给定一个类型类,其中应根据返回类型执行实例选择:
case class Monoid[A](m0: A) // We only care about the zero here
implicit def s[T] : Monoid[Set[T]] = Monoid(Set.empty[T])
implicit def l[T] : Monoid[List[T]] = Monoid(List.empty[T])
def mzero[A](implicit m: Monoid[A]) : A = m.m0
为什么Scala(2.11.6)无法解析正确的实例:
scala> mzero : List[Int]
<console>:24: error: ambiguous implicit values:
both method s of type [T]=> Monoid[Set[T]]
and method l of type [T]=> Monoid[List[T]]
match expected type Monoid[A]
mzero : List[Int]
^
当它有没有问题,使用时发现基于返回类型的隐式隐函数(我们在这里重新定义它为我来说明它是多么相似mzero)
def i[A](implicit a : A) : …在以下简化的示例代码中:
case class One[A](a: A) // An identity functor
case class Twice[F[_], A](a: F[A], b: F[A]) // A functor transformer
type Twice1[F[_]] = ({type L[?] = Twice[F, ?]}) // We'll use Twice1[F]#L when we'd like to write Twice[F]
trait Applicative[F[_]] // Members omitted
val applicativeOne: Applicative[One] = null // Implementation omitted
def applicativeTwice[F[_]](implicit inner: Applicative[F]): Applicative[({type L[?] = Twice[F, ?]})#L] = null
我可以在applicativeOne上调用applicativeTwice,并且类型推断工作,一旦我尝试在applicativeTwice(applicativeOne)上调用它,推理就会失败:
val aOK = applicativeTwice(applicativeOne)
val bOK = applicativeTwice[Twice1[One]#L](applicativeTwice(applicativeOne))
val cFAILS = applicativeTwice(applicativeTwice(applicativeOne))
scala 2.10.0中的错误是
- type mismatch; …Test.test中的错误似乎没有道理:
sealed trait A[-K, +V]
case class B[+V]() extends A[Option[Unit], V]
case class Test[U]() {
def test[V](t: A[Option[U], V]) = t match {
case B() => null // constructor cannot be instantiated to expected type; found : B[V] required: A[Option[U],?V1] where type ?V1 <: V (this is a GADT skolem)
}
def test2[V](t: A[Option[U], V]) = Test2.test2(t)
}
object Test2 {
def test2[U, V](t: A[Option[U], V]) = t match {
case B() => null // This works
}
}
有几种方法可以改变错误,或者消失: …