Object algebras in Xtend

June 29, 2014 Leave a comment

I finally found/made some time to watch the InfoQ video shot during Tijs van der Storm‘s excellent presentation during the Dutch Joy of Coding conference 2014, on object algebras. Since Tijs uses Dart for his code and I’m a bit of an Xtend junkie, I couldn’t resist to port his code. Happily, this results in code that’s at least as short and readable as the Dart code, because of the use of Xtend-specific features.

Object algebras…

In my understanding, object algebras are a way to capture algebraic structures in object-oriented programming languages in a way that allows convenient extensibility in both the algebra (structure) itself as well as in the behavior/semantics of that algebra – at the same time, even.

Concretely, Tijs presents an example where he defines an initial algebra, consisting of only integer literals and + operations. This allows you to encode simple arithmetic expressions inside of the host programming language – i.e., as an internal DSL. Note that these expressions result in object trees (or ASTs, if you will). Also, they can be seen as an example of the Builder Pattern. For this initial algebra, Tijs provides the typical print and evaluation semantics. Next, he extends the algebra with multiplication and also provides the print and evaluation semantics for that.

All of this comes at a cost. In fact, two costs: a syntactical one and a combinatorial one. The syntactical cost is that “1 + 2″ is encoded as “a.plus(a.lit(1), a.lit(2))” – at least in the Dart code example. Luckily, Xtend can help here – see below. The combinatorial cost (which seems to be intrinsic to object algebras) is that for every combination of algebra concept (i.c.: literal, plus operation, multiplication operation) and semantics (i.c.: print, evaluation) we need an individual class – although these can be anonymous if the language allows that.

Despite the drawbacks, object algebras do the proverbial trick in case you’re dealing with object trees/builders/ASTs in an object-oriented, statically-typed language and need extensibility, without needing to revert to the “mock extensibility” of the Visitor Pattern/double dispatch.

…in Xtend

…it looks like this. Go on, click the link – I’ll wait :) Note that GitHub does not know about Xtend (yet?) so the syntax coloring is derived from Java and not entirely complete - most notably, the Xtend keywords defoverride and extension are not marked as such.

To start with the biggest win, look at lines 80 and 142. Instead of the slightly verbose “a.plus(a.lit(1), a.lit(2))” you see “lit(1) + lit(2)”. This is achieved by marking the function argument of type ExpAlg/MulAlg with the extension keyword. This has the effect that the public members of ExpAlg/MulAlg are available to the function body without needing to dereference them as “a.”. In general, Xtend’s extension mechanism is really powerful especially when used on fields in combination with dependency injection. In my opinion, it’s much better than e.g. mucking about with Scala’s implicit magic, precisely because of the explicitness of Xtend’s extension.

Another win is the use of operator overloading so we can redefine the + and * operators in the context of ExpAlg/MulAlg, even usual the actual tokens: see lines 18 and 105. Further nice features are:

  • The use of the @Data annotation on classes to promote these to Value Objects, with suitable getters, setters and constructors generated automatically. Unfortunately, the use of the @Data annotation does not play nice with anonymous classes which were introduced in Xtend 2.6. So in this case, the trade-off would be to have less explicit classes versus more code in each anonymous class. In the end, I chose to keep the code close to the Dart original.
  • No semicolons ;)
  • Parentheses are not required for no-args function calls such as constructor invocations; e.g., see lines 39, 45 and 90.
  • Nice templating using decidedly non-Groovy syntax that is less likely to require escaping and also plays nice with indentation; see e.g. line 39.

All in all, even though I liked the Dart code, I like the Xtend version more.

Addendum: now with closures

As Tijs himself pointed out on Twitter, we can also use closures to do away with classes, whether they are explicit or anonymous depending on your choice of implementation language or style. This is because closures and objects are conceptually equivalent and concretely because the Xtend compiler does three things:

  • It turns closures into anonymous classes.
  • It tries to match the type of the closure to the target type, i.e.: it can coerce to any interface or abstract class which has declared only one abstract method. In our case that’s the print and eval methods of the respective interfaces.
  • It declares all method arguments to be final to match the functional programming style. As a result, the parameters of the factory methods are effectively captured by the closure.

(Incidentally, I’ve made examples of this nature before.)

The resulting code can be found here. It completely does away with explicit and anonymous classes apart from the required factory classes, saving 40 lines of code in the process. (The problem with the @Data annotation naturally disappears with that as well.) Note that we have to make explicit that the closures take no explicit arguments, only arguments captured from the scope, by using the “[| ]” syntax (nothing before |) or else Xtend will infer an implicit argument of type Object – see e.g. line 31.

A slight drawback of the closure approach is that it not only seals the details (i.e., the properties’ values – this is a good thing) but also hides them and that it limits extensibility to behavior that can be expressed in exactly one method. E.g., to do introspection on the objects one has to define a new extension: see lines 124-. Note that this make good use of the @Data annotation after all: both the constructor and a useful toString method are generated.

Categories: DSLs, Xtend(2)

The practical value of category theory (and art)

April 12, 2014 2 comments

After my presentation “A category-theoretic view of model-driven” (slides forthcoming through the conference web site) during this year’s Code Generation conference, I got the question what the practical value of category theory was. In the heat of the moment, I chanced upon an analogy with art which very soon found its way to Twitter as:

Felienne CT-tweet

This got some retweets and also some responses to the tune of me being completely wrong. Although the above is what I said, my actual opinion on this is a bit more nuanced. (As an aside: Félienne was kind enough to write a live-blog post about my presentation.)

First of all, art has quite some practical value: it improves people’s lives in a myriad ways by triggering deeply-felt emotional responses. Category theory (or CT, in short) doesn’t quite work like that in general although for some of its practitioners it might come close. CT certainly has value in providing its practitioners a mental framework which guides them to the sort of abstractions that work really well in mathematics and, increasingly, in functional programming to think and reason more effectively.

What didn’t quite made it to the tweet was my assertion that in practice CT does not relieve you of the hard work. Or to put in another way: there’s no substitute for thought. Framing something in the CT language and framework doesn’t “magically” give you the answers for the questions you might have. It can certainly help in reaching those answers more efficiently and phrasing them (much) more elegantly – which is precisely what proper abstraction should achieve. But at the same time, once you have these answers, it’s perfectly possible to “desugar away” from the use of CT. At the end of the day, everyone has to consider his/her ROI on learning enough CT to be able to take this route.

For the same reason I have a problem with the tendency in certain circles to outright declare adequate knowledge of CT as a criterion for admittance to those circles. (I hinted somewhat obliquely and humorously to this in my presentation.) It’s an entirely artificial entry barrier with those enforcing it doing so for not entirely autarkic reasons.

In conclusion: yes, CT can certainly have practical value but it requires the right context and considerable upfront effort to achieve that and does not constitute a prerequisite.

Categories: Uncategorized Tags:

Using Xtend with Google App Engine

December 6, 2012 5 comments

I’ve been using Xtend extensively for well over a year – ever since it came out, basically.

I love it as a Java replacement that allows me to succinctly write down my intentions without my code getting bogged down in and cluttered with syntactic noise – so much so that my Xtend code is for a significant part organized as 1-liners. The fact that you actually have closures together with a decent syntax for those is brilliant. The two forms of polymorphic dispatch allow you to cut down on your OO hierarchy in a sensible manner. Other features like single-interface matching of closures and operator overloading are the proverbial icing on the cake. The few initial misgivings I had for the language have either been fixed or I’ve found sensible ways to work around them. Over 90% of all my JVM code is in Xtend these days, the rest mostly consisting of interfaces and enumerations.

One of the things I’ve been doing is working on my own startup: Más – domain modeling in the Cloud, made easy. I host that on Google App Engine so I’ve some experience of using Xtend in that context as well. Sven Efftinge recently wrote a blog on using Google Web Toolkit with Xtend. Using Xtend in the context of GWT requires a recent version of Xtend because of the extra demands that GWT makes on Java code (which is transpiled from Xtend code) and Java types in order to ensure it’s possible to transpile to JavaScript and objects are serializable. However, when just using Xtend on the backend of a Google App Engine, you don’t need a recent version. However, you do need the “unsign” a couple of Xtend-related JAR files because otherwise the hosted/deployed server will trip over the signing meta data in them.

To use Xtend in a Google Web Application, do the following:

  1. After creating the Web Application project, create an Xtend class anywhere in the Java src/ folder.
  2. Hover over the class name to see the available quickfixes. Alternatively, use the right mouse click menu on the corresponding error in the Problems view.
  3. Invoke the “Add Xtend libs to classpath” quickfix by selecting it in the hover or by selecting the error in the Problems view, pressing Ctrl/Cmd 1 and clicking Finish.
  4. At this point, I usually edit the .classpath file to have the xtend-gen/ Java source folder and Xtend library appear after the src/ folder and App Engine SDK library entry.
  5. Locate the com.google.guava, org.eclipse.xtext.xbase.lib and org.eclipse.xtend.lib JAR files in the plugins/ folder of the Eclipse installation.
  6. Unsign these (see below) and place the unsigned versions in the war/WEB-INF/lib/ folder of the Web Applcation project.

Now, you’re all set to use Xtend in a GAE project and deploy it to the hosted server. In another post, I’ll discuss the benefits of Xtend’s rich strings in this context.

Unsigning the Xtend libs

The following (Bourne) shell script (which kinda sucks because of the use of the cd commands) will strip a JAR file of its signing meta data and create a new JAR file which has the same file name but with ‘-unsigned’ postfixed before the extension. It takes one argument: the name of the JAR file without file extension (.jar).

unzip $1.jar -d tmp_jar/
cd tmp_jar
awk '/^$/ {next} match($0, "(^Name:)|(^SHA1-Digest:)") == 0 {print $0}' tmp_MANIFEST.MF > MANIFEST.MF
# TODO  remove empty lines (1st clause isn't working...)

zip -r ../$1-unsigned.jar .
cd ..
rm -rf tmp_jar/

# Run this for the following JARs (with version and qualifier):
#    com.google.guava
#    org.eclipse.xtend.lib
#    org.eclipse.xtext.xbase.lib

Note that using the signed Xtend libs (placing them directly in war/WEB-INF/lib/) isn’t a problem for the local development server, but it is for the (remote) hosted server. It took me a while initially to figure out that the cryptic, uninformative error message in the logs (available through the dashboard) actually mean that GAE has a problem with signed JARs.

And yes, I know the unsign shell script is kind-of ugly because of the use of the cd command, but hey: what gives…

Categories: Xtend(2) Tags:

In the trenches: MDSD as guerilla warfare

October 1, 2012 4 comments

However much I would’ve liked for things to be different, adoption of MDSD is still quite cumbersome, even though it has been around for quite some time in different guises and various names. Particularly, the MDA variant may have caused more damage than brought goodwill because of its rigidity and reliance on UML which, frankly and paradoxically, is both unusably large and largely semantics-free at the same time.

Therefore, it’s “somewhat” unlikely that you’ll be able to ride the silver bullet train and introduce MDSD top-down as a project- or even company-wide approach to doing software. This means you’ll have to dig in and resort to guerilla tactics, so be prepared to have a lot of patience as well as suffer frustration, boredom and even a number of casualties – doing all this in the knowledge that if/when you succeed, you will have furthered the MDSD Cause and made the world a ever-so-slightly better place.

Remarkably, opposition to MDSD has -at least in my experience- equally come from both non-techies as well as techies alike but with very differing concerns. From the managers’ viewpoint, you’re introducing yet another “technology”, or more accurately: methodology, so you’ll have to convince them of the gains (e.g., increase of productivity, longer-term reliability) and placate them regarding the risks (e.g., ramp-up costs, specialty knowledge). That to be expected, as well entirely reasonable.

Your colleague developers, on the other hand, are a tougher crowd to convince and any manager or team/project lead worth his or her salt is going to take a team’s gut feeling into account when deciding on proclaiming yay or nay on adopting MDSD in a given project. What struck me, at first, as extremely odd is the reluctance of the “average developer” to use MDSD. This was at a point in my career that I still had to find out that most developers were much more interested in prolonging the usefulness of their current skill set than in acquiring new skills, and much less seeing that same skill set become much less valuable because of something that’s not a buzz word (which MDSD isn’t). From a personal economic viewpoint this makes imminent sense as it makes for a steady and predictable longer-term income.

“Unfortunately,” that’s not the way I work: I have a fair bit of Obsessive Compulsive Disorder which leaves me quite incapable of not continuously improving a code base and encourages me to find ever better ways of succinctly expressing intent in it. I stand by Ken Thompson’s quote: “One of my most productive days was throwing away 1000 lines of code.” In hindsight, it is no surprise that I immediately gravitated towards MDSD when I came to know it.

But enough about me: what would the Average Developer™ think about MDSD? As already mentioned: it can easily be perceived as a threat because adoption usually means that less of total developer time is spent on using an existing skill set (never mind that that skill set comes with dull, tedious and repetitive tasks!) which begs the question what the remainder of the developer time is spent on. The answer can be: on something which requires a new skill set or it is not spent at all because the work can be done in less hours which amounts to another blow to one’s job security.

The other reason that our AD doesn’t like MDSD is that they are, well…average, meaning that it’s probably a bit of leap for him/her to get used to it and especially to acquire some of the required skill set to actively participate in it. Meta modeling comes natural to some of us, but these people tend to occupy the far right of the bell curve of the normal distribution. So, MDSD is also a threat to one’s occupational pride and confidence.

A useful approach to convincing the AD that MDSD might hold some sway and can make life easier, is by “doing it from the inside out”: instead of converting the entire project to an MDSD approach, convert a small part of it, focusing on a low-hanging fruit which comes with a lot of daily pain for the developers involved. A fruit that usually hangs within grabbing range is the data model, together with all its derived details in the various application layers: DB schema, ORM configuration, DTOs, views, etc. It’s convenient that it’s already called a data model, so it’s not a big leap to formalize/”modelize” its description sufficiently, e.g. using a DSL, and to write a generator to generate the tedious bits of code.

It’s important to pick your battles intelligently: don’t try and subjugate entire regions of the existing code base to an MDSD regime, even if you already could with the current state-of-the-art of the model. This is for two reasons: any conversion will have impact on the project and have some unforeseen consequences, but more importantly, you’re effectively throwing away someone’s code and replacing it with generated code – most developers are uneasy with that and if you do this too harshly you will not gain traction for MDSD.

An important ingredient of guerilla tactic in general is to appease the local population: “The people are the key base to be secured and defended rather than territory won or enemy bodies counted”. For MDSD it’s no different which means it’s crucial to keep responsibility in the same place. As Daniel Pink explains, autonomy, mastery and purpose are crucial to anyone’s motivation. So, make the original developer of the replaced code responsible for the MDSD solution as well: show him/her how to change the model, the generator and (even) the DSL and demonstrate how this makes life easier.

Once small parts of a project are model-driven, you can stimulate growth in two directions: (1) “conquer” more areas of the project, and (2) glue parts together. E.g., if at first you’re only generate the DB schema from the data model, then start generating the DTOs/POJOs as well, after which you naturally can generate the ORM configuration as well. If you’ve also got a model for the backend services, you could now start writing down contracts for those services in terms of the data model.

Again, you’ll have to keep track of how your fellow programmers are doing, where you can help them with MDSD (and where you can’t), whether you can “hide” the ramp-up costs enough to not be considered a cost center, and whether they are adopting the MDSD way as their own.

A trick for speeding up Xtend building

September 24, 2012 Leave a comment

I love Xtend and use it as much as possible. For code bases which are completely under my control, I use it for everything that’s not an interface or something that really needs to have inner classes and such.

As much as I love Xtend, the performance of the compilation (or “transpilation”) to Java source is not quite on the level of the JDK’s Java compiler. That’s quite impossible given the amount of effort that has gone into the Java compiler accumulated over the years and the fact that the team behind Xtend is only a few FTE (because they have to take care of Xtext as well). Nevertheless, things can get out of hand relatively quickly and leave you with a workspace which needs several minutes to fully build  and already tens of seconds for an incremental build, triggered by a change in one Xtend file.

This performance (or lack thereof) for incremental builds is usually caused by a lot of Xtend source  interdependencies. Xtend is an Xtext DSL and, as such, is aware of the fact that a change in on file can make it necessary for another file to be reconsidered for compilation as well. However, Xtend’s incremental build implementation is not (yet?) always capable of deciding when this is the case and when not, so it chooses to add all depending Xtend files to the build and so forth – a learned word for this is “transitive build behavior”.

A simple solution is to program against interfaces. You’ve probably already heard this as a best practice before and outside of the context of Xtend, so it already has merits outside of compiler performance. In essence, the trick is to extract a Java interface from an Xtend class, “demote” that Xtend class to an implementation of that interface and use dependency injection to inject an instance of the Xtend implementation class. This works because the Java interface “insulates” the implementation from its clients, so when you change the implementation, but not the interface, Xtend doesn’t trigger re-transpilation of Xtend client classes. Usually, only the Xtend implementation class is re-transpiled.

In the following I’ll assume that we’re running inside a Guice container, so that the Xtend class is never instantiated explicitly – this is typical for generators and model extensions, anyway. Perform the following steps:

  1. Rename the Xtend class to reflect it’s the implementation class, by renaming both the file and the class declaration itself, without using the Rename Refactoring. This will break compilation for all the clients.
  2. Find the transpiled Java class corresponding to the Xtend class in the xtend-gen/ folder. This is easiest through the Ctrl/Cmd-Shift-T (Open Type) short cut.
  3. Invoke the Extract Interface Refactoring on that one and extract it into a Java interface in the same package, but with the original name of the Xtend class.
  4. Have the Xtend implementation class implement the Java interface. Compilation should become unbroken at this point.
  5. Add a Guice annotation to the Java interface:
    @com.google.inject.ImplementedBy(...class literal for the Xtext implementation class...)

Personally, I like to rename the Xtend implementation class to have the Impl postfix. If I have more Xtend classes together, I tend to bundle them up into a impl sub package.

Of course, every time the effective interface of the implementation class changes, you’ll have to adapt the corresponding interface as well – prompted by compilation errors popping up. I tend to apply this technique only as soon as the build times become a hindrance.

Categories: Xtend(2) Tags: , ,

A (slightly) better switch statement in JavaScript

September 8, 2012 2 comments

The switch statement in JavaScript suffers from the usual problems associated with C-style switch statements: fall through. This means that each case guard needs to be expressly closed with a break statement to avoid falling through to the first executable code after that – no matter which case that code belongs to. Fall through has been the source of very many bugs. Unfortunately, the static code analysis for JavaScript (JSLint, JSHint and Google’s Closure compiler) do not check for potential fall through (yet?).

Today I thought I could improve the switch statement slightly with the following code pattern:

var result = (function(it) {
switch(it) {
case 'x': return 1;
case 'y': return 2;
/* ... */
default: return 0;

(Apologies for the lack of indentation: couldn’t get that to work…)

The advantage of using return statements is two-fold:

  1. it exits the switch statement immediately,
  2. it usually comes right after the case guard, making visual inspection and verification much easier than hunting for a break either in- or outside of a nice pair of curly braces.

This approach also has a definite functional programming flavor, as we’ve effectively turned the switch statement into an expression, since the switch statement is executed as part of a function invocation.


Yes, I do write JavaScript from time to time. I usually don’t like the experience very much, mostly because of inadequate tool support and the lack of static typing (and the combination thereof: e.g. the JS plug-ins for Eclipse often have a hard time making sense of the code at all). But we do what we can to get by ;)

Groovy-type builders and JSON initializers in Xtend

September 3, 2012 Leave a comment

One of the nicer features of dynamic languages like Ruby, Groovy, etc. is the possibility to easily implement builders which are constructs to build up tree-like structures in a very succinct a syntactically noise free way. You can find some Groovy examples here – have a special look at the HTML example. Earlier, Sven Efftinge has written a blog on the implementing the same type of builders using Xtend. My blog post will expand a little on his post by providing the actual code and another example.

The main reason that builders come naturally in dynamic languages is that metaprogramming allows adding “keywords” to the language without the need to actually define them in the form of functions. In the case of HTML, these “keywords” are the tag names. Statically-typed languages like Java or Xtend do not have that luxury (or “luxury” as the construct can easily be misused) so we’ll have to do a little extra.

The HTML example

You’ll find Sven’s original example reproduced in HtmlDocumentExample . Note that because of the point mentioned above, we need to have compile-time representations of the HTML DOM elements we’re using. Sven has written these manually but I’m afraid that I’m lazy to do that so I opted for a generative approach. Apart from that, the example works exactly the same, so I’ll refer to his original blog for the magic details – some of which I’ll re-iterate for the JSON example below.


Some basic HTML DOM element types are provided by the BaseDomElements file. Note two nice features of Xtend 2.3+: one file can hold multiple Xtend classes and the use of the @Data annotation as a convenient way to define POJOs – or should those be called POXOs? ;) The POJOs for the other DOM elements are generated by the GenerateDomInfrastructure main class: note they are generated as POXOs which are then transpiled into Java. After running the GenerateDomInfrastructure main class and refreshing the Eclipse project, the  HelpDocumentExample class should compile.

A JSON example

You can find the JSON example in JsonResponseExample. For convenience and effect, I’ll reproduce it here:

class JsonResponseExample {

    @Inject extension JsonBuilder

    def example() {
            "dev"        => true,
            "myArray"    => array("foo", "bar"),
            "nested"     => object("answer" => 42)


To be able to compile this example, you’ll need my fork of Douglas Crockford’s Java JSON library with the main differences being that it’s wrapped as an Eclipse plug-in/OSGi bundle and it’s (as properly as manageable) generified. In addition to the GitHub repo, you can also directly download the JAR file.

As with the HTML example, the magic resides in the line which has a JsonBuilder Xtend class Guice-injected as an extension, meaning that you can use the functions defined in that class without needing to explicitly refer to it. This resembles a static import but without the functions/methods needing to be static themselves. The JsonBuilder class has two factory functions: object(..) builds a JSONObject from key-value pairs and array(..) builds a JSONArray from the given objects.

The fun part lies in the overloading of the binary => operator by means of  the operator_doubleArrow(..) function which simply returns a Pair object suitable for consumption by the object(..) factory function. This allows us to use the “key => value” syntax demonstrated in the example. Note that the => operator has no pre-existing meaning in Xtend – the makers of Xtend have been kind enough to provide hooks for a number of such “user-definable” operators: see the documentation.

I’m still trying to find a nice occasion to use the so-called “spaceship” or “Elvis” operators: now that would be positively…groovy ;)


I failed to notice earlier that Xtend already has an -> operator which does exactly the same thing that our => operator does. Also, the “Elvis” ?: operator already has a meaning: “x ?: y” returns x if it’s non-null and y otherwise. This makes for a convenient way to set up default values. You’ll find both overloading definitions in the org.eclipse.xtext.xbase.lib.ObjectExtensions class.

Categories: Xtend(2)

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