What does "<:" mean in Scala?

Question

I'm looking at p. 469 of "Programming in Scala" Second Edition. There is a line of code that reads:

type Currency <: AbstractCurrency 

I cannot decipher what this means.

Answer 1

It means an abstract type member is defined (inside some context, e.g. a trait or class), so that concrete implementations of that context must define that type. However, there is a constraint that this type (Currency) must actually be a subtype of AbstractCurrency. That way the abstract context can operate with Currency, knowing that it understands every operation of AbstractCurrency.

trait AbstractCurrency {   def disappearInGreece(): Unit }  abstract class Economy {   type Currency <: AbstractCurrency    def curr: Currency    // can call disappear... because `Currency`   // is an `AbstractCurrency`   def shake(): Unit = curr.disappearInGreece() } 

Trying to define Currency without constraints:

trait RadioactiveBeef  class NiceTry(val curr: RadioactiveBeef) extends Economy {   type Currency = RadioactiveBeef } 

Fails. With constraints ok:

trait Euro extends AbstractCurrency  class Angela(val curr: Euro) extends Economy {   type Currency = Euro }