Going back in time with the Undoable Monad

Slipshow is a presentation tool. One of its specificity is that it is not based on slides, but on an infinite canvas on which you can zoom or scroll. Yet, compared to Prezi, the source of a presentation is text.

The way the author defines the dynamic of its presentation is by intertwining actions in its textual content. Slipshow allows many actions, such as revealing new content, scrolling or zooming, starting an animation, or even starting a script defined by the user. This gives the author freedom in how they want to present.

However, for Slipshow itself, this is a programming challenge: How to allow to go back in your presentation’s steps? How to organize the code so that it’s not restricting to adding new actions, and that it’s easy to maintain?

Originally written in JavaScript, Slipshow has suffered from bad design in this space. However, since its rewrite in OCaml, the opportunity to tackle this problem has been taken, with great success and much more stability, using the Undoable Monad.

Monads are a way to embed a “computing” DSL in a programming language. They allow to define a “variant” of computation (for instance “concurrent programming”) and let the user program in that new way, almost transparently.

The Undoable Monad provides an excellent example of monad: it is simple enough, yet solves a real problem in an elegant way. This talk is a fun and practical introduction to monadic programming.

Paul-Elliot

After some time deep in Mathematics, I’ve been at Tarides working on the OCaml tooling. I’m now focusing on my personal project, Slipshow!