PHP: rfc:variable_syntax_tweaks

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====== PHP RFC: Variable Syntax Tweaks ======
  * Date: 2020-01-07
  * Author: Nikita Popov <nikic@php.net>
  * Target Version: PHP 8.0
  * Status: Implemented
  * Implementation: https://github.com/php/php-src/pull/5061

===== Introduction =====

The [[rfc:uniform_variable_syntax|Uniform Variable Syntax RFC]] resolved a number of inconsistencies in PHP's variable syntax. This RFC intends to address a small handful of cases that were overlooked.

===== Proposal =====

==== Terminology ====

PHP supports four principal types of "dereferencing" operations:

  * Array: ''$foo[$bar]'', ''$foo{$bar}''
  * Object: ''%%$foo->bar%%'', ''%%$foo->bar()%%''
  * Static: ''Foo::$bar'', ''Foo::bar()'', ''Foo::BAR''
  * Call: ''foo()''

A fully dereferencable construction is dereferencable by all four operations.

==== Interpolated and non-interpolated strings ====

Non-interpolated strings ''%%"foo"%%'' are currently considered fully dereferencable, i.e. constructions such as ''%%"foo"[0]%%'' or ''%%"foo"->bar()%%'' are considered legal (syntactically at least). However, interpolated strings ''%%"foo$bar"%%'' are non-dereferencable.

This RFC proposed to treat both types of strings consistently, i.e. ''%%"foo$bar"[0]%%'', ''%%"foo$bar"->baz()%%'' etc become legal.

This inconsistency was originally reported in the context of [[https://github.com/nikic/scalar_objects/issues/29|scalar objects]].

==== Constants and magic constants ====

Constants are currently array-dereferencable, that is ''FOO[0]'' is legal. However, magic constants are non-dereferencable.

This RFC proposes to treat magic constants the same way as constants, that is, writing ''%%__FUNCTION__[0]%%'' etc becomes legal.

==== Constant dereferencability ====

Constants (and class constants) are currently special cased and only dereferencable under the ''[]'' operator (and do not even support the nominally equivalent ''{}'' operator).

This RFC proposes to make constants (and class constants) fully array and object dereferencable, i.e. to allow both ''FOO{0}'' and ''%%FOO->length()%%'' (the latter only being relevant in conjunction with [[https://github.com/nikic/scalar_objects|scalar objects]]).

This makes the set of array-dereferencable and object-dereferencable operations strictly identical.

==== Class constant dereferencability =====

Currently static property accesses are array, object and static dereferencable. However, even with the change from the previous section, class constant accesses are only array and object dereferencable. This means that while ''Foo::$bar::$baz'' is legal, ''Foo::BAR::$baz'' is not.

This RFC proposes to make class constant accesses static derefencable as well, so that ''Foo::BAR::$baz'' and ''Foo::BAR::BAZ'' become legal.

It should be noted that the same is not possible for plain constants, as ''FOO::$bar'' already interprets ''FOO'' as a class name, not a constant name. Similarly, both types of constants cannot be call dereferenced, as ''FOO()'' is interpreted as a function name and ''Foo::BAR()'' as a static method call.

This inconsistency was originally reported on [[https://twitter.com/jrf_nl/status/1206541895138250753|Twitter]].

==== Arbitrary expression support for new and instanceof ====

Most syntactic constructions that normally require an identifier/name also accept a syntax variation that allows an arbitrary expression. Depending on the case, this uses either ''{expr}'' or ''(expr)'' syntax. Places that accept a simple (non-namespaced) identifier use curly braces, such as ''${expr}'', ''%%$foo->{expr}%%'', and ''FOO::{expr}()''. Places that accept a namespaced name use parentheses, such as ''(expr)()'' and ''(expr)::FOO''. These cases also accept more than just strings, in particular ''(expr)()'' additionally also allows other callables, and ''(expr)::FOO'' also allows objects.

One place where arbitrary expressions are currently not supported are class names in ''new'' and ''instanceof'' (which are currently treated as syntactically the same).

In line with the considerations above, this RFC proposes to introduce the syntax ''new (expr)'' and ''$x instanceof (expr)'' respectively.

===== Backward Incompatible Changes =====

There are not backwards incompatible changes, this RFC allows strictly more syntax than before.

===== Future Scope =====

It is in principle possible to relax the limitations on the right-hand-side of ''instanceof'' entirely: It could be treated as a normal expression, with plain constant accesses being reinterpreted as class name references instead (as this is the only ambiguity between them). While I think that this is the best option in principle, I am concerned that it will cause issues in the future if we introduce generic types, in which case a class name will no longer necessarily coincide with an ordinary expression.

===== Vote =====

Voting started 2020-01-28 and ends 2020-02-11.

<doodle title="Implement syntax changes outlined above in PHP 8?" auth="nikic" voteType="single" closed="true">
   * Yes
   * No
</doodle>
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