Scala what is <:< operator

Question

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 e.g. 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.

Possible duplicate of [Scala punctuation (AKA symbols and operators)](http://stackoverflow.com/questions/7888944/scala-punctuation-aka-symbols-and-operators) — Emil Laine, Nov 25 '15 at 23:57
A scala reddit article just came out that discusses this operator - http://blog.bruchez.name/2015/11/generalized-type-constraints-in-scala.html. — Kevin Meredith, Nov 26 '15 at 02:06
checkout this post http://stackoverflow.com/questions/3427345/what-do-and-mean-in-scala-2-8-and-where-are-they-documented — Murat Mustafin, Nov 26 '15 at 07:33

score 1 · Accepted Answer · answered Nov 26 '15 at 19:21

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.)

Scala what is <:< operator

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