Why is `contains` on collections not requiring the correct type?-CodePudding

How come this compiles:

scala> val x: Vector[Int] = Vector(1,2,3)
val x: Vector[Int] = Vector(1, 2, 3)

scala> x.contains("hello")
val res4: Boolean = false

scala> x.contains(List(Map("anything" -> 3.14)))                                                 
val res5: Boolean = false

EDIT:

This is separate from this other question in which the element type of the collection is inferred (to Any) whereas here it is explicitly set (to Int).

CodePudding user response：

The signature is (in 3.0.0; I recommend always including the language/library version in such questions, and for Scala in particular)

def contains[A1 >: A](elem: A1): Boolean

This means that the argument type must be a supertype of Int, and "hello" is inferred to be Any which is indeed such a supertype.

The reason for the signature instead of maybe more expected

def contains(elem: A): Boolean

is that that signature wouldn't allow Vector and other collections to be covariant.

Specifically, if a Vector[Int] extends Vector[Any], and you can call contains(Any) on a Vector[Any], you must also be able to call it on a Vector[Int].