Note: information on this page refers to Ceylon 1.0, not to the current release.

The language module

This is the fourteenth part of the Tour of Ceylon. The previous part introduced comprehensions. We're now going to learn about Ceylon's language module and some of the basic types it defines.

The language module is special, because it is referred to by the language specification, and some language-level constructs are defined in terms of the types it declares. Therefore, you can think of it as forming part of the language definition. In practice, the language module is implemented in a mix of Ceylon and native (Java and JavaScript) code. Of course, we've already met quite a few of the inhabitants of the language module, especially in this chapter.

An overview of the language module

The module ceylon.language contains classes and interfaces that are referred to in the language specification, other declarations they refer to, and a number of related useful functions and types. Let's meet the main characters.

Just like Java, Ceylon has a class named Object.

"The abstract supertype of all types representing 
 definite values..."
see (`class Basic`, `class Null`)
shared abstract class Object() 
        extends Anything() {

    "Determine if two values are equal..."
    shared formal Boolean equals(Object that);

    "The hash value of the value..."
    shared formal Integer hash;

    "A developer-friendly string representing the 
    shared default String string =>
            className(this) + "@" + hash.string;


In Ceylon, Object isn't the root of the type system. An expression of type Object has a definite, well-defined, non-null value. As we've seen, the Ceylon type system can also represent some more exotic types, for example Null, which is the type of null.

Therefore, Ceylon's Object has a superclass, named Anything.

"The abstract supertype of all types. A value of type 
 `Anything` may be a definite value of type `Object`, or it 
 may be the `null` value. A method declared `void` is 
 considered to have the return type `Anything`..."
shared abstract class Anything() 
        of Object | Null {}

All Ceylon types are assignable to Anything. Expressions of type Anything aren't useful for very much, since Anything has no members or operations. The one useful thing you can do with Anything is represent the signature of a method when you don't care about the return type, since a method declared void is considered to have return type Anything, as we saw in the part about functions.

The class Null also directly extends Anything.

"The type of the `null` value. Any union type of form 
 `Null|T` is considered an optional type, whose values
 include `null`. Any type of this form may be written as
 `T?` for convenience..."
shared abstract class Null() 
        of null
        extends Anything() {}

The object null is the only instance of this class.

All types that represent well-defined values extend Object, including:

  • user-written classes,
  • all interfaces, including,
  • function types, and even
  • the types that are considered primitive in Java, such as Boolean, Integer, Float, and Character.

Since an expression of type Object always evaluates to a definite, well-defined value, it's possible to obtain the runtime type of an Object, or narrow an expression of type Object to a more specific type.

Equality and identity

On the other hand, since Object is a supertype of types like Float which are passed by value at the level of the virtual machine, you can't use the === operator to test the identity of two values of type Object. The following is not allowed:

assert (x===1); //compile error: Integer is not Identifiable 

Instead, === is defined to act on instances of the interface Identifiable. Integer, Float, Character, and String don't satisfy this interface, but most classes do.

"The abstract supertype of all types with a well-defined
 notion of identity. Values of type `Identifiable` may 
 be compared using the `===` operator to determine if 
 they are references to the same object instance..."
shared interface Identifiable {

    "Identity equality comparing the identity of the two 
    shared default actual Boolean equals(Object that) {
        if (is Identifiable that) {
            return this===that;
        else {
            return false;

    "The system-defined identity hash value of the 
    see (`function identityHash`)
    shared default actual Integer hash => identityHash(this);


Identifiable implements the hash attribute and equals() method of Object, which are very similar to the equals() and hashCode() methods defined by java.lang.Object.

Just like in Java, you can refine this default implementation in your own classes. This is the normal way to get a customized behavior for the == operator, the only constraint being, that for subtypes of Identifiable, x===y should imply x==y— equality should be consistent with identity.

By default, a user-written class extends the class Basic, which extends Object and satisfies Identifiable. It's possible for a user-written class to directly extend Object, but most of the classes you write will be subclasses of Basic. All classes with variable attributes must extend Basic.

"The default superclass when no superclass is explicitly
 specified using `extends`..."
shared abstract class Basic() 
        extends Object() satisfies Identifiable {}

Operator polymorphism

Ceylon discourages the creation of intriguing executable ASCII art. Therefore, true operator overloading is not supported by the language. Instead, almost every operator (every one except the primitive ., (), is, =, ===, and of operators) is considered a shortcut way of writing some more complex expression involving other operators and ordinary function calls.

For example, the < operator is defined in terms of the interface Comparable, which has a method named compare(). The operator expression


means, by definition, === smaller

The equality operator == is defined in terms of the class Object, which has a method named equals(). So


means, by definition,


Therefore, it's easy to customize operators like < and == with specific behavior for our own classes, just by implementing or refining methods like compare() and equals(). Thus, we say that operators are polymorphic in Ceylon.

Apart from Comparable and Object, which provide the underlying definition of comparison and equality operators, the following interfaces are also important in the definition of Ceylon's polymorphic operators:

  • Summable supports the infix + operator,
  • Invertible supports the prefix + and - operators,
  • Ordinal supports the unary ++ and -- operators,
  • Numeric supports the other basic arithmetic operators,
  • Comparable supports the comparison operators,
  • Correspondence supports the index operator,
  • Ranged supports the segment and span operators,
  • Boolean is the basis of the logical operators, and
  • Set is the basis of the set operators.

Comparison operators

In addition to the traditional <, >, <=, and >= operators, which evaluate to Boolean, there is a <=> operator, which produces an instance of the enumerated type Comparison.

case (smaller) {
    return sqrt(-x);
case (equal) {
    return 0;
case (larger) {
    return sqrt(x);

Two < or <= operators may be combined to determine if a value falls withing a range:


Set operators

The operators | and & represent set union and intersection when they appear in a value expression. But, as we've already seen, when they appear in a type expression they represent type union and intersection! Indeed, there is a relationship between the two kinds of union/intersection:

Set<Integer> integers = ... ;
Set<Float> floats = ... ;
Set<Float|Integer> numbers = integers|floats;

Set<Foo> foos = ... ;
Set<Bar> bars = ... ;
Set<Foo&Bar> foobars = foos&bars;

Indexed operations

We can access an element of a Correspondence by using the index operator. Both Lists and Maps are instances of Correspondence:

"string must start with a \""
assert (exists ch = text[0], ch=='"');

All Lists are also instances of Ranged. We can produce a subrange of a Ranged by providing two endpoints:

if (text[i..i]=="/") {
    [String,String] split = [text[...i-1], text[i+1...]];

We can also produce a subrange of a Ranged by providing a starting point and a length.

String selectedText = text[selection.offset:selection.length];

Please take careful note the difference between .. and :, they have quite distinct purposes:

print("hello"[2..2]); //prints "l"
print("hello"[2:2]);  //prints "ll" 

print("hello"[2..0]); //prints "leh"
print("hello"[2:0]);  //prints "" 

Characters and character strings

We've already met the class String, way back in the first leg of the tour. Ceylon strings are composed of Characters—indeed, a String is a List of Characters.

A character literal is written between single quotes.

Character[] latinLetters = concatenate('a'..'z', 'A'..'Z');
Character newline = '\n';
Character pi = '\{#0001D452}';

An instance of Character represents a 32-bit Unicode character, not a Java-style UTF-16 char. However, under the covers, Ceylon strings are implemented using a Java char[] array (in fact, they are implemented using a Java string). So some operations on Ceylon strings are much slower than you might expect, since they must take four-byte characters into account. This includes size and item(). We think it's much better that these operations be slow, like in Ceylon, than that they sometimes give the wrong answer, like in Java. And remember, it's never correct to iterate a list using size and item() in Ceylon!

To avoid the cost of calling size(), try to use the more efficient empty, longerThan() and shorterThan() when the string might be very long.

Numeric types

As we've mentioned several times before, Ceylon doesn't have anything like Java's primitive types. The types that represent numeric values are just ordinary classes. Ceylon has fewer built-in numeric types than other C-like languages:

  • Integer represents signed integers, and
  • Float represents floating point approximations to the real numbers.

However, the compiler magically eliminates these classes, wherever possible, in order to take advantage of the high performance of the platform's native primitive types.

Therefore, the precision of these types depends on whether you're running your code on the JVM or on a JavaScript virtual machine.

  • When compiling for Java both types have 64-bit precision by default. Eventually, you'll be able to specify that a value has 32-bit precision by annotating it small. But note that this annotation is really just a hint that the compiler is free to ignore (as it currently does).
  • When compiling for JavaScript, Floats have 64-bit precision and Integers have 53-bit precision.

Overflow (on the JVM), or loss of precision (in JavaScript) occurs silently.

Numeric literals

In their simplest form the literals for Integers, and literals for Floats look as you might expect from other languages:

Integer one = 1;
Float oneHundredth = 0.01;
Float oneMillion = 1.0E+6;

However they can be a bit more sophisticated. The digits of a numeric literal may be grouped using underscores. If the digits are grouped, then groups must contain exactly three digits.

Integer twoMillionAndOne = 2_000_001;
Float pi = 3.141_592_654;

A very large or small numeric literal may be qualified by one of the standard SI unit prefixes: m, u, n, p, f, k, M, G, T, P.

Float red = 390.0n;                  // n (nano) means E-9
Float galaxyDiameter = 900.0P;       // P (peta) means E+15
Float hydrogenRadius = 25.0p;        // p (pico) means E-12
Float usGovDebt = 14.33T;            // T (tera) means E+12
Float brainCellSize = 4.0u;          // u (micro) means E-6
Integer deathsUnderCommunism = 94M;  // M (mega) means E+6

A hexadecimal integer is written using a prefix #. Digits may be grouped into groups of two or four digits.

Integer white = #FF_FF_FF;

A binary integer is written with a prefix $. Digits may be grouped into groups of four digits.

Integer sixtyNine = $0100_0101;

Whole and Decimal

The platform module ceylon.math defines the types Whole and Decimal, which represent arbitrary precision integers and arbitrary precision decimals. Both classes are subtypes of Numeric, so you can use all the usual numeric operators with them:

Decimal num = ... ;
Decimal denom = ... ;
Decimal ratio = num/denom;

Abstracting over numeric types

Since all numeric types are subtypes of Numeric, it's possible to write generic code that treats numeric values polymorphically.

Value ratio<Value>(Value num, Value denom) 
        given Value satisfies Numeric<Value> {
    return num/denom;

You can pass Floats, Integers, Wholes, Decimals or any other numeric type to ratio(). But since polymorphic numeric functions can't be optimized to use VM-level primitive types, this function is likely to be much slower than a function that accepts two Floats or two Integers.

Numeric widening

As mentioned earlier, Ceylon doesn't have implicit type conversions, not even built-in conversions for numeric types. Assignment does not automatically widen (or narrow) numeric values. Instead, we usually need to call one of the operations (well, attributes, actually) defined by the interface Number.

Float zero = 0.float; // explicitly widen from Integer

You can use all the operators you're used to from other C-style languages with the numeric types. You can also use the ^ operator to raise a number to a power:

Float diagonal = (length^2.0+width^2.0)^0.5;

Of course, if you want to use the increment ++ operator, decrement -- operator, or one of the compound assignment operators such as +=, you'll have to declare the value variable.

Since it's quite noisy to explicitly perform numeric widening in numeric expressions, the numeric operators do automatically widen their operands, so we could write the expression above like this:

Float diagonal = (length^2+width^2)^(1.0/2);

Since ceylon.language only has two numeric types the only automatic widening conversion is from Integer to Float. This is the one and only thing approaching an implicit type conversion in the whole language.


The language module includes several interfaces that represent container types: Collection, List, Map, and Set.

You might be disappointed to discover that there are no general-purpose implementations of these interfaces in the language module itself. In fact, they're only declared here so that String, Sequence, and Array can be subtypes of List.

You might be even more disappointed when you look at these interfaces and discover that they're missing half the useful operations you're used to seeing on a collection: they have no operations at all for building or mutating the collection. Actually, there's a couple of good reasons for this:

  • It's usually best for an API to return an obviously read-only collection to clients, instead of leaving the client scratching his head wondering whether mutating this collection results in mutation of the internal data structures held by the API, and whether this is safe.
  • Making these interfaces read-only means they can be declared covariant in their type parameters.

The module ceylon.collection contains general-purpose implementations of these interfaces, along with APIs for building and mutating collectons.

There's more...

The language module isn't by itself a platform for building applications. It's a minimal set of basic types that form part of the language definition itself. The Ceylon SDK is still under development, but you can already start using ceylon.file, ceylon.process, and ceylon.math.

Next we're going to come back to the subject of object initialization, and deal with a subtle problem affecting languages like Java and C#.