Many developers would like to be able to declare the return type of a function. The basic idea of declaring a return type has been included in at least three RFCs and has been discussed in a few other places (see references). This RFC proposes a different approach from previous RFC's to accomplish this goal in a simple way.
Declaring return types has several motivators and use-cases:
This proposal adds an optional return type declaration to function declarations including closures, functions, generators, and methods. This RFC does not change the existing type declarations nor does it add new ones (see differences from past RFCs).
Here is a brief example of the syntax in action:
function foo(): array { return []; }
More examples can be found in the Examples section.
Code which does not declare a return type will continue to work exactly as it currently does. This RFC requires a return type to be declared only when a method inherits from a parent method that declares a return type; in all other cases it may be omitted.
The enforcement of the declared return type during inheritance is invariant; this means that when a sub-type overrides a parent method then the return type of the child must exactly match the parent and may not be omitted. If the parent does not declare a return type then the child is allowed to declare one.
If a mismatch is detected during compile time (e.g. a class improperly overriding a return type) then E_COMPILE_ERROR
will be issued. If a type mismatch is detected when the function returns then E_RECOVERABLE_ERROR
will be issued.
Covariant return types are considered to be type sound and are used in many other languages2). This RFC originally proposed covariant return types but was changed to invariant because of a few issues. It is possible to add covariant return types at some point in the future.
Note that this topic of variance is about the declared return type of the function; this means that the following would be valid for either invariant or covariant return types:
interface A { static function make(): A; } class B implements A { static function make(): A { return new B(); } }
The class B
implements A
so it is therefore valid. Variance is about the allowed types when overriding the declared types:
interface A { static function make(): A; } class B implements A { static function make(): B { // must exactly match parent; this will error return new B(); } }
The above sample does not work because this RFC proposes only invariant return types; this could be extended in the future to be allowed.
The two major conventions in other programming languages for placing return type information are:
The former position has been proposed in the past and the RFCs were either declined or withdrawn. One cited issue is that many developers wanted to preserve the ability to search for function foo
to be able to find the definition for foo
. A recent discussion about removing the function keyword has several comments that re-emphasized the value in preserving this.
The latter position is used in several languages3); notably C++11 also places the return type after the parameter lists for certain constructs such as lambdas and auto-deducing return types.
Declaring the return type after the parameter list had no shift/reduce conflicts in the parser.
This RFC does not change the location of &
when returning by reference. The following examples are valid:
function &array_sort(array &$data) { return $data; } function &array_sort(array &$data): array { return $data; }
Consider the following function:
function foo(): DateTime { return null; // invalid }
It declares that it will return DateTime
but returns null
; this type of situation is common in many languages including PHP. By design this RFC does not allow null
to be returned in this situation for two reasons:
null
is not allowed 4).null
by default works against the purpose of type declarations. Type declarations make it easier to reason about the surrounding code. If null
was allowed the programmer would always have to worry about the null
case.The Nullable Types RFC addresses this shortcoming and more.
Class constructors, destructors and clone methods may not declare return types. Their respective error messages are:
Fatal error: Constructor %s::%s() cannot declare a return type in %s on line %s
Fatal error: Destructor %s::__destruct() cannot declare a return type in %s on line %s
Fatal error: %s::__clone() cannot declare a return type in %s on line %s
Here are some snippets of both valid and invalid usage.
// Overriding a method that did not have a return type: interface Comment {} interface CommentsIterator extends Iterator { function current(): Comment; }
// Using a generator: interface Collection extends IteratorAggregate { function getIterator(): Iterator; } class SomeCollection implements Collection { function getIterator(): Iterator { foreach ($this->data as $key => $value) { yield $key => $value; } } }
The error messages are taken from the current patch.
// Covariant return-type: interface Collection { function map(callable $fn): Collection; } interface Set extends Collection { function map(callable $fn): Set; }
Fatal error: Declaration of Set::map() must be compatible with Collection::map(callable $fn): Collection in %s on line %d
// Returned type does not match the type declaration function get_config(): array { return 42; } get_config();
Catchable fatal error: Return value of get_config() must be of the type array, integer returned in %s on line %d
// Int is not a valid type declaration function answer(): int { return 42; } answer();
Catchable fatal error: Return value of answer() must be an instance of int, integer returned in %s on line %d
// Cannot return null with a return type declaration function foo(): DateTime { return null; } foo();
Catchable fatal error: Return value of foo() must be an instance of DateTime, null returned in %s on line %d
// Missing return type on override class User {} interface UserGateway { function find($id): User; } class UserGateway_MySql implements UserGateway { // must return User or subtype of User function find($id) { return new User(); } }
Fatal error: Declaration of UserGateway_MySql::find() must be compatible with UserGateway::find($id): User in %s on line %d
// Generator return types can only be declared as Generator, Iterator or Traversable (compile time check) function foo(): array { yield []; }
Fatal error: Generators may only declare a return type of Generator, Iterator or Traversable, %s is not permitted in %s on line %d
This proposal specifically does not allow declaring multiple return types; this is out of the scope of this RFC and would require a separate RFC if desired.
If you want to use multiple return types in the meantime, simply omit a return type declaration and rely on PHP's excellent dynamic nature.
This RFC purposefully omits reflection support as there is an open RFC about improving type information in reflection: https://wiki.php.net/rfc/reflectionparameter.typehint
This proposal differs from past RFCs in several key ways:
void
, int
, string
or scalar
; this RFC does not include any new types. Note that it does allow self
and parent
to be used as return types.function foo
to find foo
's definition; all previous RFCs broke this common workflow.nullable
and more. We still require the function
keyword.This RFC is backwards compatible with previous PHP releases.
There is no impact on any SAPI.
The structs zend_function
and zend_op_array
have been changed; extensions that work directly with these structs may be impacted.
An informal test indicates that performance has not seriously degraded. More formal performance testing can be done before voting phase.
This RFC targets PHP 7.
This RFC modifies the PHP language syntax and therefore requires a two-third majority of votes.
Should return types as outlined in this RFC be added to the PHP language? Voting will end on January 23, 2015.
Dmitry and I have updated the implementation to a more current master branch here: https://github.com/php/php-src/pull/997
This RFC was merged into the master branch (PHP 7) in commit 638d0cb7531525201e00577d5a77f1da3f84811e.
Ideas for future work which are out of the scope of this RFC include:
void
in Java and C)?DateTime
). This is discussed in the Nullable Types RFC.
In the meeting in Paris on November 2005 it was decided that PHP should have return type declarations and some suggestions were made for syntax. Suggestion 5 is nearly compatible with this RFC; however, it requires the addition of a new token T_RETURNS
. This RFC opted for a syntax that does not require additional tokens so returns
was replaced by a colon.
The following (tiny) patch would allow the syntax in suggestion 5 to be used alongside the current syntax. This RFC does not propose that both versions of syntax should be used; the patch just shows how similar this RFC is to that suggestion from 2005.
ReflectionType
classReflectionType::IS_
* constants to TYPE_
*, rename ->getKind()
to ->getTypeConstant()
null
default