[英]Shapeless type disjunction for more then 2 types
如何为无形中的 3 种或更多类型提供类型析取? 例子:
import shapeless._
object Tst extends App {
sealed trait Base
final case class A() extends Base
final case class B() extends Base
final case class C() extends Base
final case class D() extends Base
def AorB[T: (A |∨| B)#λ](t: T): Unit =
t match {
case _: A => println("A")
case _: B => println("B")
}
AorB(A()) //Ok
AorB(B()) //Ok
def AorBorC[T: (A |∨| B |∨| C)#λ](t: T): Unit =
t match {
case _: A => println("A")
case _: B => println("B")
case _: C => println("C")
}
AorBorC(A()) //compile-error
AorBorC(B()) //compile-error
AorBorC(C()) //Ok
}
从 2 种类型的析取中可以看出,它完全可以正常工作。 但是对于 3 种类型的析取,它不能按预期工作。
编译错误是:
Error:(28, 10) Cannot prove that (Tst.A => Nothing) => Nothing <:< Object{type λ[X] = (X => Nothing) => Nothing <:< Tst.A => Nothing with Tst.B => Nothing => Nothing} => Nothing with Tst.C => Nothing => Nothing.
AorBorC(A())
和
Error:(29, 10) Cannot prove that (Tst.B => Nothing) => Nothing <:< Object{type λ[X] = (X => Nothing) => Nothing <:< Tst.A => Nothing with Tst.B => Nothing => Nothing} => Nothing with Tst.C => Nothing => Nothing.
AorBorC(B())
shapeless.|∨|
不适用于 2 种以上的类型。
http://milessabin.com/blog/2011/06/09/scala-union-types-curry-howard/
对于超过 2 种类型的编码变得更加复杂。
一种编码用于 2、4、8... 类型
type ¬¬¬¬[T] = ¬¬[¬¬[T]]
type |∨∨|[T, U] = {
type λ[X] = ¬¬¬¬[X] <:< (T ∨ U)
}
def AorBorC[T: ((A ∨ B) |∨∨| (C ∨ C))#λ](t: T): Unit =
t match {
case _: A => println("A")
case _: B => println("B")
case _: C => println("C")
}
AorBorC(A()) //Ok
AorBorC(B()) //Ok
AorBorC(C()) //Ok
另一个是任意数量的类型
trait Disj[T] {
type or[S] = Disj[T with ¬[S]]
type apply = ¬[T]
}
type ∨∨∨[T1, T2, T3] = Disj[¬[T1]]#or[T2]#or[T3]#apply
type |∨∨∨|[T1, T2, T3] = {
type λ[X] = ¬¬[X] <:< ∨∨∨[T1, T2, T3]
}
def AorBorC[T: |∨∨∨|[A, B, C]#λ](t: T): Unit =
t match {
case _: A => println("A")
case _: B => println("B")
case _: C => println("C")
}
AorBorC(A()) //Ok
AorBorC(B()) //Ok
AorBorC(C()) //Ok
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