Request for Comments: callable type hint

The problem

Frameworks have started to adopt Closures for callbacks, utilizing the 'Closure' typehint. That has an unfortunate sideeffect of requiring already-existing-methods to be wrapped into a closure calling that method. Additionally, the PHP documentation on closures indicates that the “Closure” class is considered an implementation detail, and may be subject to change in the future -- which means that typehinting on Closure may break in the future.

The solution

Introducing a callable typehint.

The typehint would allow a string with a function name, an array composed of classname/obj+methodname, and a closure.

Callable vs Callback

callback is callable, the opposite could not be true. A string, or a closure, is callable, but the string is not a callback.

The documentations already use `array` hinting when a function expects an array, even though it does not type hint on it. The documentations already use `callable` hiting when a function expects a callable instance, even though it does not type hint on it.

Furthermore we already have a function called is_callable(), and this new feature will share its implementation.

Introducing a different name (i..e callback) will therefore only create more inconsistencies and confusion.

The callable typehint reuses the is_callable() logic.


Only null initialize is allowed, making it consistent with classname typehinting.


Previous discussions

Discussion summary
  • The only arguments against the idea came from Stas, making the argument;
    • 'callable' is a data structure not unlike 'balanced binary tree' and 'readable file', and none of them make sense have as a part of the language syntax. They belong in userspace.
    • A “strict typing error” is hard to handle in user-space gracefully.
  • Everyone else seem to like the idea, so far.
  • As for the patch itself, Felipe pointed out that the patch is missing an method in ext/reflection to retrieve the typehint information.
  • Things to finalize:
    • Reflection support
    • Arginfo support


  • June 7th 2011, Initial writeup
  • July 27th 2011, Added “callable vs callback” and “initialization” section
rfc/callable.txt · Last modified: 2017/09/22 13:28 by