Ceylon (programming language)


Ceylon is an object-oriented, strongly statically typed programming language with an emphasis on immutability, created by Red Hat. Ceylon programs run on the Java virtual machine, and can be compiled to JavaScript.
The language design focuses on source code readability, predictability, toolability, modularity, and metaprogrammability.
Important features of Ceylon include:
The name "Ceylon" is an oblique reference to Java, in that Java and Sri Lanka, formerly known as Ceylon, are islands known for growth and export of coffee and tea.
In August 2017, Ceylon was donated to the Eclipse Foundation.

Language features

Ceylon is heavily influenced by Java's syntax, but adds many new features.

Type system

One of the most novel aspects of Ceylon compared to Java is its type system. Ceylon foregoes Java's primitive types and boxing in favor of a type system composed entirely of first-class objects. While this may cause boxing overhead in some situations, it makes the type system more uniform.
Ceylon allows for union and intersection types, in a similar fashion to TypeScript, Whiley and Flow.
Union types, written A|B, allow a variable to have more than one type. The following example shows a Ceylon function which may take either an integer or a string:

shared void integerOrString

Intersection types, written A&B, are the theoretical foundation of flow-sensitive typing:

shared void integerOrString

The condition is Integer input narrows the type of input to <Integer|String> & Integer,
which distributes to Integer&Integer | String&Integer,
which, as String and Integer are disjoint types, is equivalent to Integer&Integer | Nothing,
which simplifies to just Integer.

Null safety

Union and intersection types are used to provide null safety.
The top type of the Ceylon type hierarchy is the class Anything,
which has two subclasses: Object, the superclass of all normal classes and all interfaces, and Null, with the only instance null.
Since Object and Null are disjoint types, most regular types like Integer or List<String> are not nullable;
a nullable type is the union Integer|Null, abbreviated Integer?.
Intersection types can be used to get a non-optional type out of a possibly-optional type, such as a type parameter. For example, the signature of a function that removes null elements from a stream of values could be:

Iterable removeNulls;

When removeNulls is called with a stream of Integer|Null elements, the result will be a stream of <Integer|Null> & Object elements, which simplifies to Integer.

Functions

Similarly to many modern languages, Ceylon supports first class functions and higher order functions, including function types and anonymous functions

// A top-level higher-order function using block syntax
String process
// A top-level function calling String.reverse in expression form.
String reverse => text.reversed;
// A function reference to String.reversed but mostly equivalent to the function above.
String reverseFunctionReference = String.reversed;
// An example where the top-level function above is provided as an argument to the higher-order function above
String reversed1 = process;
// Any example where an anonymous function - => text+text - is provided to the higher-order function above.
String reversed2 = process;

Enumerated types

Similar to Java and many other languages, and with a similar mechanism as algebraic types, Ceylon supports enumerated types, otherwise known as enums. This is implemented in Ceylon with a pattern of limiting the instances of an abstract class at declaration to a limited set of objects. Another way to implement this pattern is with the new constructor feature in Ceylon 1.2 where the objects are implemented as different named constructor declarations.

// Traditional syntax for enumerated type, in this case, limiting the instances to three objects
abstract class Vehicle of plane | train | automobile
object plane extends Vehicle
object train extends Vehicle
object automobile extends Vehicle
// Compile error: type is not a subtype of any case of enumerated supertype: 'boat' inherits 'Vehicle'
//object boat extends Vehicle
// New constructor-based syntax
class Vehicle of plane | train | automobile

Type inference

Ceylon is strongly and statically typed, but also has support for type inference.
The value keyword is used to infer the type of a variable,
and the function keyword is used to infer the type of a function.
The following two definition pairs are each equivalent:

Integer i = 3;
value i = 3;
Integer add
function add

However, to make single-pass type inference possible, type inference is only allowed for non-toplevel and unshared declarations.

Entry point with names

By default the starter runs the shared run function of a module:

/* The classic Hello World program */
shared void run

but any other shared function without parameters can be used as main calling the program with the --run parameter, like this:
ceylon run --compile=force --run hello default

Versions

Versions of Ceylon released:
All parts of Ceylon are available as free software, mostly the Apache License. Part of the source code is licensed under LGPL.