Monday Morning Haskell explores a variety of topics in the Haskell programming language, from the very basics to the best tools for use in a production environment. The author, James Bowen, is a software engineer in San Francisco.

BayHac 2017!

I spent most of this last weekend at BayHac, the Bay Area Haskell Hackathon! It was hosted at Takt headquarters in San Francisco. I had been looking forward to the event for a long time and I was not disappointed. I had a blast and met a ton of smart, interesting people. I also got to speak at an event for the first time, which was a lot of fun.

The event had two primary sections of activity. First, there was a packed schedule of speakers talking about a wide array of topics. The talks ranged from some basic Stack usage all the way to some eye-opening category theory. Second, several open source maintainers coordinated some hacking on their projects.

Now, given the event was a “Hackathon”, I should perhaps have focused more on this second part than I did. That said, the litany of fascinating topics for the talks made this difficult. I managed a little bit in the end, but I'll start by going through my observations on the talks. A few major themes stood out. Many of my articles and posts try to highlight some of these themes. But it helps enormously to see concrete examples of these.

Language Safety

The first theme was Haskell’s safety as a language. The first talk of the event was given by Greg Horn, who discussed his work at Kittyhawk. He has been developing a flight-control system for small aircraft in Haskell. There isn’t a large margin for error when dealing with objects flying high above the ground. One small bug involving any kind of crash could be extraordinarily expensive. This is a large part of his motivation in using Haskell.

As I stress so often about Haskell, he is confident he can write and refactor Haskell code without breaking anything. Further there are even compile time guarantees that OTHER people can’t break things. His code also involved compiling Haskell code to C to run on embedded systems. But he managed to make most of his code pure. So the generated C code was a subset that excluded a lot of unsafe activities.

Needless to say, it was cool to see Haskell used on such a cool application. It’s even cooler to know that it’s not just being used for the sake of using Haskell. There are real language level concepts that distinguish Haskell and make it more suitable for certain tasks.

Expressiveness and Simplicity

Another talk came from Tikhon Jelvis. He discussed his work optimizing Target’s supply chain. His work is notable for tackling some fascinating mathematical problems involving non-deterministic probability. He managed to express complex probabilistic math within a monad. The simplicity of the final code was mind-boggling.

But another more prosaic issue also struck me as a real showcase of the power of Haskell's simpler ideas. Target’s database has a curious identification schema on their items. One object could potentially have three different identifiers. In dynamically typed languages, you'd likely be stuck using strings for them all. It can be a real pain to keep the different identifiers straight. You're likely to use the wrong one at some point. Tracking down bugs can be nigh impossible. Even if you make wrapper objects, you still likely won’t get compile time guarantees. And it’s quite possible a novice developer will be able to slip in incorrect usages.

Haskell avoids this with newtypes. We can simply wrap each type of ID in a different type. This gives us compile time guarantees that we are not comparing different ID types. This example strengthened my firm belief that even Haskell's simple concepts can give you some big advantages.

The Rise of Frontend Haskell

Takt’s trio of Luite Stegeman, Greg Hale, and Doug Beardsley (previously featured on Monday Morning Haskell!) all gave interesting talks on various topics related to front-end web development in Haskell. Luite discussed some of the intricacies of GHCJS. Doug shared some problems he had encountered over the course of using Reflex FRP, and how he’d solved them. I didn’t get a chance to catch much of Greg’s talk (though they’ll all be up online soon). But I gather from the title it involved showing how nice and easy our life can be when we use Haskell full-stack and get to use the same types everywhere!

Full-stack Haskell applications have existed for a while. But I'm very encouraged by the progress I see in the front-end community, especially with FRP. There seem to be a lot of new ideas and solutions out there. And they're constantly getting better and better. My fingers are crossed that Haskell could soon be widely considered a strong competitor as a full-stack web language.

Breaking Down the Complex

Haskell has a lot of complicated ideas. But several speakers did a great job breaking them down and showing concrete examples. There were a couple talks that accomplished this for some category theory concepts. Julie Moronuki (co-author of the Haskell Book) discussed monoids, semi-groups, and their connections to boolean algebra. Rúnar Bjarnason (co-author of this Scala Book) went into some details on adjunctions and showed how common they are.

Meanwhile Jon Wiegley and Sandy Maguire tackled a couple more concepts that are pretty high on my TODO list for learning: lenses and free monads. I’ve definitely tried learning about lenses before. But I still have little competency beyond playing “monkey see, monkey do” and copying from other examples in the codebase. But now it might have finally clicked. We’ll see. I’m a firm believer in the Feynman Technique. So you can probably expect to see a blog post on lenses from me sometime soon.

Baby’s First Talk!

Besides all these crazy smart people, BayHac also let me give a talk! I gave an introduction to the fantastic Servant library. In all the languages I have programmed in, it is the best tool I have come across for building or even calling an API. You start by describing your API as a type. Then you write your server side handlers. Once you've done this, Servant can generate a lot of other code for you. You'll get things like client side functions and documentation almost ready made. I’ll link to the video of the presentation once it’s up. But in the meantime you can check get the slides and code samples I used for the presentation. They should give you a decent taste of the Servant library so you can get started!

Tensor-Flow and Some Hacking

In the midst of all this I sort’ve managed to do something approaching “hacking”. Of the projects listed before hand, the one that stood out to me most was Haskell Tensor Flow. I've been focusing a lot on the Haskell language lately. This means I haven’t gotten much of a chance to focus on some other skills, particularly machine learning and AI. So I was super psyched at getting to learn more about this library, which allows you to use Google’s awesome Tensor Flow API from Haskell. I didn’t manage to hack much of anything together. I mostly spent my time wrapping my head around the Tensor Flow concepts and the Haskell types involved. Judah Jacobson’s presentation helped a lot in this regard. I’m hoping to learn enough about the library to start making contributions in the future.


So to summarize, BayHac was a fantastic event. On the plus side I learned a ton about different areas of Haskell. Meeting more people in the Haskell community was a great experience. I look forward to having more opportunities to share knowledge with others in the future. The only minus is that I now have way too many things I want to learn and not enough time. I can cope with a language that puts me in that position.

The last big takeaway from BayHac was how open and inviting the community is to newcomers. If you’ve never written a line of Haskell before, you should check out our Getting Started Checklist. It’ll walk you through installing the language and point you to some basic tools to help you on your way!

Haskell and Deliberate Practice

Compile Driven Learning