Question

在对磁体模式和隐式分辨率顺序here做了一些指示后，我遇到了一个奇怪的问题模式匹配我的磁体类。在第一个例子中，我得到有关非法循环继承的错误，但是通过切换模式匹配我不会：

sealed trait SeqOrFun[T1, T2]
case class SeqMagnet[T](itr: Iterable[T]) extends SeqOrFun[T, T]
case class FunMagnet[T1, T2](f: (T1 => T2)) extends SeqOrFun[T1, T2]

def unfurl[T1, T2](x: SeqOrFun[T1, T2], y: SeqOrFun[T1, T2]): (Iterable[T1], Iterable[T2]) = {
  (x, y) match {
    case (SeqMagnet(xs: Iterable[T1]), FunMagnet(yf: (T1 => T2))) => (xs, xs.map(yf))
  }
}

导致：

<console>:14: error: illegal cyclic inheritance involving unfurl
           case (SeqMagnet(xs: Iterable[T1]), FunMagnet(yf: (T1 => T2))) => (xs, xs.map(yf))
                             ^
<console>:14: error: illegal cyclic inheritance involving unfurl
           case (SeqMagnet(xs: Iterable[T1]), FunMagnet(yf: (T1 => T2))) => (xs, xs.map(yf))
                          ^
<console>:14: error: illegal cyclic inheritance involving unfurl
           case (SeqMagnet(xs: Iterable[T1]), FunMagnet(yf: (T1 => T2))) => (xs, xs.map(yf))
                                              ^
<console>:13: error: illegal cyclic inheritance involving unfurl
         (x, y) match {

但相反的顺序有效：

sealed trait SeqOrFun[T1, T2]
case class SeqMagnet[T](itr: Iterable[T]) extends SeqOrFun[T, T]
case class FunMagnet[T1, T2](f: (T1 => T2)) extends SeqOrFun[T1, T2]

def unfurl[T1, T2](x: SeqOrFun[T1, T2], y: SeqOrFun[T1, T2]): (Iterable[T1], Iterable[T2]) = {
  (x, y) match {
    case (FunMagnet(xf: (T2 => T1)), SeqMagnet(ys: Iterable[T2])) => (ys.map(xf), ys)
  }
}

编译

<console>:13: warning: match may not be exhaustive.
It would fail on the following input: (FunMagnet(Function1()), FunMagnet(_))
         (x, y) match {
         ^
unfurl: [T1, T2](x: SeqOrFun[T1,T2], y: SeqOrFun[T1,T2])(Iterable[T1], Iterable[T2])

（唯一的变化是

case (SeqMagnet(xs: Iterable[T1]), FunMagnet(yf: (T1 => T2))) => (xs, xs.map(yf))

到

case (FunMagnet(xf: (T2 => T1)), SeqMagnet(ys: Iterable[T2])) => (ys.map(xf), ys)

因为它们是对称的，我不明白如何编译而另一个不能编译。我仍然试图理解错误，因为其他用户似乎从递归类型定义中得到它（我不认为我在这里做）

更新：我相信如果我将磁铁定义为普通类并编写我自己的unapply方法，问题就会消失。

Scala：方法上的非法循环继承

0 个答案: