Haxe Code Cookbook
Haxe programming cookbookPrinciplesNull safety

Null safety

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NOTE: Since Haxe 4.0, the compiler provides a more natural compile-time checking for nullable values that is aware of the control flow. See official documentation for more details. We're leaving this example for the curious as it illustrates how abstract types can be used for such thing.

It's possible to write null-safe code without run-time overhead using an abstract similar to the following:

abstract Maybe<T>(Null<T>) from Null<T> {

  public inline function exists():Bool {
    return this != null;

  public inline function sure():T {
    return if (exists()) this else throw "No value";

  public inline function or(def:T):T {
    return if (exists()) this else def;

  public inline function may(fn:T->Void):Void {
    if (exists()) fn(this);

  public inline function map<S>(fn:T->S):Maybe<S> {
    return if (exists()) fn(this) else null;

  public inline function mapDefault<S>(fn:T->S, def:S):S {
    return if (exists()) fn(this) else def;


class Test {
  static function main() {
    // initialize from null...
    var value:Maybe<Int> = null;
    // ...or a value of underlying type
    value = 10;

    // compilation errors, so you can't use Maybe<T> without explicit unwrapping
    // var v:Int = value;
    // var v = value + 5;

    // get value or raise exception
    var v = value.sure();

    // get value or use default
    var v = value.or(0);

    // check whether value exists
    if (value.exists())
        trace("value exists!");

    // execute function if value exists
    value.may(function(value) trace("Value is " + value));

    // map value to Maybe<String>
    var valueString = value.map(function(value) return Std.string(value));

    // map value to String or use default string
    var message = value.mapDefault(function(value) return "Value is " + value, "No value");

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Category:  Principles