Introduction to Ceylon Part 10

Posted by    |       Ceylon

This is the tenth installment in a series of articles introducing the Ceylon language. Note that some features of the language may change before the final release.

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 declarations. Let's meet the main characters.

Just like Java, Ceylon has a class named Object.

shared abstract class Object() 
        extends Void() {
    doc "A developer-friendly string representing the instance."
    shared formal String string;
    doc "Determine if this object belongs to the given Category
         or is produced by the iterator of the given Iterable
    shared Boolean element(Category|Iterable<Equality> category) {
        switch (category)
        case (is Category) {
            return category.contains(this);
        case (is Iterable<Equality>) {
            if (is Equality self = this) {
                for (Equality x in category) {
                    if (x==self) {
                        return true;
                fail {
                    return false;
            else {
                return false;

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 Nothing, which is the type of null.

Therefore, Ceylon's Object has a superclass, named Void, which we already met in Part 1. All Ceylon types are assignable to Void. Expressions of type Void aren't useful for very much, since Void has no members or operations. You can't even narrow an expression of type Void to a different type. The one useful thing you can do with Void is use it to 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 Void, as we saw in Part 8.

As we also saw in Part 1, the type Nothing directly extends Void. All types that represent well-defined values extend Object, including:

  • user-written classes,
  • all interfaces, and
  • the types that are considered primitive in Java, such as 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 Java Virtual Machine, you can't use the === operator to test the identity of two values of type Object. Instead, there is a subclass of Object, named IdentifiableObject, which represents a type which is always passed by reference. The === operator accepts expressions of type IdentifiableObject. It's possible for a user-written class to directly extend Object, but most of the classes you write will be subclasses of IdentifiableObject. All classes with variable attributes must extend IdentifiableObject.

shared abstract class IdentifiableObject() 
        extends Object() 
        satisfies Equality {

    shared default actual Boolean equals(Equality that) {
        if (is IdentifiableObject that) {
            return this===that;
        else {
            return false;
    shared default actual Integer hash {
        return identityHash(this);
    shared default actual String string {

IdentifiableObject defines a default implementation of the interface Equality, which is very similar to the equals() and hashCode() methods defined by java.lang.Object.

shared interface Equality {
    shared formal Boolean equals(Equality that);
    shared formal Integer hash;

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 IdentifiableObject, x===y should imply x==y — equality should be consistent with identity.

Occasionally that's not what we want. For example, for numeric types, I don't care whether a value is of class Natural, Integer, or Whole when comparing it to 0. Fortunately, numeric types extend Object directly, and are not subject to the additional constraints defined by IdentifiableObject.

Thus, Ceylon is able to capture within the type system much of the behavior that Java introduces by fiat special-case rules in the language definition.

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 := operators) is considered a shortcut way of writing some more complex expression involving other operators and ordinary method calls. For example, the < operator is defined in terms of the interface Comparable<Other>, which we met in Part 5, and which has a method named smallerThan(), which is in turn defined in terms of another method named compare().


means, by definition,


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


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 Equality, 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,
  • Invertable supports the prefix + and - operators,
  • Numeric supports the other basic arithmetic operators,
  • Slots supports bitwise operators,
  • Comparable supports the comparison operators,
  • Correspondence and Sequence support indexing and subrange operators, and
  • Boolean is the basis of the logical operators.

Operator polymorphism is a little more flexible than you might imagine. Here's a quick example of this.

The Slots interface

The interface Slots is an abstraction of the idea of a set of slots which may each hold true or false. The bitwise operators &, |, and ~ are defined in terms of this interface. The most obvious subtype of Slots would be a Byte class, where the slots are the eight binary digits.

But the interface Set from the collections module also extends Slots. The slots of a Set are values which may or may not belong to the set. A slot holds true if the value it represents belongs to the Set. The practical value of this is to allow the use of the operator | for set union, the operator & for set intersection, and the infix ~ operator for set complement.

Set<Person> children = males|females ~ adults;

Yes, I realize that these aren't the traditional symbols representing these operations. But if you think carefully about the definition of these operations, I'm pretty sure you'll agree that these symbols are reasonable.

We could even define a Permission class that implements Slots, allowing us to write things like permissions&(read|execute).

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:

  • Natural represents the unsigned integers and zero,
  • Integer represents signed integers,
  • Float represents floating point approximations to the real numbers,
  • Whole represents arbitrary-precision signed integers, and
  • Decimal represents arbitrary-precision and arbitrary-scale decimals.

Natural, Integer and Float 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 (and it currently does).

Numeric literals

There are only two kinds of numeric literals: literals for Naturals, and literals for Floats:

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

The digits of a numeric literal may be grouped using underscores. If the digits are grouped, then groups must contain exactly three digits.

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

A very large or small numeric literals 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 E15
Float hydrogenRadius = 25.0p; //p (pico) means E-12
Float usGovDebt = 14.33T; //T (tera) means E12
Float brainCellSize = 4.0u; //u (micro) means E-6
Natural deathsUnderCommunism = 94M; //M (mega) means E6

Numeric widening

I mentioned earlier that 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 need to call one of the operations (well, attributes, actually) defined by the interface Number.

Whole zero = 0.whole; //explicitly widen from Natural
Decimal half = 0.5.decimal; //explicitly widen from Float

Usefully, the unary prefix operators + and - always widen Natural to Integer:

Integer negativeOne = -1;
Integer three = +3;

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 automatically widen their operands, so we could write the expression above like this:

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

The built-in widening conversions are the following:

  • Natural to Integer, Float, Whole, or Decimal
  • Integer to Float, Whole, or Decimal
  • Float to Decimal
  • Whole to Decimal

But these conversions aren't defined by special-case rules in the language specification.

Numeric operator semantics

Operators in Ceylon are, in principle, just abbreviations for some expression involving a method call. So the numeric types all implement the Numeric interface, refining the methods plus(), minus(), times(), divided() and power(), and the Invertable interface, refining inverse. The numeric operators are defined in terms of these methods of Numeric. The numeric types also implement the interface Castable, which enables the widening conversions we just mentioned.

shared interface Castable<in Types> {
    shared formal CastValue as<CastValue>()
        given CastValue satisfies Types;

The type parameter Types uses a special trick. The argument to Types should be the union of all types to which the implementing type is castable.

For example, simplifying slightly the definitions in the language module:

shared class Natural(...)
        extends Object()
        satisfies Castable<Natural|Integer|Float|Whole|Decimal> &
                  Numeric<Natural> &
                  Invertable<Integer> {
shared class Integer(...)
        extends Object()
        satisfies Castable<Integer|Float|Whole|Decimal> &
                  Numeric<Integer> & 
                  Invertable<Integer> {
shared class Float(...)
        extends Object()
        satisfies Castable<Float|Decimal> &
                  Numeric<Float> & 
                  Invertable<Float> {

These declarations tell us that Integer can be widened to Float, Whole, or Decimal, but that Float can only be widened to Decimal. So we can infer that the expression -1 * 0.4 is of type Float.

Therefore, the definition of a numeric operator like * can be represented, completely within the type system, in terms of Numeric and Castable:

Result product<Left,Right,Result>(Left x, Right y)
        given Result of Left|Right satisfies Numeric<Result>
        given Left satisfies Castable<Result> & Numeric<Left>
        given Right satisfies Castable<Result> & Numeric<Right> {

Don't worry too much about the performance implications of all this — in practice, the compiler is permitted to optimize the types Natural, Integer, and Float down to the virtual machine's native numeric types.

The value of all this — apart from eliminating special cases in the language definition and type checker — is that a library can define its own specialized numeric types, without losing any of the nice language-level syntax support for numeric arithmetic and numeric widening conversions.

There's more...

If you're interested, you can check out a complete list of Ceylon's operators along with a discussion of their precedence.

In the Part 11 we're going to come back to the subject of object initialization, and deal with a subtle problem affecting languages like Java and C#.

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