Ceylon (programming language)

Ceylon

Paradigm	Object-oriented
Designed by	Gavin King, Red Hat
First appeared	2011
Stable release	1.2.2 / March 11, 2016 (2016-03-11)
Typing discipline	Static, strong, safe
License	Apache v2
Filename extensions	.ceylon
Website	ceylon-lang.org
Influenced by
Java, Scala, Smalltalk, ML, Lisp

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 readability, predictability, toolability, modularity, and metaprogrammability.

Important features of Ceylon include:

• a type system enforcing null safety and list element existence at compile time;
• regular syntax and semantics, avoiding special cases and primitively-defined constructs in favor of syntactic sugar;
• support for generic programming and metaprogramming, with reified generics;
• modularity built into the language, based on JBoss modules, interoperable with OSGi and Maven; and
• powerful tools, including an Eclipse-based IDE.

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.

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 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 certain 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(Integer|String input) {
    if (is Integer input) {
        print("Got the integer ``input``");
    } else {
        print("Got the string '``input``'");
    }
}

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

shared void integerOrString(Integer|String input) {
    Integer added = input + 6; // illegal; the + operator is not defined on Integer|String

    if (is Integer input) {
        Integer added = input + 6; // legal; input is now known to be an Integer
        print("Got the integer ``input``");
    } else {
        print("Got the string '``input``'");
    }
}

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 (Nothing is the empty bottom type), 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<Element&Object> removeNulls<Element>(Iterable<Element> stream);

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 (not associated with any user-created classes)
String process(String text, String transformString(String toChange)) {
    return transformString(text);
}

// A top-level function calling String.reverse in expression form.
String reverse(String text) => text.reversed;

// A function reference to String.reversed but mostly equivalent to the function above.
String(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("one", reverse);

// Any example where an anonymous function - (text) => text+text - is provided to the higher-order function above. 
String reversed2 = process("one", (text) => text+text);

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 (in this case, singleton instances). 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(for this purpose: Singletons)
abstract class Vehicle(shared String name) of plane | train | automobile {}

object plane extends Vehicle("plane") {}
object train extends Vehicle("train") {}
object automobile extends Vehicle("automobile") {}
// Compile error: type is not a subtype of any case of enumerated supertype: 'boat' inherits 'Vehicle'
//object boat extends Vehicle("boat") {}

// New (as of Ceylon 1.2.0) constructor-based syntax
class Vehicle of plane | train | automobile {
    String name;

    abstract new named(String pName) {
        name = pName;
    }

    shared new plane extends named("plane") {}
    shared new train extends named("train") {}
    shared new automobile extends named("automobile") {}
    // Compile error: value constructor does not occur in of clause of non-abstract enumerated class: 'boat' is not listed in the of clause of 'Vehicle'
    //shared new boat extends named("boat") {}
}

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(Integer i1, Integer i2) {
    return i1 + i2;
}
function add(Integer i1, Integer i2) {
    return i1 + i2;
}

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

License

All parts of Ceylon are available under open source licenses, mostly the Apache License. Part of the source code is licensed under LGPL.

See also

• Dart (programming language), has its own VM, compiles to JS. Type system not very strict, supports mixins.
• Fantom (programming language), compiles to JVM. Type system not very strict, supports mixins.