How to remember the difference between conj and cons in Clojure

25 May 2014 – clojure

When I started writing Clojure, I couldn’t memorize the difference between conj and cons and always used one instead of the another. Their name are similar, but cons is used to add an element at the beginning of a vector, while conj is used to add an element at the end of it. How can one memorize this? I found a mnemonic trick over the time that helps me remember this. Here is it:

The trick is to look at the last letter of each function, s and j. As shown in the image below, the s of cons shows the right, while the j of conj shows the left.

This means that cons pushes elements from the left to the right, that is, at the beginning of a vector. conj, on the other hand, pushes elements from the right to the left, which is at the end of a vector. That’s it. Once you see this in your head, you’ll never forget the difference between cons and conj on a vector.

•

Fixing gnuplot-py’s “unknown aqua terminal” warning on OSX

03 Mar 2014 – gnuplot, osx, python

When plotting with gnuplot-py on OSX, I got an annoying warning saying that terminal aqua is unknown or ambiguous, even when I use a different terminal (e.g. postscript). This terminal doesn’t exist on my Gnuplot installation (4.6.3). In fact, gnuplot-py uses slightly different files depending on your platform. OSX’s one is exactly the same as other UNIX-flavored OSes but its default terminal is aqua. There are two ways to fix the warning, a hacky one I used before this blog post, and a clean one I discovered while writting this post. Hope this help!

The hacky way

Since OSX’s Gnuplot file is the same as Linux’s one apart from its default term, we just need to tell gnuplot-py we’re on Linux:

import sys
_platform = sys.platform # save the normal sys.platform value

if _platform == 'darwin':
    sys.platform = 'linux' # replace it with 'linux'

import Gnuplot, Gnuplot.funcutils # import Gnuplot
sys.platform = _platform # restore it

It temporarily changes sys.platform value, which gnuplot-py is relying on, to "linux", import Gnuplot and then restore it back to its previous value ("darwin").

The clean way

In fact there’s a much cleaner way to fix the warning. The default terminal is stored in GnuplotOpts.default_term, so we just need to change it to another value to fix the warning:

import Gnuplot, Gnuplot.funcutils
Gnuplot.GnuplotOpts.default_term = 'x11'

It’s cleaner than the hacky way because:

It’s clear: the second line is pretty explicit: we change default_term to 'x11'. We don’t even need to add a comment.
It’s more maintainable: if OSX’s gnuplot-py interface change in the future, this piece of code won’t break compatibility while the hacky way will prevent our code to use the new interface.
It’s shorter: one line versus five ones. It’s really easier to introduce bugs when using five lines instead of just one.

•

Using Coveralls with Clojure

15 Feb 2014 – clojure, tests

Coveralls is a service that keep track of your tests coverage for you. It can notifies you when your coverage decreases under a custom threshold, and their bot comments on pull requests to report their tests coverage. Like Travis-CI, it allows you to add a badge to your readme with an up-to-date tests coverage percentage.

If you already test your GitHub projects with a CI server like Travis, it’s very easy to add Coveralls to your workflow. Unfortunately, they have a library for Ruby, a couple user-provided libraries for other languages such as PHP, Java and Python, but nothing for Clojure. Fortunately, they provide an API for unsupported languages like Clojure. Here is how to use it.

Cloverage

Cloverage is a Clojure library and a Leiningen plugin to get form-level tests coverage of your projects. It’s dead easy to use and generates HTML reports for you as well as some other formats.

Add their plugin to your project.clj:

(defproject your-project "0.1.0"
  ; ...
  :plugins [[lein-cloverage "1.0.2"]])

Use lein cloverage to run your tests and generate various reports. You can specify which Cloverage version you want to use with CLOVERAGE_VERSION. The latest stable one is 1.0.3, but we’ll need to use 1.0.4-SNAPSHOT for this article:

CLOVERAGE_VERSION=1.0.4-SNAPSHOT lein cloverage

Coveralls

If you don’t already use it, signup on Coveralls using your GitHub account. Then, click on “Add Repo”, find your repo in the list and activate it. That’s all you have to do on their website for now.

Travis CI

You now need to custom your builds on Travis to use Coveralls. We’ll do it in a Bash script, so add this to your .travis.yml:

after_script:
  - bash -ex test/coveralls.sh

This tells Travis to run test/coveralls.sh after each build. We use Bash options -e and -x to respectively stop the script at the first failure and print each command.

Now edit test/coveralls.sh and add the following content:

COVERALLS_URL='https://coveralls.io/api/v1/jobs'
CLOVERAGE_VERSION='1.0.4-SNAPSHOT' lein2 cloverage -o cov --coveralls
curl -F 'json_file=@cov/coveralls.json' "$COVERALLS_URL"

It’ll run Cloverage and output reports in cov (use another name if you prefer), using a report for Coveralls. This generates a coveralls.json file which can then be sent to their API. That’s what we do on the next line, using cURL.

That’s all! You can now push on GitHub, Travis will run your tests and send their tests coverage to Coveralls.

Caveats

Coveralls doesn’t support partial-line tests coverage, so we’re cheating a little bit here using hits count. A line that it never covered has 0 hits, one that is partially covered has one hit, and a fully covered one has two hits. There’s an open issue regarding this.

•

← Older Newer →