Built In Types: Dynamic

This type is intended to help with code being transitioned from untyped mode to strict mode.

Although dynamic can be used as the type of a class constant or property, or a function return type, its primary use is as a parameter type.

This special type is used to help capture dynamism in the existing codebase in typed code, in a more manageable manner than mixed. With dynamic, the presence of dynamism in a function is local to the function, and dynamic behaviors cannot leak into code that does not know about it.

Consider the following:

function f(dynamic $x) : void {
  $n = $x + 5;            // $n is a num
  $s = $x . "hello!";     // $s is a string
  $y = $x->anyMethod();   // $y is dynamic

A value of type dynamic can be used in most local operations: like untyped expressions, we can treat it like a num and add it, or a string and concatenate it, or call any method on it, as shown above.

When using a dynamic type in an operation, the type checker infers the best possible type with the information it has. For example, using a dynamic in an arithmetic operation like + or - result in a num, using a dynamic in a string operation result in a string, but calling a method on a dynamic, results in another dynamic.

dynamic allows calls, property access, and bracket access without a null-check, but it can throw if the runtime value is null.


The dynamic type sits outside the normal type hierarchy. It is a supertype only of the bottom type nothing and a subtype only of the top type mixed. The type interfaces with other types via coercion (~> in the examples). All types coerce to dynamic, which allows callers to pass any type into a function that expects dynamic. Also, any type coerces to its supertypes. Coercion points include function calls, return statements, and property assignment.

function f(dynamic $d): void {}
function g(arraykey $a): void {}

function caller(int $i): void {
  f($i); // int ~> dynamic
  g($i); // int ~> arraykey by subtyping

The runtime enforces a set of types by throwing TypeHintViolationException when an incorrect type is provided. This set includes

  • primitive types
  • classes without generics, or with reified generics
  • reified generics with the <<__Enforceable>> attribute
  • transparent type aliases to enforceable types

Hack approximates this runtime behavior by allowing values of type dynamic to coerce to enforceable types at coercion points.

function enforced(int $i): void {}
function notEnforced(shape('a' => int) $s): void {}

function caller(dynamic $d): void {
  enforced($d); // dynamic ~> int, runtime will throw if $d is not an int

  notEnforced($d); // Hack error, dynamic ~/> shape('a' => int), runtime will not throw if $d is not a shape
coercion_from_dynamic = true

Unions with dynamic are also allowed to coerce to enforceable types provided that each element of the union can coerce.

function expect_int(int $i): void {}
function expect_string(string $s): void {}

function choose(bool $b, dynamic $d, int $i): void {
  if ($b) {
    $x = $d;
  } else {
    $x = $i;
  expect_int($x); // (dynamic | int) ~> int
  // dynamic ~> int because int is enforced
  // int ~> int because int <: int

  expect_string($x); // Hack error, (dynamic | int) ~/> string because int ~/> string
coercion_from_dynamic = true
coercion_from_union = true
complex_coercion = true

Notably, unlike subtyping, coercion is not transitive, meaning that int ~> dynamic and dynamic ~> string does not imply that int ~> string.

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