Understanding scala's _ vs Any/Nothing

contravariance, covariance, scala, type-systems, wildcard

Solution

It does make a little difference when generic parameter is bounded. For example, if you had

class BoundedIterable[+A <: Something]
class BoundedGrowable[-A >: Something]

then type `BoundedIterable[Any]` and `BoundedGrowable[Nothing]` would be illegal.

I don't know if there is any other difference, but I can say for sure that you should prefer the wildcard-less variant wherever possible. That is because, actually, the very purpose of declaration-site type variance is to get rid of wildcards (which are a form of usage-site variance). When you say `List[Any]` you mean "list of anything", but when you say `List[_]` then you mean "list of we-don't-know-what". So the former is just way more clear, even though they may be equivalent in some particular case.

Problem

If a class has a convariant type parameter such as `Iterable[+A]`, is there any difference between declaring ``` def foo(bar: Iterable[_]) ``` and ``` def foo(bar: Iterable[Any]) ``` ? If a class has a contravariant type parameter such as `Growable[-A]`, is there any difference between declaring ``` def foo(bar: Growable[_]) ``` and ``` def foo(bar: Growable[Nothing]) ``` ?

Original source