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我在使用f-bound類型參數時出現自定義提取器時出現問題。以下作品:模式匹配/提取器失敗(由於選項是協變的)
trait Bar
object Model {
object Foo {
def unapply[A <: Bar](foo: Foo[A]): Option[Foo[A]] = Some(foo)
}
trait Foo[A <: Bar] extends Model[A]
}
trait Model[A <: Bar]
object View {
def apply[A <: Bar](model: Model[A]): View[A] = model match {
case Model.Foo(peer) => new Foo(peer)
}
class Foo[A <: Bar](val peer: Model.Foo[A]) extends View[A]
}
trait View[A <: Bar]
但它發生故障時Bar
是F-界:
trait Bar[B <: Bar[B]]
object Model {
object Foo {
def unapply[A <: Bar[A]](foo: Foo[A]): Option[Foo[A]] = Some(foo)
}
trait Foo[A <: Bar[A]] extends Model[A]
}
trait Model[A <: Bar[A]]
object View {
def apply[A <: Bar[A]](model: Model[A]): View[A] = model match {
case Model.Foo(peer) => new Foo(peer)
}
class Foo[A <: Bar[A]](val peer: Model.Foo[A]) extends View[A]
}
trait View[A <: Bar[A]]
造成這種錯誤:
<console>:12: error: inferred type arguments [A] do not conform to method unapply's
type parameter bounds [A <: Bar[A]]
case Model.Foo(peer) => new Foo(peer)
^
<console>:12: error: type mismatch;
found : Model.Foo[A(in method unapply)]
required: Model.Foo[A(in method apply)]
case Model.Foo(peer) => new Foo(peer)
^
我懷疑的問題是,unapply
的返回類型是Option[+A]
並且A不是不變的。這是這個問題的起源嗎?我怎麼能解決這個問題?