New language features

[theosotr]

• [ ] Nullable types (maybe implemented as a variance of Option, https://en.wikipedia.org/wiki/Option_type).
• [ ] When expressions (Kotlin only).
• [ ] Higher-order functions (including lambdas, parameterized functions, and extending types for supporting functions).
• [ ] Singleton classes (i.e., object declarations).
• [ ] Nested class declarations.
• [ ] Object-oriented features (e.g., getters, setters, private, protected, etc.).
• [ ] Delegation (Kotlin only).
• [ ] where keyword (Kotlin only).
• [ ] Modelling of standard library (e.g., data structures, lists, sets, maps, etc.).
• [ ] Variance at use-site (Kotlin only).
• [ ] Enums.
• [ ] Extension functions (Kotlin only).
• [ ] Inline functions (Kotlin only).
• [ ] this and super expressions.
• [ ] traits
• [ ] bi-variant type parameters

[StefanosChaliasos]

We can use the OOPSLA paper to prioritise which features we could implement in the future. Anyway, some of these features have already been implemented.