rfc:true-type

PHP RFC: Add true type

Introduction

PHP now has support for null and false as standalone types. However, true which is the natural counter part of false does not even exist as a type.

The motivation in the Union Types 2.0 RFC to include false but not true was:

While we nowadays encourage the use of null over false as an error or absence return value, for historical reasons many internal functions continue to use false instead. As shown in the statistics section, the vast majority of union return types for internal functions include false.

A classical example is the strpos() family of functions, which returns int|false.

While it would be possible to model this less accurately as int|bool, this gives the false impression that the function can also return a true value, which makes this type information significantly less useful to humans and static analyzers both.

For this reason, support for the false pseudo-type is included in this proposal. A true pseudo-type is not part of the proposal, because similar historical reasons for its necessity do not exist.

We believe there are now sufficient reasons to include support for the true type.

With the introduction of ValueError and many E_WARNING being promoted to Errors/Exceptions in PHP 8.0.0, a large amount of internal functions which used to return false on failure/error no longer do. And they now always return true. Ideally these function would be converted to void but doing so is a BC break. Moreover a void function always returns null which is a falsy value. Therefore the following code would now never hit the happy path:

if (function_which_could_fail_before()) {
    // Do useful stuff
} else {
   // Bailout and handle error
}

Marking these internal functions as having a return type of true could allow tools, such as static analysers, to mark part of this code unreachable.

Another motivation is providing more precise type information for humans and static analysers while satisfying LSP. For example a child method which always returns true instead of bool:

class User {
    function isAdmin(): bool
}
 
class Admin extends User
{
    function isAdmin(): true
    {
        return true;
    }
}

Finally, being able to add a true return type would help the optimizer and also prevent manually maintaining specialized information for the optimizer.

Proposal

Add support for using true as a type declaration, wherever type declarations are currently allowed. The true type does not allow coercions, exactly as how the false type currently behaves.

class Truthy {
    public true $truthy = true;
 
    public function foo(true $v): true { /* ... */ *}
}

Compile time redundancy checks

As it is the case currently for false a compile time error will be emitted if true and bool are present in union. For example, the following function signature:

<?php
function test(): bool|true {}
?>

Will emit the following error:

Fatal error: Duplicate type true is redundant in %s on line %d

Compile time error for using true|false instead of bool

A compile time error is also emitted when true|false (or equivalently false|true) is used instead of bool. The following function signature:

<?php
function test(): false|true {}
?>

will emit the following error:

Fatal error: Type contains both true and false, bool should be used instead in %s on line %d

The rationale is that the subtyping relation between false|true and bool needs to be defined. As bool coerces scalar values but false and true currently do not. It is possible to make true|false follow the same semantics as bool, however this might impact the implementation of literal types. Therefore the conservative approach of banning the usage of true|false is chosen.

Examples

There are many examples of functions which only return true being internal or in userland.

Some examples of internal functions which always return true are the array_sort() functions, many mysqli_*() functions, SplFixedArray::setSize(), etc.

An example of a userland project which can benefit of this change is Composer, which has various cases where they could use a true type. [1] [2] [3] [4]

Reflection

The new true type will behave as any other type in regards to Reflection.

Backward Incompatible Changes

This RFC does not contain any backwards incompatible changes.

Proposed PHP Version

Next minor version, i.e. PHP 8.2.

Proposed Voting Choices

As per the voting RFC a yes/no vote with a 2/3 majority is needed for this proposal to be accepted.

Voting started on 2022-03-12 and will end on 2022-03-26.

Accept Add true type RFC?
Real name Yes No
Final result: 0 0
This poll has been closed.

Future Scope

The features discussed in the following are not part of this proposal.

Literal Types

The true (and previously false) pseudo-type introduced in this RFC is a special case of a “literal type”, such as supported by TypeScript. They allow specifying enum-like types, which are limited to specific values.

type ArrayFilterFlags = 0|ARRAY_FILTER_USE_KEY|ARRAY_FILTER_USE_BOTH;
array_filter(array $array, callable $callback, ArrayFilterFlags $flag): array;

A benefit of using a union of literal types instead of an enum, is that it works directly with values of the underlying type, rather than an opaque enum value. As such, it is easier to retrofit without breaking backwards-compatibility.

However, the opinion of the authors is that enums are superior to literal types and the reason to add true is due to the extremely low implementation complexity, as the PHP engine already treats true and false as separate types. And that only having false and not true is strange.

Allowing usage of true|false

To allow the usage of false|true the subtyping relation between false|true and bool will need to be defined.

Type Aliases

As types become increasingly complex, it may be worthwhile to allow reusing type declarations. There are two general ways in which this could work. One is a local alias, such as:

use int|float as number;
 
function foo(number $x) {}

In this case number is a symbol that is only visible locally and will be resolved to the original int|float type during compilation.

The second possibility is an exported typedef:

namespace Foo;
type number = int|float;
 
// Usable as \Foo\number from elsewhere

Implementation

GitHub pull request: https://github.com/php/php-src/pull/8326

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

References

rfc/true-type.txt · Last modified: 2022/04/20 16:26 by girgias