Musings about FP and CS

A log of my journey through FP and CS

Hammertime and real world haskell, part 2

by Clement Delafargue on February 28, 2013

Tagged as: fp, haskell, hammertime.

After a few months of complete inactivity on Hammertime, I’ve finally found the time to get back on it. These last weeks, I have also played a lot with Hakyll. Writing some Haskell gave me the material needed for the part 2 of my “Real World Haskell” series. Hence this post.

Build environment. Again.

Cabal layout

As I said in the previous post, I had some trouble finding the good source code layout, especially with respect to tests integration. Well, I’ve finally managed to do that, but I wish there were better guidelines. As of now, there is no “right way” of structuring code in a cabal setup. On one hand it provides flexibility. On another hand it’s a bit difficult for newcomers to decide how to structure their files.

Some people in the Haskell community think default project layout could be useful too (see

I took inspiration from the Lens package and asked for advice on #haskell. Now Hammertime has a reasonably clean layout.


It was on the top of my todo list, so I took the time to integrate tests in the build environment.

To integrate tests in cabal, the most simple way is to make it run an executable which executes tests. If this executable returns EXIT_SUCCESS, then cabal knows the tests passed.

While this can seem a bit tedious, this is quite well handled by the tools.

I used TestFramework to integrate tests. It allows to easily generate the test executable, which is very convenient. Another nice feature is that it can group HUnit tests and Quickcheck properties in tests suites.

I’ve not written Quickcheck properties yet, only HUnit tests. HUnit is on par with conventional Unit testing frameworks. The only annoying thing is that you have to manually add the tests to the test suite.

I’ve not used Quickcheck a lot for now and I think it will deserve its own blog post.

Continuous Integration

Once hammertime had its brand new test suite, the first thing I did was to setup CI. I’ve been impressed by the ease of use of Travis CI. Travis was setup in roughly 10 minutes and its github integration is amazing.

I also have to try Drone.IO which seems promising and is backed by a very reactive team. I asked them about haskell support Tuesday evening and on Wednesday morning it was in production.

For small open-source projects, CI is free, so it’d be a shame not to use it.

Thought process

Working with Hakyll was really interesting. It gave me the confirmation that the best way to discover a new library is via its types. Thinking about types upfront allowed me to solve problems with a very limited cognitive load. Thinking only about types allowed me to combine lots of unknown bricks whithout forcing me to think about how they worked. When working with a new codebase, it’s priceless.

I tend to only reason about types. It allows me to quickly combine functions from the standard lib (with help from Hakyll), and care about writing code only at the end of the process. When working with Scala and JS, I often resort to reading the libraries’ source code to understand how to use them. With haskell, types are sufficient.

Regular unit tests are still very useful but not as important as in languages with weaker type systems. Property-based tests with Quickcheck are really interesting and deserve a dedicated article, but I’m not yet comfortable enough with it.

Wrap up

Writing “Real World” haskell (as opposed to playing with one-liners for code challenges) is really interesting and helps me a lot to get more effective with Haskell. It may not be as fun as writing high-level, ultra-generic code with advanced concepts but it puts haskell on the same ground as the other languages and shows its productivity boost in real-life situations.


I’ve been able to put some work in Hammertime. It now has and a clean code layout and unit tests, a working CLI parser and provides simple reports. I’m working on turning -Wall on during the compiling phase. There are just a few warnings left.

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