Update: This article was written as an April Fool’s joke, taking a stab at the fact that JSON is a closed, dumb, and pretty anemic data format. There are other interesting data formats available, such as extensible data notation (edn). JSON (JavaScript Object Notation) is widely known as JavaScript’s data format, often used as a language-independent data interchange format. However, what I consider to be the three most powerful features of JSON are little-known. The main reason for this is…

I recently rolled my own Decimal to Roman Numeral converter in Clojure. I was inspired by some tweets about a roman numeral code kata. The Decimal to Roman Numeral converter takes a decimal number (like 2012) and returns a string of roman numerals (like “MMXII”). The code that I came up with is around ten lines that look like this: (def mapping (sorted-map-by > 1000 "M" 900 "CM" 500 "D" 400 "CD" 100 "C" 90 "XC" 50 "L" 40 "XL"…

Imagine that you one day were asked to implement the following requirement: “As an Administrator, I want to know what has changed in the database since a given timestamp, and also who made the changes, so that I can feel secure that nobody will screw up the database.” In most projects, this would give you a major headache. There is no notion of time in a database. There might be a transaction log you could look in, or perhaps some…

This post is the final part of a series of articles about LISP, a function called SUBST, Clojure, and other interesting stuff. You probably want to read the previous posts before continuing with this. Part 1: Introducing LISP Part 2: Walking through SUBST Part 3: Clojure version of SUBST In part 1 of this series, we learned about the nine special forms of LISP, the building blocks that make up the language. In part 2, we went through how the…

This post is part of a series of articles about LISP, a function called SUBST, Clojure, and other interesting stuff. You may want to read the previous posts before continuing with this post. Part 1: Introducing LISP Part 2: Walking through SUBST In part 1 of this series, we learned about the nine special forms of LISP, the building blocks that make up the language. In part 2, we went through how the SUBST function works on a small tree….

This post is part of a series of articles about LISP, a function called SUBST, Clojure, and other interesting stuff. You may want to read the previous post before continuing with this post. Part 1: Introducing LISP In part 1 of this series, we learned about the nine special forms of LISP, the building blocks that make up the language. In this part we will go through how the SUBST function works on a small tree. When John McCarthy in…

When John McCarthy in 1960 wrote his famous paper on the programming language LISP, he used a particular function to illustrate what you could do with the language. The language consisted of only nine “special forms”, primitive building blocks by which any computable function could be created. The function was called SUBST, and it included all nine of the special forms. It was a remarkable function. It could rip through any tree and replace all leaves that had one value…

ShowOff is a presentation software where you create your slides by writing simple text files containing markdown formatting. Styling is done using CSS. The slides are served by a web server and can be displayed in a web browser. With a simple command, a presentation can be deployed on Heroku for everyone to see. ShowOff can even execute code and show the result (currently that’s possible for Ruby and JavaScript). ShowOff is great if you are fed up with the…

According to this blog entry on Heroku, Clojure becomes the third language officially supported on the Cedar stack, after Ruby and Node.js. They write: – “Clojure combines the expressiveness of Lisp, the agility of a dynamic language, the performance of a compiled language, and the wide applicability of the JVM in a robust, production-ready package. Clojure is a practical language designed to support high-performance, concurrent applications which efficiently interoperate with other software in the JVM ecosystem. All of this combines…

Section 1.3 in Structure and Interpretation of Computer Programs is about Formulating Abstractions with Higher-Order Procedures. As an example, the authors use three simple sums: a sum of an integer range a sum of the cubes of an integer range a sum of a series that converges to π/8 The purpose is to highlight what is common between them and what differs. The differences boil down to: calculating the current term calculating the next value The authors show that all…

Here’s the development of a tiny little macro that is actually pretty useful. The post is quite verbose, because I explain a lot. I want a debug function that first prints the “quoted” (unevaluated) code and then what it evaluates to, so I can write something like this:

This can be useful in iterations, for example, to check whether you get what you expect in each step:

Quoting is best explained using an example:

I’m also going…

I just upgraded my OpenLDAP to 2.4.21 and suddenly I couldn’t load an LDIF that we in Spring LDAP have used successfully for years.

After some time of swearing and random changes, I managed to find out the following little piece of information (which seems rather obvious in hindsight): country (c) is supposed to be the two-letter ISO 3166 country code. But hang on now, we have in the Spring LDAP samples successfully been using c=Sweden and c=Norway. What…

Also known as the “Divine Quotient”, the Golden Ratio was given an almost magical meaning during the renaissance, but it’s actually much older than that. Leonardo DaVinci used it. Euclid used it. It was supposedly discovered by Pythagoras. So, what is it? It’s very simple. Take a length and divide it into two parts: a and b: When the ratio between a and b is equal to the ratio between a+b and a, then you have the Golden Ratio. The…

The purpose of Unit Testing is to verify for the developer that a software unit does what it is supposed to and is fit for use. The confidence that the developer gets, gives the developer courage to do other useful practices like Refactoring. Unit testing is often used to test complex units with one or more collaborators, usually with the help of mock objects. But the greatest value with unit testing, in my opinion, lies in testing the lowest level…

When I tried to upgrade Groovy from 1.6.1 to 1.6.2 using MacPorts, it failed with an OutOfMemoryError. I managed to get it to build using some manual fixing in the build file. I’ll explain what I did in this blog. This was the result that I got:

Looking in the directory mentioned above, we find a build.properties file that contains some settings:

OK, plenty of memory, but nothing seems related to javac compilation. Let’s check the line 173,…

Unit testing in my world is basically defined by being able to run the tests wherever and whenever; on the train, on the plane, at work, or at home. If you also have integration tests, chances are that they require some external data source or something that simply makes it impossible to run them everywhere. If that’s the case, they should probably not be included in the unit test suite. Having code coverage of your unit tests is important, because…

Properties are used in many Java applications as a simple way of separating parts that are likely to change, from the parts that are not that likely to change. Consider for example this typical bean definition in a Spring configuration file:

In order to simplify deployment and maintenance, it’s quite common to extract properties related to server names, ports, and user credentials from the Spring configuration file into a separate property file, like in this ldap.properties:

In the…

I just checked out the old Squid again, the worlds most famous caching proxy. If you direct all your web access through the Squid proxy server, it will cache stuff after the first access. This would simplify for example for labs where fifty people simultaneously begin retrieving stuff from a Maven repo somewhere or downloading required libraries, and we have a very limited bandwidth. On the Mac it was easy to get running (if you have MacPorts):

Then it…

Amazon Simple Storage Service (S3) is cheap on-line storage with a Web Service interface. You just log in with your Amazon id, sign up for S3, designate a credit card, and that’s it. You now have access to pretty much unlimited storage space, managed by Amazon. The price is $0.15 per GB-Month of storage used (plus transfer costs). It was too tempting; I simply had to test it. I used the Jungle Disk software to provide the mapping from the…