PHP: rfc:stringable

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====== PHP RFC: Add Stringable interface ======
  * Version: 0.9
  * Date: 2020-01-15
  * Author: Nicolas Grekas, nicolasgrekas@php.net
  * Status: Implemented
  * First Published at: http://wiki.php.net/rfc/stringable

===== Introduction =====

This RFC introduces a new ''Stringable'' interface that is automatically added to classes that implement the ''%%__toString()%%'' method.

It has two goals:
  - allow using ''string|Stringable'' to express ''string|object-with-%%__toString()%%''
  - provide a forward upgrade path from PHP 7 to 8

===== Proposal =====
Goal 1. is to allow using the ''string|Stringable'' union type in PHP 8, to accept both strings and objects that implement ''%%__toString()%%''. This is critically missing currently, on codes that deal with stringable objects: they can't be made type-safe.

Classes that implement ''%%__toString()%%'' can also declare the interface explicitly. Classes that don't declare the interface explicitly will still declare it implicitly. This allows both forward compatibility and backward compatibility: using a polyfill, classes can declare the interface on PHP 7; and on PHP 8, classes that don't do so will still be compatible with the ''string|Stringable'' union type.

Once a polyfill becomes widely available (e.g. [[https://github.com/symfony/polyfill/pull/224|as part of symfony/polyfill-php80]]), code style checkers could be able to enforce declaring the interface when ''%%__toString()%%'' is declared explicitly, for people that prefer doing so.

Here is the stub declaration of the interface:

<PHP>
interface Stringable
{
   public function __toString(): string;
}
</PHP>

Because it adds the ''string'' return type, this interface has the potential to force a BC break on any existing libraries that want to adopt it.

In order to ease forward and backward compatibility, this RFC also proposes to automatically add the return type at compile time when a ''%%__toString()%%'' method doesn't do it explicitly already. Returning a string is already enforced by the engine so this doesn't change any semantics.

This way, code moving to PHP8 won't be forced to add the return type explicitly (which would break BC on their side), and code in PHP < 8 can adopt a polyfill interface immediately (one that doesn't declare the return type for the same BC reasons.)

Providing an easy forward-path is the second goal of this RFC.

For reference, [[https://github.com/symfony/symfony/search?q=%22%40param+string%7Cstringable%22|here are some annotations in Symfony]], added by contributions from real-world use cases and that currently cannot be expressed precisely enough using any union types in PHP 8.

===== Backward Incompatible Changes =====
Codes that declare a symbol named ''Stringable'' in the root namespace will have to rename it.

===== Proposed PHP Version(s) =====
PHP 8.0

===== RFC Impact =====
==== To Existing Extensions ====
Extensions will need to declare both the interface and the ''string'' return type when they declare ''%%__toString()%%'' methods if they want to pass the ''string|Stringable'' union type.

===== Open Issues =====
none

===== Unaffected PHP Functionality =====
Declaring this interface is not mandatory to benefit from the magic of the ''%%__toString()%%'' method.

===== Future Scope =====
void

===== Proposed Voting Choices =====
yes/no

===== Vote =====
<doodle title="Adopt the Stringable interface as proposed in this RFC?" auth="nicolasgrekas" voteType="single" closed="true">
   * Yes
   * No
</doodle>

===== Patches and Tests =====
https://github.com/php/php-src/pull/5083/files

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature
  - a link to the language specification section (if any)

===== References =====
https://github.com/php/php-src/pull/5083

===== Rejected Features =====

  - Adding a new ''%%stringable%%'' special type (like ''iterable'', ''callable'', etc.) is not considered in this RFC because it would require adding a new reserved keyword in the language. This would break BC more heavily and would defeat goal #2 mentioned previously (ability to polyfill on PHP7.)
  - Consistently, this RFC doesn't embed any ''%%is_stringable()%%'' function. If we were to consider one, defining what happens when e.g. an int is passed [[https://externals.io/message/98424|has no simple answers]].
  - it has been proposed [[https://github.com/php/php-src/pull/5083#issuecomment-573899924|on the GitHub PR]] that the method attached to the interface could be named ''%%toString()%%'' instead of ''%%__toString()%%''. This idea goes further than strictly required to achieve goal #1 and has been objected as not necessary in the following comments. For reference, [[https://github.com/php/php-src/pull/5088|here is a naive implementation]] of the proposal. What the patch highlights is that this increases the complexity of the engine, for a reason that would need to be justified.
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