2.6 Dynamic

While Haxe has a static type system, it can essentially be disabled by using the Dynamic type. A dynamic value can be assigned to anything and anything can be assigned to it. This has several drawbacks:

  • The compiler can no longer type-check assignments, function calls and other constructs where specific types are expected.
  • Certain optimizations, in particular when compiling to static targets, can no longer be employed.
  • Some common errors such as typos in field accesses cannot be caught at compile-time and likely cause errors at runtime.
  • Dead Code Elimination cannot detect used fields if they are used through Dynamic.

It is very easy to come up with examples where the usage of Dynamic can cause problems at runtime. Consider compiling the following two lines to a static target:

var d:Dynamic = 1;

Trying to run a compiled program in the Flash Player yields an error Property foo not found on Number and there is no default value. Without Dynamic, this would have been detected at compile-time.

Use of Dynamic should be minimized as there are often better options available. However, it is occasionally the practical solution; parts of the Haxe Reflection API make use of it. Additionally, using Dynamic can be the best choice to handle custom data structures that are not known at compile-time.

Dynamic behaves in a special way when being unified with a monomorph. Monomorphs are never bound to Dynamic which can have surprising results in examples such as this:

class Main {
  static function main() {
    var jsonData = '[1, 2, 3]';
    var json = haxe.Json.parse(jsonData);
    $type(json); // Unknown<0>
    for (i in 0...json.length) {
      // Array access is not allowed on
      // {+ length : Int }

Although the return type of Json.parse is Dynamic, the type of local variable json is not bound to it and remains a monomorph. It is then inferred as an anonymous structure upon the json.length field access, which causes the following json[0] array access to fail. In order to avoid this, the variable json can be explicitly typed as Dynamic by using var json:Dynamic.

Trivia: Dynamic Inference before Haxe 3

The Haxe 3 compiler never infers a type to Dynamic, so users must be explicit about it. Previous Haxe versions used to infer arrays of mixed types, e.g. [1, true, "foo"], as Array<Dynamic>. We found that this behavior introduced too many type problems and thus removed it for Haxe 3.

Trivia: Dynamic in the Standard Library

Dynamic was quite frequent in the Haxe Standard Library before Haxe 3. With the continuous improvements of the Haxe type system, the occurrences of Dynamic were reduced over the releases leading to Haxe 3.