Musings about FP and CS

A log of my journey through FP and CS

Hammertime and real world haskell, part 1

by Clément Delafargue on October 31, 2012

Tagged as: fp, haskell, hammertime.

I’ve spent the last few evenings on a pet project, Hammertime. It’s a CLI time-tracker which allows me to collect data on how I use my time. I intend to generate SVG reports later on.

For instance, before starting to read my news feeds:

hammertime start personal news

Then, proceeding to read work-related emails:

hammertime start work emails

For now, hammertime is just a helper to register events associated to a timestamp. Its model is fully sequential (no parallel tasks). It uses a flat file to store data.

Hammertime’s feature set is arguably not the more important part of this project. Hammertime is my first “real” haskell project. By that I mean a project that I will really use and which is inherently impure (to some extent at least). Most of the haskell I write is usually based on one-liners typed in ghci. With hammertime, I intend to bridge the gap between fun, purely functional, highly abstract code (the kind of code I’ve never written outside of haskell-land) and more concrete, everyday code, to see how Haskell’s beauty can help me in a more mundane task.

I intend to track my work on hammertime on this blog to write about how real world haskell development compares to other, mainstream workflows.

Development environment

Build tool

Most of the time I just load a .hs file from ghci and I get running. This time, I’ve put up a cabal setup, just to keep track of the dependencies and to quickly provide a usable executable (even though I can’t visualize data, I’ve started to collect some). However, hammertime is still in one file, so I’ve kept cabal from my development workflow. Also, I must admit that I’m a bit scared of its dependency management, which is bad, to say the least. Fortunately, I use cabal only for development, not for installing software. My distro has a fairly good support of haskell (see Exherbo’s haskell repository).

That said, having fiddled with snap and yesod, cabal has always been a PITA with conflicting dependencies. I’ve tried cabal-dev but rebuilding everything for every new install was a huge time sink. I somehow miss the java world with sbt and maven which handle these kinds of problems quite well.

There is no real equivalent of sbt’s REPL (which starts with the right classpath), but I use a .ghci file wich makes ghci execute statements when starting.

My reference for a good cabal layout was Tony Morris’ tic-tac-toe example, until I found (precisely when writing down this very sentence) Tony Morris’ haskell-package. This skeleton package provides a very good starting point for a package.

I’ve not yet written unit tests for hammertime (which is bad, I know), and I must admit I’m a bit put off by how they’re integrated in cabal. Again, I miss sbt for this.

Cabal is able to provide all of that, but not out of the box.

Playing with the code

I don’t really like IDEs for lots of reasons (cf IDEs are a language smell) and I’ve not used one for years. For a full-blown java dev, I would certainly use one, but right now, I’m using languages expressive enough to avoid IDEs (I even wrote a small java project from sbt the other day and played with it via the REPL, but that’s another story).

Overall, I’ve been able to keep my habits: vim + ghci is a great combo to play with code. One nice side effect is that it gives a strong incentive to write really short top-level methods, which are easier to use in ghci.

Playing with side-effecting code in ghci is quite simple too. For instance:

Prelude Data.Time> :t getCurrentTime
getCurrentTime :: IO UTCTime

Prelude Data.Time> getCurrentTime
2012-10-30 22:30:32.980922 UTC

Prelude Data.Time> :t it
it :: UTCTime

With the special it variable, I directly have access to the unwrapped UTCTime value after it has been evaluated. This makes ghci even more useful when trying new code.

I use pointfree from time to time to refactor lambdas into a more elegant style. I’m quite cautious with this tool because it can produce cryptic formulations, but from time to time it’s useful.


Once again, it’s quite the same for haskell or scala or JS, I just browse the reference websites when I want to have precise info on some part of the API. Since I’m quite new to haskell, my knowledge of the standard library is a bit thin, and that’s where Hoogle is a huge time-saver. Hayoo! is more useful for packages outside the standard lib, but for these I prefer to read thoroughly the documentation. These tools allow me to find functions based on their type. Since haskell’s type system is really expressive and functions are highly generic, most often the type describes exactly the feature.

Even though I almost know LYAH by heart and I have a decent knowledge of common haskell constructs, I don’t know anything about basic stuff like writing to a file or reading command line args. That’s where Real World Haskell is of tremendous help. I’ve not read it completely yet because it frightened me a bit, but the more I read it, the more I like the choices taken by its authors. They offer a good vision of their process in end-to-end case studies.

I just wish they explained how to package Haskell software with cabal.

Wrap up

Apart from the problems with cabal, developing in Haskell is a nice experience. ghci and hoogle are making me insanely more productive when I’m in need of documentation. Hoogle is one of the best proofs that a good type system is essential for readability. I may not have been as productive with new libraries as I would have with an IDE and some C^Space magic, but I’ve learned more about the libraries I’m using.

Next part will be about the thought process when prototyping haskell code. I’ve found it to be quite different from the one I adopt when writing scala or JS.

The code is available on github. Feel free to browse it and give me feedback. As I said, it’s my first haskell project and I’m eager to improve.

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