Subtype in Scala: what is “type X <: Y”?

Question

Can anyone explain the subtype(<:) in the following code? Why it could be used like that? When we use that? Thanks.

trait SwingApi {

    type ValueChange         


        

Answer 1

By virtue of the abstract type members ValueChanged and ButtonClicked, trait SwingApi is itself uninstantiable (all traits are, but if they are fully implemented, they are trivially turned into a concrete class that can be instantiated).

These constraints say that instantiable subtypes of SwingApi must define ValueChanged and ButtonClicked as subtypes of Event.

The type aliases TextField and Button are constrained as structural types (they need not particular subclass relationship, but simply must supply the specified members with the specified types).

The why of it is simply generality. This imposes the minimal constraints on the implementors of trait SwingApi necessary for it to be used by code that demands a SwingApi.