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

Dynamic typing

Interoperation with a dynamic language like JavaScript poses a special challenge for Ceylon. Since no typing information for dynamically typed values is available at compile time, the compiler can't validate the usual typing rules of the language. Therefore, Ceylon lets us write dynamically typed code where typechecking is performed at runtime.

Partially typed declarations

The keyword dynamic may be used to declare a function or value with missing type information. Such a declaration is called partially typed.

dynamic xmlHttpRequest = ... ;

void handle(dynamic event) { ... }

dynamic findDomNode(String id) { ... }

Note that dynamic is not itself a type. Rather, it represents the absence of typing information. Therefore any value is considered assignable to a dynamic value or returnable by a dynamic function.

Dynamically typed expressions

A dynamically typed expression is an expression that involves references to program elements for which no typing information is available. That includes references to values and functions declared dynamic, along with things defined in a dynamic language like JavaScript.

A dynamically typed expression may only occur within a dynamic block. The dynamic block serves to suppress certain type checks that the compiler normally performs.

dynamic xmlHttpRequest;
dynamic {
    xmlHttpRequest = XMLHttpRequest();

void handle(dynamic event) {
    dynamic {

Note: you cannot make use of a partially typed declaration outside of a dynamic block.

Runtime type checking

When a dynamically typed expression is evaluated, certain runtime type checks are performed, which can result in a typing exception.


This example demonstrates the use of a native JavaScript API. Try it:

dynamic { 
    dynamic req = XMLHttpRequest();"HEAD", "", true);
    req.onreadystatechange = void () {
        if (req.readyState==4) {

There's more ...

Well actually, we've finished the tour! Of course, there's still plenty of scope for you to explore Ceylon on your own. You should now know enough to start writing Ceylon code for yourself, and start getting to know the platform modules.

Alternatively, if you want to keep reading you can browse the reference documentation or (if you're sitting comfortably) read the specification.