Time: 13:30 - 14:20

Room: Stokerij

Abstract

Every developer that has ever dealt with contra- and covariance in Java or Scala shivers with fear when they have to deal with these concepts in their code. Typically the solution is to click, click, click, click on the 'suggested fix' by the IDE until the error messages finally go away. All this panic is for nothing since at runtime all generics is erased anyway, and for this reason the Dart language made all generic types covariant.

However, fear no more!

In this talk we will provide trauma recovery therapy for victims of variance by explaining the concepts from first principles using real world examples such as vending machines and garbage cans. For additional fun, we will throw in side-effects and show how the simplest possible covariant and contra-variant types are nothing more than the primitive collection types Iterator[+T] and Observer[-T]. Add the idea of self-application, Iterator[Iterator[T] c.q. Observer[Observer[T]], on top of that and we arrive at well known types Iterable[+T] and Observable[+T] that we cannot not live without.

And in case you wondered why Scala uses + for covariance and - for contravariance, and why a contravariant type in a contravariant position becomes a covariant type, well, that is all part of the joy of coding.