Search code examples
scalainvariance

Scala Set, what is happening with invariant types?

While doing a refactor in a scala application I came across a situation where changing from List to Set raised a question which I didn't have before. I have some idea about variance, but I would like to understand what does it mean for the compiler exactly.

I had something similar to this, which compiles and works just fine:

case class MyClassList(s: List[Any])
val myList = List(("this", false)) // List[(String, Boolean)]
val listWorks = MyClassList(myList)

Then I changed my list to set:

case class MyClassSet(s: Set[Any])
val mySet = Set(("this", false)) // Set[(String, Boolean)]
val setFails = MyClassSet(mySet)

At this point, creating an object of MyClassSet type is no longer ok with me passing a Set as the argument, even when it accepts a Set of Any. Now, it got a bit confusing when the following worked (note that the set is "the same" as the previous mySet):

val setWorks1 = MyClassSet(Set(("this", false)))

I believe that the simple explanation is that the compiler is inferring mySet val as a Set[(String, Boolean)], but when I instantiate it directly in the arguments list of setWorks1, because it accepts a Set[Any], the compiler is inferring it as a Set[Any]. This makes the first example fail and the second one pass. These ones also work, which points to the previous being correct:

val setWorks2 = MyClassSet(mySet.toSet[Any])
val mySetOfAny: Set[Any] = Set(("this", false), ("that", true), ("other", false))
val setWorks3 = MyClassSet(mySetOfAny)

The actual error shown by the compiler is:

Error:(15, 55) type mismatch;
found   : Set[(String, Boolean)]
required: Set[Any]
Note: (String, Boolean) <: Any, but trait Set is invariant in type A.
You may wish to investigate a wildcard type such as `_ <: (...)

Lists and Sets are defined as follows:

type List[+A]  = scala.collection.immutable.List[A]
type Set[A]    = immutable.Set[A]
  • Is this difference in the type variance which allows me to pass a List of a "more restricted type than Any" as the argument but not a in the case of Set?
  • Is this difference only preventing the casting or conversion between types?
  • Is this mostly a compiler "limitation" or an expected property of an invariant type?
  • Are there any other differences between invariant types "in practice" or do they boil down to casting such as this?
Solution

  • 1) It's explained here: Why is Scala's immutable Set not covariant in its type?

    Basically, Set[T] is also a Function1[T, Boolean]. The signature of Function1 is [-In, +Out], so T couldn't be both +T and -T at the same time as scala doesn't allow bivariance (it would significantly weaken type system).

    2) You can easily cast it using .toSet[Any] (which is a wrapper over asInstanceOf). There is also a way to skip variance check.

    3, 4) It's expected property of generic (polymorphic) types. They can be invariant/covariant/contravariant (not only) and it's formally described by simple rules. You can read explanation here: https://stackoverflow.com/a/27627891/1809978