Quick introduction

It's impossible to get to the essence of a programming language by looking at a list of its features. What really makes the language is how all the little bits work together. And that's impossible to appreciate without actually writing code. In this section we're going to try to quickly show you enough of Ceylon to get you interested enough to actually try it out. This is not a comprehensive feature list!

Support for Java and JavaScript virtual machines


Ceylon modules may be deployed on Ceylon's own JVM-based module runtime, on any OSGi container, on the Node.js module system, on Vert.x, or in a browser using require.js.

A familiar, readable syntax

Ceylon's syntax is ultimately derived from C. So if you're a C, Java, or C# programmer, you'll immediately feel right at home. Indeed, one of the goals of the language is for most code to be immediately readable to people who aren't Ceylon programmers, and who haven't studied the syntax of the language.

Here's what it looks like to define and call a simple function:

function distance(Point from, Point to) {
    return ((from.x-to.x)^2 + (from.y-to.y)^2)^0.5;

value dist = distance(Point(0.0, 0.0), Point(2.0, 3.0));

Here's how we create and iterate a sequence:

String[] names = ["Tom", "Dick", "Harry"];
for (name in names) {

If these code examples look boring to you, well, that's kinda the idea - they're boring because you understood them immediately!

Here's a simple class:

class Counter(count=0) {
    variable Integer count;
    shared Integer currentValue {
        return count;
    shared void increment() {

If that's a little too boring, you're allowed to write it more compactly using fat arrows instead of brace-delimited blocks:

class Counter(count=0) {
    variable Integer count;
    shared Integer currentValue => count;
    shared void increment() => count++;

Declarative syntax for treelike structures

Hierarchical structures are so common in computing that we have dedicated languages like XML for dealing with them. But when we want to have procedural code that interacts with hierarchical structures, the "impedence mismatch" between XML and our programming language causes all sorts of problems. So Ceylon has a special built-in "declarative" syntax for defining hierarchical structures. This is especially useful for creating user interfaces:

Table table = Table {
    title = "Squares";
    rows = 5;
    Border {
        padding = 2;
        weight = 1;
    Column {
        heading = "x";
        width = 10;
        String content(Integer row) {
            return row.string;
    Column {
        heading = "x^2";
        String content(Integer row) {
            return (row^2).string;

But it's much more generally useful, forming a great foundation for expressing everything from build scripts to test suites:

Suite tests = Suite {
    Test { 
        "sqrt() function";
        void run() {
    Test {
        "sqr() function";
        void run() {

Any framework that combines Java and XML requires special purpose-built tooling to achieve type-checking and authoring assistance. Ceylon frameworks that make use of Ceylon's built-in support for expressing treelike structures get this, and more, for free.

Principal typing, union types, and intersection types

Ceylon's conventional-looking syntax hides a powerful type system that is able to express things that other static type systems simply can't. All types in Ceylon can, at least in principle, be expressed within the type system itself. There are no primitive types, arrays, or anything similar. Even Null is a class. Even Tuple is a class.

The type system is based on analysis of "best" or principal types. For every expression, a unique, most specific type may be determined, without the need to analyze the rest of the expression in which it appears. And all types used internally by the compiler are denotable - that is, they can be expressed within the language itself. What this means in practice is that the compiler always produces errors that humans can understand, even when working with complex generic types. The Ceylon compiler never produces error messages involving mystifying non-denotable types like Java's List<capture#3-of ?>.

An integral part of this system of denotable principal types is first-class support for union and intersection types. A union type is a type which accepts instances of any one of a list of types:

Person | Organization personOrOrganization = ... ;

An intersection type is a type which accepts instances of all of a list of types:

Printable & Sized & Persistent printableSizedPersistent = ... ;

Union and intersection types are often useful as a convenience in ordinary code. (Once you start using Ceylon, you'll be amazed just how often!) More importantly, they help make things that are complex and magical in other languages—especially generic type argument inference—simple and straightforward in Ceylon.

For example, consider the following:

value stuff = { "hello", "world", 1.0, -1 };
value joinedStuff = concatenate({"hello", "world"}, {1.0, 2.0}, {});

The compiler automatically infers the types:

  • Iterable<String|Float|Integer> for stuff, and
  • Sequential<String|Float> for joinedStuff.

These are the correct principal types of the expressions. We didn't need to explictly specify any types anywhere.

We've worked hard to keep the type system quite simple at its core. This makes the language easier to learn, and helps control the number of buggy or unintuitive corner cases. And a highly regular type system also makes it easier to write generic code.

Mixin inheritance

Like Java, Ceylon has classes and interfaces. A class may inherit a single superclass, and an arbitrary number of interfaces. An interface may inherit an arbitrary number of other interfaces, but may not extend a class other than Object. And interfaces can't have fields, but may define concrete members. Thus, Ceylon supports a restricted kind of multiple inheritance, called mixin inheritance.

interface Sized {
    shared formal Integer size;
    shared Boolean empty => size==0;

interface Printable {
    shared void printIt() => print(this);

object empty satisfies Sized & Printable {
    shared actual Integer size => 0;

What really distinguishes interfaces from classes in Ceylon is that interfaces are stateless. That is, an interface may not directly hold a reference to another object, it may not have initialization logic, and it may not be directly instantiated. Thus, Ceylon neatly avoids the need to perform any kind of "linearization" of supertypes.

Polymorphic attributes

Ceylon doesn't have fields, at least not in the traditional sense. Instead, attributes are polymorphic, and may be refined by a subclass, just like methods in other object-oriented languages.

An attribute might be a reference to an object:

String name = firstName + " " + lastName;

It might be a getter:

String name {
    return firstName + " " + lastName;

Or it might be a getter/setter pair:

String name {
    return fullName;

assign name {
    fullName = name;

In Ceylon, we don't need to write trival getters or setters which merely mediate access to a field. The state of a class is always completely abstracted from clients of the class: we can change a reference attribute to a getter/setter pair without breaking clients.

Typesafe null and flow-sensitive typing

There's no NullPointerException in Ceylon, nor anything similar. Ceylon requires us to be explicit when we declare a value that might be null, or a function that might return null. For example, if name might be null, we must declare it like this:

String? name = ...

Which is actually just an abbreviation for:

String | Null name = ...

An attribute of type String? might refer:

  • to an actual string, the String half of the union type, or
  • the value null, the only instance of the class Null.

So Ceylon won't let us do anything useful with a value of type String? without first checking that it isn't null using the special if (exists ...) construct.

void hello(String? name) {
    if (exists name) {
        //name is of type String here
    else {
        print("Hello, world!");

Similarly, there's no ClassCastException in Ceylon. Instead, the if (is ...) and case (is ...) constructs test and narrow the type of a value in a single step. Indeed, the code above is really just a clearer way of writing the following:

void hello(String? name) {
    if (is String name) {
        //name is of type String here
    else {
        print("Hello, world!");

The ability to narrow the type of a value using conditions like is and exists is called flow-sensitive typing. Another scenario where flow-sensitive typing comes into play is assertions:

if ('/' in string) {
    value bits = string.split("/");
    value first = bits.first; //first may be null, according to its type
    value second = bits.rest.first; //second may be null, according to its type
    //assert that first and second are in 
    //fact _not_ null, since we happen to
    //know that the string contains a /
    assert (exists first); 
    assert (exists second);
    value firstLength = first.size; //first is not null
    value secondLength = second.size; //second is not null

Enumerated subtypes

In object-oriented programming, it's usually considered bad practice to write long switch statements that handle all subtypes of a type. It makes the code less extensible. Adding a new subtype to the system causes the switch statements to break. So in object-oriented code, we usually try to refactor constructs like this to use an abstract method of the supertype that is refined as appropriate by subtypes.

However, there's a class of problems where this kind of refactoring isn't appropriate. In most object-oriented languages, these problems are usually solved using the "visitor" pattern. Unfortunately, a visitor class actually winds up more verbose than a switch, and no more extensible. There is, on the other hand, one major advantage of the visitor pattern: the compiler produces an error if we add a new subtype and forget to handle it in one of our visitors.

Ceylon gives us the best of both worlds. We can specify an enumerated list of subtypes when we define a supertype:

abstract class Node() of Leaf | Branch {}

And we can write a switch statement that handles all the enumerated subtypes:

Node node = ... ;
switch (node)
case (is Leaf) { 
    //node is of type Leaf here
case (is Branch) {
    //node is of type Branch here

Now, if we were to add a new subtype of Node, we would be forced to add the new subtype to the of clause of the declaration of Node, and the compiler would produce an error at every switch statement which doesn't handle the new subtype.

Type aliases and type inference

Fully-explicit type declarations very often make difficult code much easier to understand, and they're an invaluable aid to understanding the API of a library or framework. But there are plenty of occasions where the repetition of a verbose generic type just detracts from the readability of the code. We've observed that:

  1. explicit type annotations are of much less value for local declarations, and
  2. repetition of a parameterized type with the same type arguments is common and extremely noisy in Java.

Ceylon addresses the first problem by allowing type inference for local declarations. For example:

value names = LinkedList { "Tom", "Dick", "Harry" };

function sqrt(Float x) => x^0.5;

for (item in order.items) { ... }

On the other hand, for declarations which are accessible outside the compilation unit in which they are defined, Ceylon requires an explicit type annotation. We think this makes the code more readable, not less, and it makes the compiler more efficient and less vulnerable to stack overflows.

Type inference works somewhat better in Ceylon than in other languages with subtyping, because Ceylon has union and intersection types. Consider a map with a heterogeneous key type:

value numbers = HashMap { "one"->1.0, "zero"->0.0, 1->1.0, 0->0.0 };

The inferred type of numbers is HashMap<String|Integer,Float>.

Ceylon addresses the second problem via type aliases, which are very similar to a typedef in C. A type alias can act as an abbreviation for a generic type together with its type arguments:

interface Strings => List<String>;

We encourage the use of both these language features where - and only where - they make the code more readable.

Higher-order functions

Like most programming languages, Ceylon lets you pass a function to another function.

A function which operates on other functions is called a higher-order function. For example:

void repeat(Integer times, 
        void iterate(Integer i)) {
    for (i in 1..times) {

When invoking a higher-order function, we can either pass a reference to a named function:

void printSqr(Integer i) {

repeat(5, printSqr);

Or we can specify the argument function inline, either like this:

repeat(5, (i) => print(i^2));

Or, using a named argument invocation, like this:

repeat {
    times = 5;
    void iterate(Integer i) {

It's even possible to pass a member method or attribute reference to a higher-order function:

value names = { "Gavin", "Stef", "Tom", "Tako" };
value uppercaseNames = names.map(String.uppercased);

Unlike other statically-typed languages with higher-order functions, Ceylon has a single function type, the interface Callable. There's no need to adapt a function to a singe-method interface type, nor is there a menagerie of function types F, F1, F2, etc, with some arbitrary limit of 24 parameters or whatever. Nor are Ceylon's function types defined primitively.

Instead, Callable accepts a tuple type argument that captures the parameter types of the function. Of course, there's also a single class Tuple that abstracts over all tuple types! This means that it's possible to write higher-order functions that abstract over functions with parameter lists of differing lengths.


A tuple is a kind of linked list, where the static type of the list encodes the static type of each element of the list, for example:

[Float,Float,Float,String] origin = [0.0, 0.0, 0.0, "origin"];

We can access the elements of the list without needing to typecast:

[Float,Float,Float,String] xyzWithLabel = ... ;

[Float,Float] xy = [xyzWithLabel[0], xyzWithLabel[1]];
String label = xyzWithLabel[3];

Tuples are useful as a convenience, and they really come into play if you want to take advantage of Ceylon's support for typesafe metaprogramming. For example, you can take a tuple, and "spread" it across the parameters of a function.

Suppose we have a nice function for formatting dates:

String formatDate(String format, 
                  Integer day, 
                  Integer|String month, 
                  Integer year) { ... }

And we have a date, held in a tuple:

[Integer,String,Integer] date = [25, "March", 2013];

Then we can print the date like this:

print(formatDate("dd MMMMM yyyy", *date));

Of course, Ceylon's support for tuples is just some syntax sugar over the perfectly ordinary generic class Tuple.


Filtering and transforming streams of values is one of the main things computers are good at. Therefore, Ceylon provides a special syntax which makes these operations especially convenient. Anywhere you could provide a list of expressions (for example, a sequence instantiation, or a "vararg"), Ceylon lets you write a comprehension instead. For example, the following expression instantiates a sequence of names:

[ for (p in people) p.firstName + " " + p.lastName ]

This expression gives us a sequence of adults:

[ for (p in people) if (p.age>=18) p ]

This expression produces a Map of name to Person:

HashMap { for (p in people) p.firstName + " " + p.lastName -> p }

This expression creates a set of employers:

HashSet { for (p in people) for (j in p.jobs) j.organization }

Here, we're using a comprehension as a function argument to format and print the names:

print(", ".join { for (p in people) p.firstName + " " + p.lastName });

Simplified generics with fully-reified types

Ceylon's type system is more powerful than Java's, but it's also simpler. The Ceylon compiler never even uses any kind of "non-denotable" type to reason about your code. And there's no wildcard capture, implicit constraints on type arguments, nor "raw" types. Compared to other languages, generics-related error messages are much more understandable to humans.

Furthermore, Ceylon's type system is fully reified at runtime. In particular, generic type arguments are reified, eliminating many frustrations that result from type argument erasure in Java. For example, Ceylon lets us write if (is Map<String,Object> map).

Finally, there's a cleaner, more regular syntax for generic type constraints. The syntax for declaring type constraints on a type parameter looks very similar to a class or interface declaration.

shared Value sum<Value>({Value+} values) 
        given Value satisfies Summable<Value> { ... }

Declaration-site and use-site variance

Ceylon supports two approaches to variance of generic types:

  • declaration-site variance, which is used throughout the Ceylon language module and SDK, and is considered the idiomatic approach, and
  • use-site variance, which is used mainly for interoperating with Java's generic types.

With use-site variance, a type argument is marked covariant (out) or contravariant (in). Thus, this type is contravariant in its first argument, and covariant in its second argument:

Map<in String, out Number>

(In Java, this type would be written Map<? super String, ? extends Number>.)

With declaration-site variance, the system we strongly prefer in Ceylon, a type parameter may be marked as covariant or contravariant by the class or interface that declares the type parameter. Consider:

shared interface Dictionary<in Key, out Item> {
     shared formal Item? get(Key key);

Given this declaration of Dictionary:

  • a Dictionary<String,Integer> is also a Dictionary<String,Number>, since get() produces an Integer, which is also an Number, and
  • a Dictionary<List<Character>,Integer> is also a Dictionary<String,Integer>, since get() accepts any key which is an List<Character>, and every String is a List<Character>.

So code which uses the Dictionary interface doesn't have to worry about variance.

Operator polymorphism

Ceylon features a rich set of operators, including most of the operators supported by C and Java. True operator overloading is not supported. Sorry, you can't define the pope operator <+|:-) in Ceylon. And you can't redefine * to mean something that has nothing to do with numeric multiplication. However, each operator predefined by the language is defined to act upon a certain class or interface type, allowing application of the operator to any class which extends or satisfies that type. We call this approach operator polymorphism.

For example, the Ceylon language module defines the interface Summable.

shared interface Summable<Other> of Other
        given Other satisfies Summable<Other> {
    shared formal Other plus(Other that);

And the + operation is defined for values which are assignable to Summable. The following expression:


Is merely an abbreviation of:


Likewise, < is defined in terms of the interface Comparable, * in terms of the interface Numeric, and so on.

Typesafe metaprogramming and annotations

Ceylon provides sophisticated support for meta-programming, including a unique typesafe metamodel. Generic code may invoke members reflectively without the need for unsafe typecasts and string passing.

Class<Person,[Name]> personClass = `Person`;
Person gavin = personClass(Name("Gavin", "King"));

Ceylon supports program element annotations, with a streamlined syntax. Indeed, annotations are even used for language modifiers like abstract and shared - which are not keywords in Ceylon - and for embedding API documentation for the documentation compiler:

"The user login action"
by ("Trompon the Elephant")
throws (`class DatabaseException`,
        "if database access fails")
see (`function LogoutAction.logout`)
scope (session)
action { description="Log In"; url="/login"; }
shared deprecated
void login(Request request, Response response) {

Well, that was a bit of an extreme example!


Ceylon features language-level package and module constructs, along with language-level access control via the shared annotation which can be used to express block-local, package-private, module-private, and public visibility for program elements. There's no equivalent to Java's protected. Dependencies between modules are specified in the module descriptor:

"This module is just a silly example. You'll 
 find some proper modules in the community 
 repository [Ceylon Herd][Herd].

 [Herd]: http://modules.ceylon-lang.org

 Happy Herding!"
module org.jboss.example "1.0.0" {         
    import ceylon.math "1.1.0";
    import ceylon.file "1.1.1";

For execution on the Java Virtual Machine, the Ceylon compiler directly produces .car module archives in module repositories. You're never exposed to unpackaged .class files. The .car archives come with built-in metadata for the Ceylon module runtime, for OSGi containers, and for Maven.

At runtime, modules are loaded according to a peer-to-peer classloader architecture, based upon the same module runtime that is used at the very core of JBoss AS 7. Alternatively, Ceylon modules are compatible with OSGi, and with Vert.x.

For execution on JavaScript Virtual Machines, the Ceylon compiler produces CommonJS modules, which are compatible with node.js and require.js.

Ceylon Herd is a community module repository for sharing open source modules.

Interoperation with native Java and JavaScript

Code written in Ceylon interoperates elegantly with native code written for the platform. For example, we can make use of Java's collections, which are exposed to Ceylon code in the module java.base:

import java.util { HashMap }

value javaHashMap = HashMap<String,Integer>();
javaHashMap.put("zero", 0);
javaHashMap.put("one", 1);
javaHashMap.put("two", 2);

Notice that basic types like String and Integer may be passed completely transparently between the two languages.

We can even call untyped native JavaScript APIs, inside a dynamic block:

dynamic {
    dynamic req = XMLHttpRequest();
    req.onreadystatechange = void () {
        if (req.readyState==4) {
                    .innerHTML = req.status==200
                            then req.responseText
                            else "error";
    req.open("GET", "/sayHello", true);

Try it!

dynamic { alert("Hello, World!"); }

A real specification

The Ceylon language is defined by an exhaustive, but highly readable, 144-page specification. The specification predates the compiler, and functions as its foundation.

Take the Tour

We're done with the introduction. Take the tour of Ceylon for a full in-depth tutorial.