Leporello.js follows a pragmatic functional approach. You can use IO functions and throw exceptions as you typically would, without the need for an IO monad or an Option monad. However, there is one critical limitation – you cannot mutate your data structures. Mutations are not allowed because they would make time-travel debugging impossible. For instance, the following code is forbidden:

  const point = {x: 1, y: 2}
  point.x = 2

And instead you should use this:

  const point = {x: 1, y: 2}
  const another_point = {...point, x : 2}

In Leporello.js, it's vital to avoid mutating data structures; instead, you create new ones. You can watch Rich Hickey's insightful talk [0], where he discusses the advantages of immutability in functional programming.

In many ways, Leporello.js bears a resemblance to Clojure. Clojure served as a source of inspiration for Leporello.js, to the extent that I even contemplated building Leporello.js for Clojure first.

[0] https://www.youtube.com/watch?v=-6BsiVyC1kM