HHVM 4.108 is released! HHVM 4.103–4.107 remain supported, as do the 4.80 and 4.102 LTS releases.


  • The Hack Standard Library is now a built-in part of HHVM. Hack projects can now call any HSL functions without downloading and installing any extra packages.
    • This includes HSL IO, which is no longer considered experimental. Other parts of hsl-experimental are not yet included.
  • Constraints (as/super) on abstract context constants are now supported.
  • The HHVM runtime currently doesn’t allow using meth_caller() with a private or protected method (even within the same class hierarchy), but these runtime errors are not detected by the Hack typechecker. A new .hhconfig option, meth_caller_only_public_visibility = true, can be used to raise Hack errors in these cases. However, the plan is to support private and protected methods in meth_caller() in the future, at which point this .hhconfig option will become unnecessary.

Future Changes

  • Functions in the Str\ namespace in the (now built-in) Hack Standard Library will no longer respect the system/request locale. Instead, they will default to using the POSIX (also known as C) locale.
    • In the future, we expect most of them to accept additional parameter(s) overriding the default locale and encoding, but these may not be available immediately.
    • Removing the dependency on a global locale setting will allow these functions to be considered “pure” (they can be called from other functions with any contexts, including an empty context list).