存在类型的类型推断

Question

I have a generic trait SomeTrait defined as so:

trait SomeTrait[T] {
  def foo(t: T): String
}

And methods bar and qux as so:

def bar[T](t: SomeTrait[T]): T
def qux: List[SomeTrait[_]]

I do not have control over the above. I am trying to operate on the list returned by qux , like so

qux map { x => x.foo(bar(x))}

However, the compiler complains that the types don't match up. As far as I know this should be fine.

I have tried adding a generic method (signature [T](SomeTrait[T])String ), and calling that to do the work, but the compiler still complains. I can cast my way around it like this:

qux map { x =>
  val casted = x.asInstanceOf[SomeTrait[T forSome { type T }]] // !!!
  casted.foo(bar(casted))
}

But this is even more perplexing, as x already has the type SomeTrait[_] and SomeTrait[T forSome { type T }] means the same thing. The only difference that I'm aware of is that the former is a shorthand for the latter that makes the compiler create its own synthetic names. I'm hoping there's a better way to do this. I have seen this question however I don't think it applies.

Answer 1

The correct way to do this is to use a type variable to give a name to T :

qux map { case x: SomeTrait[t] => x.foo(bar(x)) }

This way the compiler knows bar(x): t and so it's an acceptable argument to x.foo .

Or, alternately, you can combine foo and bar into one method (remember that methods can be local, so you can just define it where you need it):

def fooOfBar[T](x: SomeTrait[T]) = x.foo(bar(x))
qux map { fooOfBar(_) }