A guest post by Sigrid Keydana telling us the backstory behind the very cool trading app notebook she put together… Keras, ggplot2, Haskell and R all in one Jupyter notebook! Post originally appeared here.

Earlier today, I presented at UseR! 2017 about HaskellR: a great piece of software, developed by Tweag I/O, that allows to seamlessly use R from Haskell.

It was my first UseR!, it was a great experience, and if I had the time I’d like to write a separate blog post about it, as there were things that did not quite align with my prior expectations… Stuff for thought, but not the topic of this post. (Mainly this would be about how the academic talks compared to the non-academic ones.)

So, why HaskellR? If you allow me one personal note… For the ex-psychologist, ex-software-developer, ex-database administrator, now “in over my head” data scientist and machine learning/deep learning person that I am (see this post for that story), there has always been some fixed point of interest (ideal, you could say), and that is the elegance of functional programming. It all started with SICP, which I first read as a (Java) programmer and recently read again (partly) when preparing R 4 hackers, a talk focused to a great part on the functional programming features of R.

For a database administrator, unless you’re very lucky, it’s hard to integrate use of a functional programming language into your work. How about deep learning and/or data science? For deep learning, there’s Chris Olah’s wonderful blog post linking deep networks to functional programs, but the reality (of widely used frameworks) looks different: TensorFlow, Keras, PyTorch… it’s mostly Python around there, and whatever the niceties of Python (readability, list comprehensions…) writing Python certainly does not feel like writing FP code at all (much less than writing R!).

So in practice, the connections between data science/machine learning/deep learning and functional programming are scarce. If you look for connections, you will quickly stumble upon the Tweag I/O guys’ work: They’ve not just created HaskellR, they’ve also made Haskell run on Spark, thus enabling Haskell applications to use Spark’s MLLib for large-scale machine learning.

What, then, is HaskellR? It’s a way to seamlessly mix R code and Haskell code, with full interoperability in both directions. You can do that in source files, of course, but you can also quickly play around in the interpreter, appropriately called H (no, I was not thinking of its addictive potential here ;-)), and even use Jupyter notebook with HaskellR! In fact, that’s what I did in the demos.

If you’re interested in the technicalities of the implementation, you’ll find that documented in great detail on the HaskellR website (and even more, in their IFL 2014 paper), but otherwise I suggest you take a look at the demos from my talk: First, there’s a notebook showing how to use HaskellR, how to get values from Haskell to R and vice versa, and then, there’s the trading app scenario notebook: Suppose you have a trading app written in Haskell – it’s gotta be lightning fast and as bug-free as possible, right? But, how about nice visualizations, time series diagnostics, all kinds of sophisticated statistical and machine learning algorithms… Chances are, someone’s implemented that algorithm in R, already! Let’s take ARIMA – one line of code with R.J. Hyndman’s auto.arima package! Visualization? ggplot2, of course! And last not least, an easy way to do deep learning with R’s keras package (interfacing to Python Keras).

Besides the notebooks, you might also want to check out the slides, especially if you’re an R user who hasn’t had much contact with Haskell. Ever wondered why the pipe looks the way it looks, or what the partial and compose functions are doing?

Last not least, a thousand thanks to the guys over at Tweag I/O, who’ve been incredibly helpful in getting the whole setup to run (the best way to get it up and running on Fedora is using nix, which I didn’t have any prior experience with… just at a second level of parentheses, I think I’d like to know more about nix, the package manager and the OS, now too ;-)). This is really the great thing about open source, the cool stuff people build and how helpful they are! So thanks again, guys – I hope to be doing things “at the interface” of ML/DL and FP more often in the future!