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exit-as-function.md

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PHP RFC: Transform exit() from a language construct into a standard function

Introduction

The exit language construct (and its alias die) can be used on its own as a "constant" ((We are using this terminology as it can be used in any place where an expression is expected, like a constant: https://3v4l.org/sL9Q5)) to terminate a PHP script with status code 0, or it can be used like a "function" which accepts an optional argument $status that can either be an integer, in which case the PHP script will be terminated with the given integer as the status code, or it can be a string, in which case the PHP script is terminated with status code 0 and the string is printed to STDOUT.

However, because exit() is not a proper function it cannot be called with a named argument, passed to functions as a callable, does not respect the strict_types declare, and most confusingly, it does not follow the usual type juggling semantics.

Indeed, any value which is not an integer, is cast to a string. This means passing an array or a resource to exit() will not throw a TypeError but print Array or Resource id #%d respectively with the relevant warning being emitted. However, it does throw a TypeError for non-Stringable objects.

Furthermore, arguments of type bool are cast to string instead of int, violating the standard type juggling semantics for a string|int union type. This is something that we find especially confusing for CLI scripts which may have a boolean $has_error variable passed to exit() with the assumption that false will be coerced to 0 and true coerced to 1.

Finally, the need for exit() to be a language construct with its own dedicated opcode is not a requirement anymore since PHP 8.0, as (in order to unwind the stack normally) the opcode throws a special kind of exception which cannot be caught, nor executes finally blocks. ((php/php-src#5768))

Proposal

We propose to make exit() a proper function with the following signature:

function exit(string|int $status = 0): never {}

Secondly, die() will be made an alias of exit(), and "constant" usages of exit/die transformed to function calls at compile time.

It will continue to be impossible to declare exit or die functions in namespaces, or disable/remove them via the disable_functions INI directive.

Moreover, as exit and die will no longer be keywords, it will be possible to use them as goto labels.

Another slight improvement is that the AST printing (seen via assert()) now reflects what has actually been written in the source code rather than always using exit.

Backward Incompatible Changes

The impact of this RFC is deemed to be low.

The T_EXIT token will be removed because exit will no longer need to be parsed specially by the lexer. As most PHP libraries that deal on an AST level use Nikita Popov's php-parser which creates its own AST, this should have minimal impact on userland tooling.

Projects that directly use the tokenizer extensions, like Exakat, will need some straight-forward adaptation.

And the behaviour of values of different types passed to exit() will be altered to match the usual type juggling semantics:

Argument passed Current behaviour New behaviour Consequences
int int int No change, interpreted as exit code
string string string No change, interpreted as status message
bool string int Was status message, now exit code
float string int Was status message, now exit code, with a possible "Implicit conversion from float to int loses precision" deprecation notice
null string int Was status message, now exit code, with `"Passing null to parameter #1 ($status) of type string
stringable object string string No change, interpreted as status message
non-stringable object TypeError TypeError None
array string TypeError Was status message with warning, now TypeError
resource string TypeError Was status message with warning, now TypeError

Future scope

These are ideas for future proposals that are not part of this RFC:

  • Deprecate using exit as a "constant"
  • Execute finally blocks for exits
  • Allow disabling exit()/die() functions via the disable_functions INI directive, similarly to how it is possible to disable assert()

Version

Next minor version, PHP 8.4.

Vote

VOTING_SNIPPET

Notes