编译时检查某些属性 [英] Compile time check on some property
问题描述
给定以下 Scala 代码:
Given the following scala code:
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 Red
和 isBroken = true
的 Car.否则它应该发出编译错误.此外,输出 B
应该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,它应该相应地编译.
That is, what should A
and B
be? The method should only accept a Car that is both Red
and isBroken = true
. Otherwise it should emit a compile error. Moreover, the output B
should contain information on its type such that if I create another method destroyRedCar(c: B)
and apply it on the output, it should compile accordingly.
推荐答案
你应该将你的数据移动到类型级别然后:
You should move your data to the type-level then:
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$1@67d9a642
摧毁"它:
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]{})
^
如果您需要变异"Cr
类型(从另一种类型构造一种类型,更准确地说):
If you need to "mutate" Cr
type (construct one type from another, more precisely):
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
你也可以在 Car
的 trait 中定义一些 def copy[C <: Color, B <: Bool]: Copy[C, B]
方法(就像在 case 类中)而不仅仅是 asInstanceOf
.
You may also define some def copy[C <: Color, B <: Bool]: Copy[C, B]
method inside Car
's trait (like in case classes) instead of just asInstanceOf
.
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