即使在使用 isInstanceOf 检查类型后 Scala 类型不匹配错误

Question

I have a function unlist taking as first argument a list of any type:

def unlist(xs: List[Any], ys: List[Any]): List[Any] = ...

and I call this function on the first element of an external list, which may or may not be a list of lists. Thus, I first need to check if this head element is itself a list, in which case I can call my unlist function. I do it with InsintanceOf method, like so:

...
if (xs.head.isInstanceOf[List[Any]]) unlist(xs.head, ys)

However, this does not compile because of a type mismatch on xs.head:

Error: type mismatch;
found   : Any
required: List[Any]

What am I doing wrong?

---

PS : since many of you have suggested to avoid type Any , I have to specify that this is part of a coding exercise aiming at having a function as general as possible

Answer 1

To illustrate Mateusz's comment

isInstanceOf is not being remembered

it is meant we would have to follow up with asInstanceOf like so

if (list.head.isInstanceOf[List[_]]) unlist(list.head.asInstanceOf[List[_]])

Pattern matching, as demonstrated by Tim, implicitly performs isInstanceOf/asInstanceOf combination. Also consider related answer .

If you are unable to refactor out Any , which is the weakest of types, maybe try to recover as much typing information as early as possible, perhaps like so

val lists: List[List[_]] = list.collect { case xs: List[_] => xs }
val ints: List[Int] = list.collect { case i: Int => i }
val strings: List[String] = list.collect { case s: String => s }

Note due to type erasure we cannot easily do much better than List[List[_]] , for example, the following assertion passes

val list: List[Any] = List(List("woo", "hoo"), 42, "kerfuffle")
assert(list.head.isInstanceOf[List[Double]])

In this last case, a compiler warning will tell you that type argument Double in type List[Double] (the underlying of List[Double]) is unchecked since it is eliminated by erasure . It is usually not a good idea to ignore such warnings.

Answer 2

As mentioned in the comments, using Any like this is the sign of some bad design choices. But for the specific problem you have , you can change your if to a match like this:

def unlist(xs: List[Any], ys: List[Any]): List[Any] = ???

val xs: List[Any] = ???

xs.head match {
  case l: List[Any] => unlist(l, ???)
  case _ =>
}

The match checks that the head value is List[Any] then assigns the value to a variable l which has type List[Any] and can therefore be used in the unlist call.

Note : This only works because you are testing for List[Any] . You cannot test for a list of a specific type (eg List[Int] ) because a process called type erasure removes the runtime information that would be required for this to work. The compiler will warn you when it can't implement a match for this reason.

The real solution to this question is to fix the design to use specific types rather than Any .