Level L2: Senior library designer

Variance annotations

Existential types (e.g., to interface with Java wildcards)

Self type annotations and the cake pattern for dependency injection

Here are some links

• http://java.dzone.com/articles/scala-self-type-annotations-0
• http://downgra.de/2010/07/15/scala_self-types/

Structural types (aka static duck typing)

Defining map/flatmap/withFilter for new kinds of for-expressions

Extractors