Maybe Monads Might Not Matter

This post isn’t really about the Maybe Monad of course. It is more focused on the State Monad, but I have a weakness for alliteration.

What do

space suits
nuclear waste containers
romantic conquests
monsters
macros
containers
conversations

have in common? They’ve all been used as metaphors for monads.

Last time I looked, the Haskell wiki listed 29 tutorials on the subject, and that is where all these allusions come from.

Such a wealth of explanatory fauna demands its own (meta-)explanation. Maybe monads are so wildly popular that there is monadic gold rush to cash in on the monad education and training market. And yet, the long-awaited landmark tome, “Category Theory for Dummies in 21 days and 1001 nights” is nowhere to be found.

Tangent: There is of course, Benjamin Pierce's delightfully slim book on the topic which is as close to a gentle introduction as one can come.

Could it just be that people just have a hard time understanding monads? If so, what are the prospects of mass adoption? Or making Just(something) out of Nothing am I?

By now you realize that if monads were a stock, I’d be shorting it. I’m going to go get myself in a huge amount of trouble now, just as I did when I took a hideously pragmatic tack on continuations some years ago.

The most important practical contribution of monads in programming is, I believe, the fact that they provide a mechanism to interface pure functional programming to the impure dysfunctional world.

The thing is, you don’t really need them for that. Just use actors. Purely functional actors can interact with the stateful world, and this has been known since before Haskell was even conceived.

Tangent: Before you crucify me for being so narrow minded, pray consider the mitigating circumstance that I have used the words "practical" and "pure functional programming" in the same sentence. There are many who regard that, rather than my disrespectful attitude toward monads, as grounds for my institutionalization.


Some kind soul will doubtless point out to me how you can view actors as monads or some such. Be that as it may, it is beside the point. You can invent, build and most importantly, use, actors without ever mentioning monads. Carl Hewitt and his students did that decades ago.

Tangent: I have to say how amazing that is. Actors were first conceived by Hewitt in 1973(!), and Gul Agha's thesis has been around for 25 years. I firmly believe actors are the best answer to our concurrency problems, but that is for another post.

You can write an actor in a purely functional language, and have it send messages to file systems, databases or any other other stateful actor. Because the messages are sent asynchronously, you never see the answer in the same activation (aka turn) of the actor, so the fact that these actors are stateful and may give different answers to the same question at different times does not stain your precious snow white referential transparency with its vulgar impurity. This is pretty much what you do with a monad as well - you bury the stateful filth in a well marked shallow grave and whistle past it.

Of course, your troubles are by no means over. Actors or monads, the state is out there and you will have to reason about it somewhere. But better you reason about it in a well bounded shallow grave than in C.

What is important to me is that the notion of actors is intuitive (a pesky property of Dijkstra’s hated anthropomorphisms, like self) for many people. Yes, there are many varieties of actors and I have my preferences - but I’ll take any one of them over a sheaf of categories.

Speaking of those preferences, look at the E programming language (I often point at Mark Miller’s PhD thesis) or on AmbientTalk. I would like to have something similar in Newspeak (and in its hypothetical functional subsets, Avarice and Sloth).

Of course, there is much to be said for a programming culture that excludes anyone without at least the potential of a PhD. Indeed, if you can surround yourself with such people, you can do amazing things with just Java, C++ and Python (though they will still be more productive if they have the good taste to use something nicer). So perhaps the true value of monads lies in their exclusionary nature.

Nevertheless, there is more work to be done than some small, celebrated priesthood can or will do all by itself. There is real value in functional programming in some contexts, and it needs to integrate with stateful programming. Actors provide a model that is much easier for most humans to relate to.