问题描述
假设我正在编写一个向量乘法程序.遵循本文中的要求:
Assuming that I'm writing a program for vector multiplication. Following the requirement in this article:
https://etrain.github.io/2015/05/28/type-safe-linear-algebra-in-scala
只有当两个向量的维数相等时，乘法才能成功编译.为此，我定义了一个通用类型 Axis，它使用一个无形的文字类型(维数)作为类型参数:
The multiplication should only compile successfully if the dimension of both vectors are equal. For this I define a generic type Axis that uses a shapeless literal type (the number of dimension) as the type parameter:
import shapeless.Witness
trait Axis extends Serializable
case object UnknownAxis extends Axis
trait KnownAxis[W <: Witness.Lt[Int]] extends Axis {
def n: Int
def ++(that: KnownAxis[W]): Unit = {}
}
object KnownAxis {
val w1 = Witness(1)
val w2 = Witness(2)
case class K1(n: Witness.`1`.T) extends KnownAxis[w1.type]
case class K2(n: Witness.`2`.T) extends KnownAxis[w2.type]
// K2(2) ++ K1(1) // doesn't compile
K2(2) ++ K2(2)
}
到目前为止一切顺利，但是当我尝试将其概括为所有 n 时出现了问题:
So far so good, the problem however appears when I try to generalise it for all n:
case class KN[W <: Witness.Lt[Int]](n: W#T) extends KnownAxis[W]
KN(1)
以上代码触发编译错误:
The above code triggers a compilation error:
Axis.scala:36: type mismatch;
found : Int(1)
required: this.T
[ERROR] KN(1)
[ERROR] ^
[ERROR] one error found
我的问题是:为什么 Spark 无法专注于更精细的类型 Witness.`1`.T，而是使用类型 Int?覆盖此行为需要什么才能成功定义案例类 KN?
My question is: why Spark is incapable of focusing on the more refined type Witness.`1`.T, instead it is using type Int? What does it take to override this behaviour so case class KN can be successfully defined?
更新 1: 后续已移至新问题:
当利用Scalaless的单例类型特性，如何强制编译器使用窄/单例类型作为隐式参数?
推荐答案
Scala 无法推断 W 给定 W#T 这并不奇怪，因为通常也就是说，两个不同的 W 可能具有相同的 W#T.不适用于见证类型，但编译器不会对其进行特殊处理.
It isn't surprising that Scala can't infer W given W#T, because generally speaking it's possible for two different Ws to have the same W#T. Not for witness types, but they aren't treated specially by the compiler.
我期望的工作(但不确定为什么不这样做)是指定类型参数:
What I expected to work (and not sure why it doesn't) is to specify the type parameter:
KN[Witness.`1`](1)
// error: type arguments [scala.this.Any] do not conform to method apply's type parameter bounds [W <: shapeless.this.Witness.Lt[scala.this.Int]]
或者更有可能
KN[w1.type](1)
// error: type mismatch;
// found : scala.this.Int(1)
// required: .this.T
什么有效:
case class KN[W <: Witness.Lt[Int]](w: W) extends KnownAxis[W] {
val n = w.value
}
KN(Witness(1))
它似乎符合您问题中的要求，但我不知道它是否适用于您的其余代码.
It seems to suit the requirements in your question, but I don't know if it'll work with the rest of your code.
您可能还想考虑这种在 Scala 2.13 中不需要 Shapeless 的替代方案:
You may also want to consider this alternative which doesn't require Shapeless in Scala 2.13:
trait Axis extends Serializable
case object UnknownAxis extends Axis
trait KnownAxis[W <: Int with Singleton] extends Axis {
def n: W
def ++(that: KnownAxis[W]): Unit = {}
}
case class KN[W <: Int with Singleton](n: W) extends KnownAxis[W]
KN(1)
对于 2.12
import shapeless.syntax.singleton._
...
KN(1.narrow)
这篇关于在 scala shapeless 中，是否可以将文字类型用作泛型类型参数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！