====== PHP RFC: Arbitrary string interpolation ======

  * Date: 2022-03-17
  * Author: Ilija Tovilo, tovilo.ilija@gmail.com
  * Status: Withdrawn
  * Target Version: PHP 8.2
  * Implementation: https://github.com/php/php-src/pull/8256

===== Proposal =====

PHP has four types of string interpolation. The two most common forms are ''"$foo"'' and ''"{$foo}"''. For details on the two other forms of string interpolation see the [[https://wiki.php.net/rfc/deprecate_dollar_brace_string_interpolation|deprecate ${} string interpolation RFC]].

All the current forms of string interpolation are limited to variables or variable chains (array access, properties, method calls, etc). This probably covers 80% of use cases for string interpolation (unverified figure). However, there are times when this is not enough, namely you might want to embed any of the following in your string:

  * Function calls
  * Constants
  * Class constants
  * Static method calls

<code php>
$world = 'world';
echo "Hello {strtoupper($world)}!";
// Hello {strtoupper(world)}
// :(
</code>

This RFC proposes to add a new form for string interpolation that works for arbitrary expressions. 

<code php>
$world = 'world';
echo "Hello {$:strtoupper($world)}!";
// Hello WORLD!
// :)

echo "{$:T_PAAMAYIM_NEKUDOTAYIM}";
echo "{$:Foo::class}";
echo "{$:self::doSomething()}";
</code>

The proposed syntax works in all cases where existing string interpolation is allowed, namely double quoted strings (''""''), heredoc (''<<<TXT TXT'') and the execution operator (''``'').

===== Motivation =====

Of course, this can already be achieved in two other ways:

  * By creating a temporary local variable
  * By stopping the string and concatenating the value with ''.''

Creating local variables is not a terrible option. It does pollute the variable symbol table a bit and it requires more visual jumping to see what value is being embedded.

Concatenation is a fair bit worse. The quotes and ''.'' symbols lead to a symbol soup, and the approach will not work nicely with heredoc at all.

<code php>
echo <<<TXT
    Hello 
TXT /* <-- Must be on a new line */ . strtoupper('world') . <<<TXT
TXT;
</code>

===== Syntax choice =====

The primary advantage of the syntax ''"{$:expr}"'' is that is it is fully backwards compatible as it leads to a syntax error in PHP 8.1 rather than being interpreted literally. It also looks familiar while being distinctly different from other forms of string interpolation to avoid confusion. The '':'' could be swapped with another character or even white space, but I believe that white space has a bigger chance of leading to confusion.

A different syntax could be chosen with a given BC break.

<code php>
echo "Hello #{strtoupper('world')}";
</code>

This could be mitigated by adding a prefix to the string itself.

<code php>
echo $"Hello #{strtoupper('world')}";
</code>

I think this is unfortunate because:

  * Converting an existing string to ''$'' will potentially alter its behavior and require escaping of other parts of the string (similar to switching from ''''' to ''"'')
  * The prefix will need to work for all existing string types that allow interpolation (''"'', heredoc, execute operator), doubling the types of interpolated string we have

===== Escaping =====

Escaping works by adding a backslash ''\'' after the brace ''{''.

<code php>
echo "{\$:foo}";
// {$:foo}
</code>

If you're thinking why not before the ''{'', PHPs existing escaping of ''\{$foo}'' could be considered buggy. While it escapes the ''{}'', it also gets printed itself.

<code php>
$foo = 'bar';
echo "\{$foo}";
// \{bar}
</code>

https://3v4l.org/oEWFC

That said, escaping the proposed string interpolation ''{$:'' should be very rare in practice.

===== Backwards incompatible changes =====

None

===== Comparison to other languages =====

Most modern languages have string interpolation that allow arbitrary expressions.

^ Language                                                                                           ^ Syntax                        ^
| [[https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated|C#]]       | ''$"x + y = {x + y}"''        |
| [[https://api.dart.dev/stable/2.16.1/dart-core/String-class.html|Dart]]                            | '''x + y = ${x + y}'''        |
| [[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals|JavaScript]] | ''`x + y = ${x + y}`''        |
| [[https://kotlinlang.org/docs/basic-types.html#string-templates|Kotlin]]                           | ''"x + y = ${x + y}"''        |
| [[https://peps.python.org/pep-0498/|Python]]                                                       | ''f'x + y = {x + y}'''        |
| [[https://docs.raku.org/language/quoting.html#Interpolation:_qq|Raku (Perl6)]]                     | ''"\$x + \$y = { $x + $y }"'' |
| [[http://ruby-for-beginners.rubymonstas.org/bonus/string_interpolation.html|Ruby]]                 | ''"x + y = #{x + y}"''        |
| [[https://docs.scala-lang.org/overviews/core/string-interpolation.html|Scala]]                     | ''s"x + y = ${$x + $y}"''     |
| [[https://docs.swift.org/swift-book/LanguageGuide/StringsAndCharacters.html#ID292|Swift]]          | ''"x + y = \(x + y)"''        |

===== Credits =====

Credits to Rowan Tommins (IMSoP) for the syntax suggestion and comparison to other languges.

===== Vote =====

Voting starts 2022-xx-x and ends 2022-xx-xx. 

As this is a language change, a 2/3 majority is required.

<doodle title="Accept arbitrary string interpolation in PHP 8.2?" auth="ilutov" voteType="single" closed="true">
   * Yes
   * No
</doodle>