Tools: Transpiler

The Hack transpiler, h2tp, allows you to write code in Hack and then off your code to folks who only deploy on an engine that doesn't support Hack, but rather supports only PHP. This gives you the benefits of writing your code in Hack, allowing users who run code on an Hack supported runtime like HHVM to use Hack code, while also allowing users who haven't made the switch to something like HHVM to use your code as well.

h2tp <source directory of Hack code> <destination directory of transpiled code>

The source and destination must be directories. And the destination directory must not exist.

Hack Library

Imagine we have this Hack file in /tmp/original:


class A {
  protected ?int $x = null;

  public function foo(): bool {
    return true;

function bar(Vector<int> $vec): int {
  if ($vec->count() > 0) {
    return $vec[0];
  return -1;

We run the Hack transpiler as such:

h2tp /tmp/original /tmp/transpiled

If you examine the transpiled file in /tmp/transpiled:

require_once ($GLOBALS["HACKLIB_ROOT"]);
class A {
  protected $x = null;
  public function foo() {
    return true;
function bar($vec) {
  if ($vec->count() > 0) {
    return $vec[0];
  return -1;

Notice the following:

  • There are no more type annotations.
  • We have a new line require_once ($GLOBALS["HACKLIB_ROOT"]);

The new require_once line is important. After you use the transpiler, you must copy the Hack library to the transpiled directory. The Hack library is called hacklib and is normally located in /usr/share/hhvm/hack/hacklib.

cp -r /usr/share/hhvm/hack/hacklib /tmp/transpiled

Finally, add a piece of code to a top-level file somewhere in the root of your transpiled directory that will run before any of the transpiled code will run:

$GLOBALS['HACKLIB_ROOT'] = __DIR__ . '/hacklib/hacklib.php';


The Hack transpiler will examine any file with <?hh in the hopes of converting it to a non-Hack <?php. It will try to perform the following conversions:

  • Remove type annotations.
  • Lambdas are changed to closures.
  • Shapes are changed to arrays.
  • Enums are converted to classes.
  • Attributes are removed.
  • Collection are supported via the Hack library in PHP. Any collection literal syntax is replaced with new.

Unsupported Conversions

The following conversions are unsupported:

  • Async functions require Hack-aware runtime support (e.g., HHVM) and thus are not converted.
  • The __Memoize attribute requires a Hack-aware runtime and are not converted.
  • Traits that implement interfaces.
  • Collection literals that have initial values for non-static properties since we have to convert those literals to new, and new is not supported on property initializers. Static properties can be moved outside the class.


  • Do not edit a transpiled file. Comments are stripped. Formatting may not be preserved. Treat the transpiled code as machine generated, which it is.
  • All non-Hack <?hh files in the transpiler directory path are copied to the destination directory unmodified.
  • If one file cannot be converted correctly, the transpiler gives up on that file. There are no partial conversions.