Types

Concrete types must always begin with an uppercase letter (like Int, Float, or String). Type parameters are passed via angle brackets (e.g. List<Int>).

Tuples can be written using the special notation (X, Y) instead of Tuple(X, Y).

Function types are written with the (A, B) -> C syntax.

Underscore types mean type variables. Unlike generics in languages like typescript, type variables are implicitly universally quantified. For example, the type of the identity function is (a) -> a.

In the example above, a is a type variable that can be instantiated to any possible type, as long as every occurrence is instantiated to the same type. This is roughly the same as writing <T>(x: T) => T in typescript.