A Haskeller in Pythonland

20 Jul 2015programming

I recently did some work for Andy Ridgwell, an old colleague from Bristol, writing a build and configuration system and GUI for a medium-sized climate model called GENIE. GENIE is an EMIC, an Earth system Model of Intermediate Complexity. It’s about 55,000 lines of Fortran and includes models of the atmosphere and ocean plus models of atmospheric chemistry and biogeochemistry in the ocean and ocean sediments.

This model had been in use for some years by different groups, and the infrastructure around it had become quite baroque. Andy wanted this tidied up and made nice (i.e. rewritten...) to make the model easier to set up and use. He also wanted a cross-platform GUI for configuring and running the model, allowing you to keep track of the model state in real-time, to pause and restart model runs, changing the model configuration in between, and so on.

A major consideration for this work was that as well as being easy to use the new system had to be easy to install (on both Linux and Windows) and easy for scientists to hack on. That ruled out Haskell, my usual tool of choice. I decided to use Python instead, for a couple of reasons.

The first reason (one that I didn’t tell Andy until after we were done!) was that I’d never written a line of Python in my life, and this seemed like a good project to use to learn–quite a bit of file handling and munging, some process communication, GUI programming. That seemed like a good set of requirements to get a basic feel for the language and its ecosystem. The second reason was that I wanted to try SCons as a build system. This is written in Python, so I’d have to do some Python coding in any case, and limiting the number of tools used seemed like a good idea. And why SCons? GNU autotools didn’t seem like a good fit here, and I’ve just never really got on with CMake (I’ve tried it on a couple of projects, but always ended up dropping it in favour of something else).

So, Python it was. Being used to Haskell (compiled, static typing, sophisticated type system), Python is going to feel very different (interpreted, dynamic typing, run-time instead of compile-time errors). I was expecting it to be much more painful than it was.

Here are the Good and the Bad (I can do the Ugly myself...).

The Good

The Bad

The “bad” about Python really comes down to one thing, which is that it’s a dynamic language. The holy wars between proponents of static and dynamic languages aren’t very interesting, but it’s important to recognise the trade-offs.

For Python, the positive side of the trade-off is pretty clear–dynamic languages are easy to use, and they’re especially suited for interactive development and data analysis (which is why all the scientific Python stuff is so popular). It’s hard to imagine how you could replicate that flexibility in a statically typed language, although that shouldn’t stop people from trying.

However, dynamic languages, particularly compared to languages like Haskell with a sophisticated and expressive type system, do lose out in that they lack a certain kind of “rigidity” that static languages can have. When people talk about static languages being “rigid”, they usually mean it in a negative sense, but what I’m talking about is a positive kind of rigidity. In Python, if you have a working piece of code and decide to refactor it, you can make changes in one part of your code, rerun it and you’ll get run-time errors, i.e. your code will just fail to work. You have to track down the other places in your code that are affected by your refactoring yourself. In Haskell, if you have a working piece of code and you refactor it somewhere, the Haskell compiler will tell you all the places that you need to change other things to fit with your refactoring. If you’re used to dynamic languages or languages with weaker type systems, this kind of refactoring can feel very strange–you start with working code, hack away at it intensively for a couple of hours until the compiler says you’re good to go, you run the code and... it works, just the same as before. The compiler really is your friend in this situation, and the “rigidity” that demands that all the parts of your code fit together in compatible ways is a huge advantage.

This aspect of Haskell is one of my favourite things about the language. It encourages a much more exploratory approach to programming, which is kind of paradoxical, given that you might expect dynamic languages to be more, well, “dynamic” in that sense. If you know that making big structural changes to your code is rendered more or less painless by the requirements imposed by the type system and support from the compiler, the barrier to refactoring is lowered and you can experiment with different design approaches without worrying that you’ll lock yourself into one way of doing things simply because you can’t face the work of refactoring to do things a better way.

This is why test-driven development is popular in dynamic languages: it’s easy to break your code, so you need lots of tests to ensure that doesn’t happen. Simon Peyton Jones (one of the original developers of Haskell) often describes static typing as a form of “lightweight formal methods”–a mechanism for proving certain properties about programs that is invoked every single time you compile your code. You can think of it as the compiler automatically doing a whole bunch of tests that you would probably have to write by hand in a dynamic language. But because they’re always there and they’re completely automated, you can have a higher degree of assurance that those things really are getting tested. (Of course, you still need tests, but the tests that you write tend to involve higher-level invariants of your code that should be preserved, rather than checking argument types or ranges.)

When I was working on the GENIE GUI code, I did a couple of small-scale refactors that really brought home to me how much I depend on the type system in Haskell. My usual working practice is to change what I want to change then run the compiler to tell me where I should look to make things fit together again. In Python, I have to find those places myself, either by thinking, grepping, through tests (that I have to write) or by crashing my code and working out what went wrong. It feels a bit primitive.


Well, the obvious conclusion is that I wish I could have my cake and eat it too. It would be great if someone could port all the scientific computing infrastructure that’s available in Python to Haskell. The problem, of course, is where to find those “someones”. There are only a small number of research groups or companies working on numerical applications of Haskell and, of those that I know about, I can’t think of any who could or would push the development of the huge open-source scientific computing infrastructure that Python has. The companies doing numerically oriented work are mostly in industries where proprietary software development is the norm, with strong IP protection policies. The research groups are mostly coming at things from a computer science perspective, so they have fewer resources to put into developing libraries focused on applications. It’s quite notable that the original developers of the core of the Python scientific computing infrastructure were scientists who wanted to use this stuff...

Back in the real world, I’ve written numerical code in Haskell (notably arb-fft and a stiff ODE solver I wrote for FP Complete) and it feels natural and clean and the compiler helps a lot. But writing all the code to get from where the Haskell numerical infrastructure is now to being able to write something like Fenics (and a Haskell Fenics would be great!) seems like an almost Sisyphean task.

My personal take on this is that I think I’ll probably end up writing more Python. I want to explore some of the packages that are out there (a talk from the SciPy 2015 conference gives an overview) and use them for some fun stuff. I think that the key thing is to focus more on applications than technology–doing things in Haskell could be better if decent libraries existed, but the amount of work you currently need to do to get to the “fun stuff” is prohibitive. Any discomfort I feel about Python the language, I think I can assuage by the fact that getting fun stuff done with tools you don’t like so much is immeasurably better than not getting fun stuff done at all because the tools you like would need years of effort to build the infrastructure you need.