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Convert untyped data to typed data

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v4.2.2 2021-09-13 15:44 UTC


Continuous Integration

Hack library for converting untyped data to typed data.

Warning for TypeAssert\matches_type_structure()

TypeStructure<T>, type_structure(), and ReflectionTypeAlias::getTypeStructures() are experimental features of HHVM, and not supported by Facebook or the HHVM team. This means that matches_type_structure() may need to be removed in a future release without warning.

We strongly recommend moving to TypeAssert\matches<T>() and TypeCoerce\match<T>() instead.


composer require hhvm/type-assert


TypeAssert provides functions that take a mixed input, and will either return it unmodified (but with type data) or throw an exception; for example:

<?hh // strict
use namespace Facebook\TypeAssert;
function needs_string(string $bar): void {

function main(): void {
  needs_string(TypeAssert\string('foo')); // type-safe and works fine
  needs_string(TypeAssert\string(123)); // type-safe, but throws

These include:

  • string(mixed): string
  • int(mixed): int
  • float(mixed): float
  • bool(mixed): bool
  • resource(mixed): resource
  • num(mixed): num
  • arraykey(mixed): arraykey
  • not_null<T>(?T): T
  • instance_of<T>(classname<T>, mixed): T
  • classname_of<T>(classname<T>, mixed): classname<T>
  • matches<T>(mixed): T
  • matches_type_structure<T>(TypeStructure<T>, mixed): T


TypeAssert also contains the Facebook\TypeCoerce namespace, which includes a similar set of functions:

  • string(mixed): string
  • int(mixed): int
  • float(mixed): float
  • bool(mixed): bool
  • resource(mixed): resource
  • num(mixed): num
  • arraykey(mixed): arraykey
  • match<T>(mixed): T
  • match_type_structure<T>(TypeStructure<T>, mixed): T

These will do 'safe' transformations, such as int-ish strings to int, ints to strings, arrays to vecs, arrays to dicts, and so on.


You can also assert/coerce complex types (except for shapes and tuples) without a type_structure:


use namespace Facebook\TypeSpec;

$spec = TypeSpec\dict(
$x = $spec->assertType(dict['foo' => 123]); // passes: $x is a dict<string, int>
$x = $spec->assertType(dict['foo' => '123']); // fails
$x = $spec->assertType(dict[123 => 456]); // fails
$x = $spec->assertType(dict[123 => 456]); // fails

$x = $spec->coerceType(dict[123 => '456']); // passes: $x is dict['123' => 456];

Shapes and tuples are not supported, as they can not be expressed generically.

matches_type_structure<T>(TypeStructure<T>, mixed): T

Asserts that a variable matches the given type structure; these can be arbitrary nested shapes. This is particular useful for dealing with JSON responses.

<?hh // strict

use namespace Facebook\TypeAssert;

class Foo {
  const type TAPIResponse = shape(
    'id' => int,
    'user' => string,
    'data' => shape(
      /* ... */

  public static function getAPIResponse(): self::TAPIResponse {
    $json_string = file_get_contents('https://api.example.com');
    $array = json_decode($json_string, /* associative = */ true);
    return TypeAssert\matches_type_structure(
      type_structure(self::class, 'TAPIResponse'),

You can use type_structure() to get a TypeStructure<T> for a type constant, or ReflectionTypeAlias::getTypeStructure() for top-level type aliases.

not_null<T>(?T): T

Throws if it's null, and refines the type otherwise - for example:

<?hh // strict
use namespace \Facebook\TypeAssert;

function needs_string(string $foo): void {}
function needs_int(int $bar): void {}

function main(?string $foo, ?int bar): void {
  needs_string(TypeAssert\not_null($foo)); // ?string => string
  needs_int(TypeAssert\not_null($bar)); // ?int => int

is_instance_of<T>(classname<T>, mixed): T

Asserts that the input is an object of the given type; for example:

use namespace Facebook\TypeAssert;

class Foo {}

function needs_foo(Foo $foo): void {}

function main(mixed $foo): void {
  needs_foo(TypeAssert::is_instance_of(Foo::class, $foo));

main(new Foo());

is_classname_of<T>(classname<T>, mixed): classname<T>

Asserts that the input is the name of a child of the specified class, or implements the specified interface.

<?hh // strict
use namespace Facebook\TypeAssert;

class Foo {
  public static function doStuff(): void {}
class Bar extends Foo {
  public static function doStuff(): void {
    // specialize here

function needs_foo_class(classname<Foo> $foo): void {

function main(mixed $class): void {
  needs_foo_class(TypeAssert::is_classname_of(Foo::class, $class));



This library is a reimplementation of ideas from:

  • @admdikramr
  • @ahupp
  • @dlreeves
  • @periodic1236
  • @schrockn

Security Issues

We use GitHub issues to track public bugs. Please ensure your description is clear and has sufficient instructions to be able to reproduce the issue.

Facebook has a bounty program for the safe disclosure of security bugs. In those cases, please go through the process outlined on that page and do not file a public issue.


Type-Assert is MIT-licensed.