "mutate" Cr 类型（从另一种类型构造一种类型，更准确地说）：

trait Car[Clr <: Color, Brkn <: Bool] { 
   type Copy[C <: Color, B <: Bool] <: Car[C,B] // define "copy" type-method
}

trait BrandedCar[Clr <: Color, Brkn <: Bool] extends Car[Clr, Brkn] {
   type Copy[C <: Color, B <: Bool] = BrandedCar[C, B] // implement "copy" type-method
   def brand: String = "default"
}

def fixBrokenCar[Cr <: Car[Red, T]](c: Cr) = c.asInstanceOf[Cr#Copy[Red, F]]

def checkBrandedCar(c: BrandedCar[_, F]) = true // accepts only branded and fixed


scala> checkBrandedCar(new BrandedCar[Red, F]{})
res10: Boolean = true

scala> checkBrandedCar(new Car[Red, F]{})
<console>:15: error: type mismatch;
 found   : Car[Red,F]
 required: BrandedCar[?, F]
              checkBrandedCar(new Car[Red, F]{})
                              ^

scala> checkBrandedCar(fixBrokenCar(new BrandedCar[Red, T]{}))
res12: Boolean = true

您还可以在 Car 的特征中定义一些 def copy[C <: Color, B <: Bool]: Copy[C, B] 方法（例如类），而不仅仅是 asInstanceOf.

Answer 2

我假设您可以更改 class 结构。

实现您想要的目标的一种方法是使用类型级编程。这里有一个很好的堆栈溢出 post：Scala type programming resources

下面是一些基于您的原始代码的示例代码，演示了如何使用 Scala 中的类型系统实现这一点。

/* 
 * Color traits I've left the same except converted the objects 
 * to classes. This would work just as well with traits. It might 
 * even be better if the Colors such as Red and Blue are traits
 * themselves that extend Color, I'm still just learning this 
 * method myself.
 */
sealed trait Color
class Red extends Color
class Blue extends Color

/* New trait to represent whether something is broken or not */
sealed trait IsBroken    
class Broken extends IsBroken
class NotBroken extends IsBroken

/* Change Car trait to have two type parameters, broken and color */   
trait Car[T <: Color, S <: IsBroken]

/* fixBrokenCar signature includes whether a car is broken and it's color */
def fixBrokenRedCar(c: Car[Red, Broken]): Car[Red, NotBroken]
    = new Car[Red, NotBroken]{}

val brokenRedCar = new Car[Red, Broken]{}
val fixedRedCar = new Car[Red, NotBroken]{}
val brokenBlueCar = new Car[Blue, Broken]{}

/* Compiles */
fixBrokenRedCar(brokenRedCar)

/* Doesn't compile */
fixBrokenRedCar(fixedRedCar)

/* Doesn't compile */
fixBrokenRedCar(brokenBlueCar)

编译时检查一些属性

Compile time check on some property

types

scala

编译时检查一些 属性

Compile time check on some property

types

scala

编译时检查一些属性