In a couple of days Ceylon 1.2 will be released. Ceylon 1.2 is out now, after a year of development. That’s exciting for us, but we think it would be interesting to summarize our thoughts about why you should be excited about Ceylon, and why you might consider it over other programming languages designed to run on the Java and JavaScript virtual machines.

Ceylon is a rather ambitious programming language, so sometimes when people ask us to summarize its advantages, it can be a bit difficult to know where to start!

A truly cross-platform language

Ceylon is a language that is equally at home on the JVM and on any JavaScript VM. Furthermore, implementation of a compiler backend for the Dart VM is already quite advanced, and work has begun on a native backend based on LLVM. The architecture of the compiler and tooling make addition of a new runtime environment, if not easy, at least straightforward.

Also important: Ceylon is rigorously defined by a complete language specification. The very act of writing a specification forces the language designer to really think through the semantics and corner cases, and acts as an essential reference as to what the behavior of the compiler should be.

Unlike other languages designed first for the JVM and then, belatedly, ported to JavaScript, Ceylon doesn’t feature JVM-specific numeric types with semantics that aren’t relevant to—or can’t be honored on—other VMs. And its language module isn’t polluted with dependencies to types like java.lang.Exception that don’t exist in other runtime environments. Instead, Ceylon’s language module completely abstracts the underlying runtime, and offers a set of elegant APIs that vastly improve on those available natively.

But this abstraction doesn’t come at the cost of lowest-common-denominator performance characteristics. On the contrary, you can expect Java-like performance when Ceylon executes on the JVM, Dart-like performance when Ceylon executes on the Dart VM, and JavaScript-like performance when Ceylon executes on a JavaScript VM like Node.js.

Nor does this abstraction limit the potential for interoperation with native code. Ceylon features excellent interoperation with Java and Maven, and it’s very easy to use most Java libraries in Ceylon. Similarly, Ceylon’s battery of dynamic constructs make interoperation with native JavaScript straightforward. With the new native functions and classes in Ceylon 1.2, it’s even possible to write a cross-platform module that interoperates with native Java and JavaScript code!

Finally, Ceylon's module system is already compatible with OSGi, with Maven, with Vert.x, with npm , and with requirejs . When Jigsaw finally arrives, we'll support it too.

This all amounts to an impressive engineering achievement, especially when taking into account the sophistication of the language itself.

Truly disciplined static typing

Chances are you have some experience writing code in a language with static typing. In combination with the right tooling, static typing makes code more robust, more understandable, and much more maintainable. But most languages go out of their way to include features and APIs which undermine their own static type system. A handful of languages such as ML and Haskell eschew this, offering a principled, disciplined approach to static typing. And programmers working with these languages report a curious thing: that their programs have a strange tendency to work first or second time they run them. Sure, it takes a little longer to produce a program that the compiler accepts, but once the compiler is satisfied, so many common bugs have already been eliminated, that the program is often already correct or at least almost correct.

Ceylon follows this same philosophy and, even though it’s a very different sort of language to ML, our experience is that our programs have the very same tendency to just work, almost immediately. You surely won’t believe this until you experience it yourself.

Ceylon’s type system itself is state-of-the-art, including generic types with both use-site and declaration-site covariance and contravariance, mixin (multiple) inheritance, principal instantiation inheritance, sum types, disjointness analysis, experimental support for higer-order and higher-rank generics, and, best of all, union and intersection types.

Union and intersection types

Ceylon was the first modern language to introduce a full implementation of union and intersection types, and the first language to realize that union and intersection types make flow-typing and type inference, especially generic type inference, very much simpler, more predictable, and more elegant. As of today, it’s still the language with the most sophisticated support for unions and intersections.

What’s most difficult to explain to folks who’ve never written any significant amount of code in Ceylon is just how powerful union and intersection types really are. Far from being an exotic construct that you encounter occasionally in tricky library code, they form an essential part of the everyday experience of writing code in Ceylon, and there is almost no significant Ceylon program which doesn’t use union types somewhere. It’s no exaggeration to say they will totally change the way you think about types.

Flow-sensitive typing

Ceylon was also the first modern language to embrace the notion of flow-sensitive typing, where the conditional logic in the body of a function affects the types inferred by the compiler. This approach eliminates a significant amount of verbosity and a significant number of runtime type casting errors. In combination with Ceylon’s powerful coverage and disjointness analysis, it also helps detect certain other kinds of logic errors.

The cleanest solution to the problem of null

The null value has perplexed language designers for decades. For a long time the “best” solution to the problem was an ML- or Haskell-style Maybe type, which offered type safety and eliminated the hated “null pointer” error. That solution works quite well for languages without subtyping, but it’s not the best approach for a language with subtyping and union types. Ceylon’s approach to null is just as type safe, but, with flow-sensitive typing, is more user-friendly, and it doesn’t require the overhead of separate Option -style wrapper objects. Unlike some other languages, Null isn’t a special case in Ceylon’s type system. Rather, it’s just one further example of the power of union types.

An elegant and powerful representation of tuple and function types

Like most other modern languages, Ceylon features tuples and function references. Unlike other languages Ceylon doesn’t represent these things as special cases in the type system, nor as an explosion of interface types, one for each tuple/function -arity. Instead, there is just one class Tuple , and one interface Callable which expresses function types in terms of tuple types. This approach allows the language to abstract over tuple or function -arity, and write higher-order functions that simply can’t be expressed in many other modern languages.

Reified generics

Along with Gosu, Ceylon is one of only two languages to offer reified generics on the JVM. And it’s the only language to offer reified generics on JavaScript virtual machines. You’ll sometimes run into attempts to “backsplain” the lack of reified generics in Java or other JVM languages: claims that reified generics perform badly, aren’t useful, or don’t work correctly with variance or with higher-order generics. Ceylon is an existence proof that none of these assertions is true.

The system of reified generics backs Ceylon's unique typesafe metamodel, which enables powerful runtime metaprogramming (reflection). It also makes debugging easier: you can see the runtime type arguments of every generic type or generic function in the Ceylon debugger!

A clean, efficient, highly readable syntax

A program written in idiomatic Ceylon is usually much more compact than the same program written in Java. But it’s also usually more readable. Ceylon’s syntax appears, at first glance, to be rather boring and conservative: it uses familiar (and cleaner) prefix type annotations instead of postfix types; it uses plain English keywords instead of cryptic strings of symbols; its naming convention eschews the use of abbreviations. When you look at a program written in Ceylon, you’ll probably understand more or less what it’s doing, even if you have only a passing familiarity with the language.

But that first impression misses a surprising amount of syntactic flexibility. Type inference, named arguments, comprehensions, tuples and sequences, “fat arrow” function definitions, anonymous functions, let , switch , if , case , and object expressions, flow-sensitive typing, and a very powerful facility for stream processing all work together to lend a level of expressiveness rarely seen outside of dynamic languages.

Operator overloading is a feature that, while useful, is widely abused in languages that offer it in unadulterated form. Instead of untrammelled operator overloading, Ceylon supports operator polymorphism, a more disciplined approach, where its operators have fixed, well-defined semantics, but are defined in terms of interfaces that user-written types may satisfy. Thus, you can have the + operator apply to your own class, but it always means some kind of addition-like operation.

We often don’t need to write down the types of things explicitly (type inference takes care of that), but when we do, Ceylon features a uniquely sophisticated expression syntax for types, along with type aliases, eliminating the verbosity of generic type declarations in other languages.

Awesome modularity and tooling

Ceylon’s suite of command line tools, including an incremental compiler, program runner, test runner, documentation compiler, code formatter, and WAR archive packager, are all accessible via the ceylon command, which features a plugin architecture, and excellent usability.

But Ceylon really comes into its own when authoring code using its powerful Eclipse-based IDE. Ceylon’s IDE has a feature set that easily surpasses other modern languages, and competes with much more mature languages like Java and C#. For those who prefer IntelliJ, an IntelliJ-based IDE is already in the works, with a first release planned in the next few months.

If you’ve ever spent time waiting for mvn to finish, you’ll immediately appreciate Ceylon’s powerful module architecture, which comes fully integrated into the command line tools and IDE. Modularity is a problem that is utterly central to the discipline of software engineering, and Ceylon, at least arguably, does it better than any other language out there. You don’t usually need to interact directly with the module repository architecture, since the command line compiler, IDE, and runtime all know how to fetch dependencies transparently.

Finally, Ceylon Herd is an fantastic tool that makes it ridiculously easy to share your work with other Ceylon developers.

Seamless web development

Ceylon is evolving into a compelling platform for web development, allowing you to reuse code written for a Java server on the JavaScript client and vice-versa, while still interoperating cleanly with native code on both sides. With our new serialization facility, you'll be able to transparently move objects between client and server. The Ceylon SDK comes with a HTTP server module based on Undertow, and a transactions module based on JBoss Transactions, but if you're looking for something "heavier", Ceylon modules can already be deployed on RedHat's OpenShift cloud platform, on WildFly, or on Vert.x 2 (with Vert.x 3 support coming soon). Alternatively you could use Ceylon in Node.js!

Developing web applications in Ceylon is actually enjoyable. With Ceylon, you'll spend much less time redeploying and/or refreshing things, or wrestling with inadequate tooling, and more time focused on writing elegant code.

A helpful and open-minded community

The Ceylon community is friendly and helpful, and you can usually get your questions answered quickly at just about any time of day. And we're extremely open to feedback, and don't mind explaining the reasoning behind decisions we've made. Of course we have the traditional mailing lists like any other open source project, but actually we're much more active on Gitter and on the GitHub issue tracking system. So come join us on our Gitter channel, and ask us anything!

If you're motivated to contribute, there's plenty of places to get involved. Some fairly significant parts of the Ceylon ecosystem were originally written by community contributors. Currently, the development of two new compiler backends is being driven by the community. Note that a whole lot of development, including the IntelliJ IDE, and the new compiler backends, is now being done in Ceylon, so you can learn Ceylon by contributing to the Ceylon project itself!

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