Thursday, June 5, 2014

Xtend, the Gentle and Functional Java

I didn't want to learn yet another programming language.  But I had to.

I needed to create an Eclipse editor for a domain-specific language, and I needed to customize content-assist and completion.  A friend recommended Xtext as a quick way to get an Eclipse editor with all the goodies up and running.  And it was quite easy to start with; I had to translate my ANTLR grammar into Xtext, which wasn't too painful, and within a day or so I had a working editor for my DSL.

The next step was to add custom content-assist to the editor.  This turned out to be relatively easy as well, except that the file Xtext generated for me was in this new language, Xtend.  So I had to learn the language.  Unlike quite a few other people I know, I first read the manual.  I admit I did this reluctantly, but as I kept reading I got more and more excited.

I've been using Java for many years, both in teaching and in developing applications.  It is certainly nicer than C++, and the refactoring support in Eclipse is wonderful, but there were other languages I used in the past that I enjoyed much more than Java.  Common Lisp, especially on the Lisp Machine, was a great language with a great environment.  Scheme was a special pleasure, being a very compact yet extremely powerful language.  In contrast, Java is very limiting and extremely verbose!

First, the substance.  I am a great fan of functional programming; it is a limiting yet at the same time liberating paradigm.  You don't get to make any assignments or other state changes; in return, the computation model is very simple and similar to standard mathematics.  Functional languages enjoy referential transparency, which means that the same expression in the same environment will always yield the same value.  In contrast, when side effects are possible, it is possible to call the same function or method multiple times and get different results each time, since something in the environment may have changed.  While object-oriented programming seems to be all about changing state, it is still possible to use functional-programming techniques to minimize state changes to those places where they are really useful.  Item 15 of Joshua Bloch's excellent book Effective Java (2nd Edition) is "Minimize mutabililty."  And Bertand Meyer, designer of the Eiffel programming language, expounds in his classic text Object-Oriented Software Construction (2nd Edition) on his Command-Query Separation Principle.  This principle advocates minimizing state changes, and using functional programming techniques inside object-oriented programs as much as possible.  The Scala language is a more recent attempt to reconcile the functional and object-oriented programming styles.  Java allows a functional programming style, but makes it unnecessarily difficult.  (A ray of hope, though: Java 8 is going to have lambda notation!)

And then, there is the form.  Java is obnoxiously verbose, especially in its requirements for types everywhere.  I'm not necessarily objecting to strong typing here, but even with strong typing there are many type inferences that the compiler can make.  For example, the fact that I have to write something like this is infuriating:

HashMap<String, List<String, Integer>> map = new HashMap<String, List<String, Integer>>();

One way of thinking about Xtend is as a simpler syntax for Java.  The Xtend compiler does extensive type inference, and most type specifications are optional.  But Xtend is much more than that.  It provides many conveniences, and is particularly well-geared to the use of functional techniques.  Functions ("lambda expressions") are an essential part of the language, and are a pleasure to use and read.  They simplify a lot of the boilerplate code needed in Java for callbacks of various kinds.  In addition, Xtend libraries supply many utilities in the functional style, and these look like an integral part of the language due to Xtend's extension capabilities, which allow external libraries to add methods that can be called as though they existed in the original Java classes.  Another great convenience is a much enhanced switch statements, which, among other things, can switch on types without requiring type casts.

At the same time, Xtend compiles directly into (readable) Java, not even into bytecode.  So it is 100% compatible with Java, and can be used in any combination with Java classes.  For example, Xtend classes can inherit from Java classes as well as the other way round.  Xtend is well-integrated with Eclipse, and offers most of the conveniences of Java development, including debugging the Xtext sources.  The major lack is in refactoring, although Rename, for example, is available using the Alt-Shift-R shortcut.

If you are stuck with Java but want a nicer syntax, I recommend that you take a good long look at Xtend.  You can start with the 50-second video on the Xtend website, then read the manual; surprisingly, it is quite readable!

#Xtend #FunctionalProgramming #Java

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