Purely functional sets in TypeScript, inspired by an assigment in the course "Functional Programming Principles in Scala" by Martin Odersky, with a few added features.

Functional representation of sets based on the mathematical notion of characteristic functions. The goal is to gain practice with higher-order functions.

As an example to motivate our representation, how would you represent the set of all negative integers? You cannot list them all... one way would be to say: if you give me an integer, I can tell you whether it's in the set or not: for 3, I say 'no'; for -1, I say yes.

Mathematically, we call the function which takes an integer as argument and which returns a boolean indicating whether the given integer belongs to a set, the characteristic function of the set. For example, we can characterize the set of negative integers by the characteristic function (x: Int) => x < 0.