Scala what is <:< operator
There is some explanation here
Types and objects can also have symbolic names; in particular, it should be mentioned that for types with two type parameters the name can be written between parameters, so that eg Int <:< Any is the same as <:<[Int, Any] .
But, i still couldn't quite get it. I'd appreciate if someone can provide an explanation with an example.
This is a class defined in object Predef :
abstract class <:<[-From, +To] extends Function1[From, To]
An instance of A <:< B witnesses that A is a subtype of B .
That the class name is symbolic has no particular consequences, it could have been named IsSubType[From, To] aka From IsSubType To .
You get 'evidence' instances of this class by way of Predef.$conforms :
implicit def $conforms[A]: <:<[A, A]
The use case of this is where you have a type A but you want to operate on sub-type B <: A . Then you can simply ask for this implicit evidence:
trait Foo[A] {
def x: A
// def fails: Int = x * x -- not possible, we don't know that A is an Int
def succeeds(implicit ev: A <:< Int): Int = x * x
}
From the use-site, the compiler only allows us to call succeeds if A <: Int , since otherwise we wouldn't be able to get the ev parameter. The implicit look-up works because of how the compiler can infer the upper bounds of the sought type based on the variance of the type parameters From and To of <:< .
(Of course Int is a final type, so we could have also asked for equality A =:= Int , this is just to have a simple example.)
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