Null safety for Haxe
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Safety Build Status

This library implements a few features for writing Haxe code which generates less null pointer errors. These features are: safe navigation operator, safe arrays, safe api (automatic checks of method arguments for null) and null safety.

Most features are compatible with Haxe 3.4 except null safety.

Null safety is implemented as a plugin for Haxe compiler. At least Haxe 4.0.0-preview.3 is required to enable null safety.

It's an attempt to push Haxe towards null safety implementation in the compiler. Use this plugin, report bugs and share your thoughts in issues. Hopefully we can find the best approach to null safety together. And then with all the collected experience we will be able to propose a solid implementation to the compiler.


Compatible compiler build is official 4.0.0-preview.3:

Install Safety from haxelib:

haxelib install safety

or the latest development version from github:

haxelib git safety

Prebuilt plugin binaries are compatible with Haxe 4.0.0-preview.3 and x64 systems Windows, OSX and Linux. If you want to use null safety plugin with another OS, arch or another version of Haxe you need to setup desired version of Haxe for development (see Building Haxe from source) and then

cd path/to/haxe-source/
make PLUGIN=path/to/safety/src/ml/safety_plugin plugin


Add -lib safety to your hxml file. Use following compiler arguments:

  • --macro Safety.enable(dotPath, enableAdditionalFeatures) (requires Haxe 4.0.0-preview.3 or later) - Enable null safety for the specified path. E.g. --macro Safety.enable('my.pack') to enable it for all the types in my.pack and in subpackages. Or --macro Safety.enable('my.pack.MyClass') to enable it for my.pack.MyClass only. Additional features are safe navigation operator, safe api and SafeArray. Each feature can be enabled separately if you pass false to enableAdditionalFeatures. The default value is true. More on the features below.
  • --macro Safety.safeNavigation(dotPath, recursive) - Enables safe navigation operator !. in the specified path. If recursive is true (it is by default), then !. operator is also enabled for subpackages in dotPath.
  • --macro Safety.safeArray(dotPath, recursive) - Makes all array declarations to be typed as SafeArray. See feature description for details.
  • --macro Safety.safeApi(dotPath, recursive) - Adds runtime checking for not-nullable arguments of public methods in the specified path. If null is passed to such an argument, then safety.IllegalArgumentException is thrown. Details
  • -D SAFETY_SILENT - do not abort compilation on safety errors. You can handle safety errors manually at macro time in Safety.plugin.onComplete(() -> trace(Safety.plugin.getErrors()))
  • -D SAFETY_DEBUG - prints additional information during safety checking.

All --macro Safety.* arguments can be used multiple times with different dotPath values.

You can pass empty string "" as any dotPath to apply a feature to the whole codebase (not recommended, because a lot of compilation errors will come from std lib).


Compile time null safety

  • Safety makes sure you will not pass nullable values to the places which are not explicitly declared with Null<SomeType> (assignments, return statements, array access etc.);
function fn(s:String) {}
var nullable:Null<String> = getNullableStr();
var str:String = null; //Compilation error
str = nullable; //Compilation error
fn(nullable); //Compilation error. Function argument `str` is not nullable
  • Using nullables with unary and binary operators (except == and !=) is not allowed;
  • If a field is declared without Null<> then it should have an initial value or it should be initialized in a constructor (for instance fields);
  • Passing an instance of parameterized type with nullable type parameter to a place with the same type, but with not-nullable type parameter is not allowed:
var nullables:Array<Null<String>> = ['hello', null, 'world'];
var a:Array<String> = nullables; //Compilation error. Array<Null<String>> cannot be assigned to Array<String>
  • Local variables checked against null are considered safe inside of a scope covered with that null-check:
var nullable:Null<String> = getSomeStr();
var s:String = nullable; //Compilation error
if(nullable != null) {
    s = nullable; //OK
s = nullable; //Compilation error
s = (nullable == null ? 'hello' : nullable); //OK
switch(nullable) {
    case null:
    case _: s = nullable; //OK
  • Control flow is also taken into account:
function doStuff(a:Null<String>) {
    if(a == null) {
    //From here `a` is safe, because function execution will continue only if `a` is not null
    var s:String = a; //OK

Excluding code from null safety

These are the ways to disable safety in certain situations. You should avoid such cases where possible. Use at your own risk.

  • To skip null safety checking for a method, field or a whole class, add @:safety(unsafe) meta to that field or class:
class Dummy {
    /** no initial value is ok, because this field is not checked */
    @:safety(unsafe) var unsafeField:String;

    @:safety(unsafe) function unsafeMethod() {
        var s:String = null; //ok

class FreeZone {
    static function method() {
        var s:String = null; //ok
  • Unsafe<T> is a special type which is an alias for T. Nullable values will be passed to/from this type without any checks.
var u:Unsafe<String> = null; // ok
var s:String = u; // ok
  • To skip null safety checking for an expression, wrap it in a typecheck with the special Unsafe<T> type:
var s:String;
(s = null : Unsafe<String>); //ok
  • Untyped cast is not checked for null safety
var s:String = cast null; // ok

Safe navigation operator

Adds safe navigation operator !. to Haxe syntax. Compiler argument to enable it: --macro Safety.safeNavigation(my.pack)

var obj:Null<{ field:Null<String> }> = null;
trace(obj!.field!.length); //null
obj = { field:'hello' };
trace(obj!.field!.length); //5

Safe array

SafeArray<T> (abstract over Array<T>) behaves exactly like Array<T> except it prevents out-of-bounds reading/writing (throws safety.OutOfBoundsException). Writing at an index of array's length is allowed.

See Limitations to find out why you need it.

If --macro Safety.safeArray(my.pack) is in effect, then all array declarations become SafeArray:

var a = ['hello', 'world'];
$type(a); //SafeArray<String>

You can use stdArray() method to get a reference to the Array<T>:

var a = ['hello', 'world'];
$type(a.stdArray()); //Array<String>

Safe api

Compiler argument to enable this feature: --macro Safety.safeApi(my.pack).

If enabled it adds runtime checking against null for all not-nullable arguments of public methods:

public function method(arg:String) {}
method(null); //throws safety.IllegalArgumentException

It's pretty cheap performance wise, because it just adds a simple line to the body of a method: if(arg == null) throw new IllegalArgumentException();

If argument is nullable, no check is generated.

Also safe api does not generate such checks for Int, Float, Bool (and other basic types) on static targets, because it's impossible to assign null to such types on static targets.

This feature is especially useful if you are creating a library, and you don't want your users to pass nulls to your API. You can add --macro Safety.safeApi('my.lib') to extraParams.hxml.

Static extensions for convenience

using Safety;

var nullable:Null<String> = getSomeStr();
var s:String = nullable.or('hello');

Available extensions:

*  Returns `value` if it is not `null`. Otherwise returns `defaultValue`.
static public inline function or<T>(value:Null<T>, defaultValue:T):T;
*  Returns `value` if it is not `null`. Otherwise throws an exception.
*  @throws NullPointerException if `value` is `null`.
static public inline function sure<T>(value:Null<T>):T;
*  Just returns `value` without any checks, but typed as not-nullable. Use at your own risk.
static public inline function unsafe<T>(value:Null<T>):T;
*  Applies `callback` to `value` and returns the result if `value` is not `null`.
*  Returns `null` otherwise.
static public inline function let<T,V>(value:Null<T>, callback:T->V):Null<V>;
*  Passes `value` to `callback` if `value` is not null.
static public inline function run<T>(value:Null<T>, callback:T->Void):Void;
*  Applies `callback` to `value` if `value` is not `null`.
*  Returns `value`.
static public inline function apply<T>(value:Null<T>, callback:T->Void):Null<T>;
*  Prints `true` if provided expression can not evaluate to `null` at runtime. Prints `false` otherwise.
*  Always prints `false` if invoked outside of a path passed to `Safety.enable()`
macro static public function isSafe(expr:Expr):ExprOf<Void>


var a:Array<String> = ["hello"];
$type(a[100]); // String
trace(a[100]); // null
var s:String = a[100]; // Safety does not complain here, because `a[100]` is not `Null<String>`, but just `String`
  • Out-of-bounds array write fills all positions between the last defined index and the newly written one with null. Safety cannot save you in this case.
var a:Array<String> = ["hello"];
a[2] = "world";
trace(a); //["hello", null, "world"]
var s:String = a[1]; //Safety cannot check this
trace(s); //null
  • Haxe was not designed with null safety in mind, so it's always possible null will come to your code from 3rd-party code or even from std lib.
  • Nullable fields and properties are not considered null-safe even after checking against null. Use safety extensions instead:
using Safety;

class Main {
    var nullable:Null<String>;
    function new() {
        var str:String;
        if(nullable != null) {
            str = nullable; //Compilation error.
        str = nullable.sure();
        str = nullable.or('hello');
  • If a local var is captured in a closure, it cannot be safe inside that closure:
var a:Null<String> = getSomeStr();
var fn = function() {
    if(a != null) {
        var s:String = a; //Compilation error
  • If a local var is captured and modified in a closure with a nullable value, that var cannot be safe anymore:
var nullable:Null<String> = getSomeNullableStr();
var str:String;
if(nullable != null) {
    str = nullable; //OK
    doStuff(function() nullable = getSomeNullableStr());
    if(nullable != null) {
        str = nullable; //Compilation error