Type inference in ActionScript

Using the -infer-types compiler option

Apache Royale adds support for type inference, which enables the ActionScript compiler to automatically fill in missing type declarations on variables, fields, and function returns. The compiler can typically detect an appropriate type based on a variable’s initializer or a function’s return statements. This feature gives developers the ability to purposefully omit type declarations — which allows them to write less code, while still gaining the performance and productivity benefits of stricter types.

Traditionally, ActionScript has treated the type of a symbol with no explicitly declared type as the any type *, and the compiler would emit a warning for the missing type declaration. When type inference is enabled, the compiler will skip the warning if a type other than * can be inferred from the initializer or return statements. If the intended type should actually be *, an explicit type declaration is encouraged.

Compiler option

Royale does not enable type inference by default, to avoid potential backwards compatibility issues with existing AS3 code. To enable type inference in your application, use the -infer-types compiler option.

mxmlc -infer-types=true MyApp.mxml

Code examples

The examples below demonstrate how the types of variables, fields, and function returns are automatically detected when type inference is enabled.

Local variables

The following local variable is typed as String because it is initialized with a string value.

var localStr = "hello";

Similarly, the next local variable is typed as Number because it is initialized with a numeric value.

var localNum = 123.4;


The following member variable is typed as Boolean because it is initialized with the value true.

public var memberVar = true;

The property defined below includes both getter and setter functions, and they are used to store data in a private member variable. The member variable has an inferred type of String, which means that the getter return type and setter parameter type are inferred to be of type String too. A setter’s return type is always inferred as void.

private var _memberProp = "hello";

public function get memberProp() {
	return _memberProp;

public function set memberProp(value) {
	_memberProp = value;


The return type of the function below is Number because the result of the return statement is numeric.

function add(a:Number, b:Number) {
	return a + b;

If a function contains no return statements, the inferred type is void.

function log(text:String, source:String = null)
	if (source)
		trace("[" + source + "] " + text);

If a function contains multiple return statements, the common base class of all returned values is used as the inferred type. In the following example, let class B extend class A. The function’s return type will b inferred as A because that’s the common base class between A and B.

function getResult(a:A, b:B, preferB:Boolean = true)
	if (preferB)
		return b;
	return a;

Similar to variable types, function parameter types may be inferred from default values. The parameter below is typed as String.

function callback(result = "success"):void

Special case: null and undefined

If the initializer or return value is null, then the * type will be inferred because null may be used with many types. This will result in the compiler emitting a warning, even when type inference is enabled.

// Warning: variable 'defaultsToNull' has no type declaration.
var defaultsToNull = null;

It is recommended to declare a type when initializing to null.

var defaultsToNull:String = null

Similarly, if the initializer or return value is undefined, then the * type will be infered, and a warning will be reported.

// Warning: variable 'defaultsToUndefined' has no type declaration.
var defaultsToUndefined = undefined;

To use the * type without a warning, it should declared explicitly.

var defaultsToUndefined:* = undefined;

Special case: Number, int, and uint

While the common base class of Number, int, and uint is technically Object, ActionScript allows int and uint values to be assigned to Number. As a special case, the common base class for a mix of Number, int, or uint values will be detected as Number instead of Object.

The return type of the function below is Number because the function returns a mix of Number or int values.

function round(value:Number) {
	var integer = int(value);
	var difference = value - integer;
	if (difference >= 0.5) {
		return integer + 1.0;
	return integer;

Special case: Interfaces

ActionScript allows an interface to extend one or more other interfaces. When inferring the return type of a function, the case of multiple interface inheritance can result in a situation where determining the correct return type to infer from two interfaces becomes ambiguous, if you were to consider common extended interfaces. This is because two interfaces could both extend more than one common interface. With this in mind, finding the common base type between two interfaces considers only those two interfaces, and not any of the other interfaces that they extend.

Consider the following interfaces:

interface A {}
interface B extends A {}
interface C extends A {}

The base interface between A and B is A because B extends A. However, in the context of type inference, there is no common base interface between B and C because B does not extend C and C does not extend B. Both B and C extend A, but as you’ll see in the next example, that’s where things can get ambiguous.

Consider the following interfaces:

interface J {}
interface K {}
interface L extends J, K {}
interface M extends J, K {}

Interface L extends J and K, and interface M also extends J and K. If type inference were to find a common base interface between L and M by checking which interfaces they both extend, should it choose J or should it choose K? Neither J nor K takes precedence in any way, so neither is the better option. With that in mind, type inference must fall back to the * type, and the developer can explicitly declare a type to prevent a compiler warning.