Scala-如何在类型的泛型子类型上调用方法？

Question

A colleague wrote the following code:

  def nonEmpty[Coll[_], T](implicit ev: Coll[T] <:< TraversableOnce[T]): Coll[T] => Boolean =
      (coll: Coll[T]) => coll.nonEmpty

The intent being to be able to call nonEmpty on any subtype of Traversable[T] . However, this feels overly complicated. I am trying to simplify it, but with no luck so far. I tried eg:

def nonEmpty2[Coll[T] <:< TraversableOnce[T]](coll: Coll[T]): Boolean = coll.nonEmpty

But that is rejected with Wrong parameter on Coll[T] already. Is it just a syntactical (or series of, syntactical issues)? What is the simplest way to write that nonEmpty method?

Answer 1

There's no need to specify that you're dealing with a generic (higher-kinded) type that contains some other type. Just one type parameter is needed:

def nonEmpty2[T](coll: T)(implicit ev: T <:< TraversableOnce[_]): Boolean = coll.nonEmpty