Type Notation

The "arrow" type constructor -> denotes function application.

For example if an entity has type int -> int it is a function that takes an int and returns an int.

Arrow type constructors are right-associative. As another example the function:

fn add(a, b) { a + b }

has type int -> int -> int which you can parse as int -> (int -> int) which makes sense in the presence of currying, e.g. it is a function that takes an int and produces a function that takes an int and produces an int.

This notation is more useful when it is applied to more complex types, for example informally map is a function that takes a function and a list and produces another list, but more precisely we can say that map is a function that takes a function that transforms some type a into some type b, and a list of a's, and produces a list of b's. That can be expressed quite neatly with this notation as

(a -> b) -> list(a) -> list(b)