# RFC: Callable type hint

- Status: Implemented (PHP 5.4)
- Authors: Hannes Magnusson bjori@php.net
- Date: 2011-06-07
- Version: 1.0
- Original: https://wiki.php.net/rfc/callable
- Page: https://externals.io/rfc/callable
- More for agents: search with https://externals.io/search.md?q={query} · RFCs at https://externals.io/rfc.md · JSON API at https://externals.io/api · documentation at https://externals.io/llms.txt

## Discussions on the mailing list

No thread found.

---

## 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.

## Initialization

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

## Patch

- [http://php.markmail.org/download.xqy?id=ya3f6abxw2u6asdp&number=1](http://php.markmail.org/download.xqy?id=ya3f6abxw2u6asdp&number=1)

## Previous discussions

- <http://php.markmail.org/message/gdas65h3im52sleg>
- <http://php.markmail.org/message/ya3f6abxw2u6asdp>

##### Discussion summary

- The only arguments against the idea came from Stas, [making the argument](http://php.markmail.org/message/hhdjdfo3o34mcj2t?q=callable); 
    - '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

## Changelog

- June 7th 2011, Initial writeup
- July 27th 2011, Added “callable vs callback” and “initialization” section