有趣的是,如果
HList
中没有适当类型的元素可用,编写一个不会编译的版本要容易得多。
import shapeless._, ops.hlist.Selector
def applyWithHList2[A1, A2, R, L <: HList](l: L, f: (A1, A2) => R)(implicit
selA1: Selector[L, A1],
selA2: Selector[L, A2]
): R = f(selA1(l), selA2(l))
如果您确实希望在没有适用的配对时(在Try
中)出现运行时错误,那么可以使用默认的null
实例技巧:
import scala.util.{ Failure, Success, Try }
def applyWithHList2[A1, A2, R, L <: HList](l: L, f: (A1, A2) => R)(implicit
selA1: Selector[L, A1] = null,
selA2: Selector[L, A2] = null
): Try[R] = Option(selA1).flatMap(s1 =>
Option(selA2).map(s2 => f(s1(l), s2(l)))
).fold[Try[R]](Failure(new MatchError()))(Success(_))
如果你觉得这不太好(确实如此),你可以使用自定义类型类:
trait MaybeSelect2[L <: HList, A, B] {
def apply(l: L): Try[(A, B)] = (
for { a <- maybeA(l); b <- maybeB(l) } yield (a, b)
).fold[Try[(A, B)]](Failure(new MatchError()))(Success(_))
def maybeA(l: L): Option[A]
def maybeB(l: L): Option[B]
}
object MaybeSelect2 extends LowPriorityMaybeSelect2 {
implicit def hnilMaybeSelect[A, B]: MaybeSelect2[HNil, A, B] =
new MaybeSelect2[HNil, A, B] {
def maybeA(l: HNil): Option[A] = None
def maybeB(l: HNil): Option[B] = None
}
implicit def hconsMaybeSelect0[H, T <: HList, A](implicit
tms: MaybeSelect2[T, A, H]
): MaybeSelect2[H :: T, A, H] = new MaybeSelect2[H :: T, A, H] {
def maybeA(l: H :: T): Option[A] = tms.maybeA(l.tail)
def maybeB(l: H :: T): Option[H] = Some(l.head)
}
implicit def hconsMaybeSelect1[H, T <: HList, B](implicit
tms: MaybeSelect2[T, H, B]
): MaybeSelect2[H :: T, H, B] = new MaybeSelect2[H :: T, H, B] {
def maybeA(l: H :: T): Option[H] = Some(l.head)
def maybeB(l: H :: T): Option[B] = tms.maybeB(l.tail)
}
}
trait LowPriorityMaybeSelect2 {
implicit def hconsMaybeSelect2[H, T <: HList, A, B](implicit
tms: MaybeSelect2[T, A, B]
): MaybeSelect2[H :: T, A, B] = new MaybeSelect2[H :: T, A, B] {
def maybeA(l: H :: T): Option[A] = tms.maybeA(l.tail)
def maybeB(l: H :: T): Option[B] = tms.maybeB(l.tail)
}
}
然后:
def applyWithHList2[A1, A2, R, L <: HList](l: L, f: (A1, A2) => R)(implicit
ms2: MaybeSelect2[L, A1, A2]
): Try[R] = ms2(l).map(Function.tupled(f))
但这样做太麻烦了,只是为了放弃一些编译时的安全性。
请注意,这些方法都没有强制执行函数可以应用于HList
中最多只有一对元素的约束条件,因为我将其视为您问题中的前提条件。肯定有可能编写一个在编译时强制执行该约束条件的解决方案(甚至可能比上面的MaybeSelect2
实现更短)。