Request for Comments: Scalar Type Hinting With Casts

• Version: 0.1
• Date: 2012-07-03
• Author: Anthony Ferrara ircmaxell@php.net
• Status: Draft
• First Published at: http://wiki.php.net/rfc/scalar_type_hinting_with_casts

Currently, PHP has no way to provide type hinting for function parameters which are not classes or arrays. This is on often requested feature that has been discussed on the internals list many many times. This RFC discusses a new implementation of this feature that attempts to stay close to php's type shifting roots, and attempts to mirror zend_parse_parameters as much as possible.

This RFC discusses a method of adding scalar type hints to PHP while attempting to embrace the dynamic nature of PHP variables. This means that passing a type that does not exactly match the hinted type will cause a cast to happen. This cast will only succeed if the argument can be cleanly converted to the requested type. If it cannot be converted without significant data-loss, an E_RECOVERABLE_ERROR will be raised.

For consistency, this patch attempts to follow zend_parse_parameters() for the validation rules, except for disallowing lossy conversion from float to int (1.5 -> int generates an error).

This implementation does not change the parser at all. The parser still detects the type hints as object type hints. The compiler (zend_compile.c) will then detect the exact value for the type hint, and change the stored hint from IS_OBJECT to the proper type (freeing the string).

Therefore this patch does not introduce new reserved words.

One potential side-effect, is that with the current patch, function a(int $a) hints for an integer primitive, but function a(Int $a) hints for a class named Int. This can be resolved by forcing a strtolower on the hint prior to string comparison.

Five new type hints are introduced with this patch:

• int - Matching integers only
• float - Matching floating point numbers
• boolean - Matching boolean parameters only
• string - Matching strings only
• resource - Matching resources only

Conversion is allowed only if data-loss does not happen. There are a few exceptions (objects using __toString, strings containing leading numerics, etc). Here's a table of examples.

• fail indicates an E_RECOVERABLE_ERROR
• pass indicates no error and a conversion
• notice indicates an E_NOTICE and a conversion

It's important to note that passing `12.5` as a float or string to a int type hint will presently fail, since data-loss would occur (this diverges from zend_parse_parameters which would truncate the value).

If a provided hint does not match at all (“foo” passed to an int hint), an E_RECOVERABLE_ERROR is raised.

If a non-well-formed string is passed to an int hint (“12.5foo”), an E_NOTICE is raised on a non-well-formed numeric (the same as zend_parse_parameters).

Any value can be entered as a default. Presently even array() is allowable for an int type hint. The default is converted at run-time when it is accessed.

This can lead to odd bugs, so in the future it would be good to validate the default in zend_compile.c (casting it where appropriate, checking for a valid cast).

The current implementation treats references like any other value. If it casts, the referenced value is casted.

This current proposal adds a series of conversion functions to the core:

• int convert_to_{type}_safe(zval **ptr) - Convert the zval to {type}. Return value indicates if conversion was “clean”. (FAILURE indicates unclean conversion)
• int convert_to_{type}_safe_ex(zval **ptr) - Separate zval if not a reference, and convert to {type}. Return indicates clean conversion (FAILURE indicates unclean conversion).

These functions pairs exist for long, double, string, boolean. Resource doesn't need one, as no type other than resource can presently be converted safely to a resource.

For consistency, the following new methods have been added to ReflectionParameter

• isInt() - boolean to determine if parameter is type-hinted as an integer.
• isFloat() - boolean to determine if parameter is type-hinted as a float.
• isBoolean() - boolean to determine if parameter is type-hinted as a boolean.
• isString() - boolean to determine if parameter is type-hinted as a string.
• isResource() - boolean to determine if parameter is type-hinted as a resource.

The modifications necessary to implement this feature exist on the scalar_type_hints branch of ircmaxell's github fork. It is still a work-in-progress, and should be considered unstable at this time.

At present, the cast from float to int results in an error if the int doesn't exactly represent the float (satisfying a double cast: val = (double) (long) val). And a cast from an int to a float follows the same semantics (as on 64 bit platforms PHP_INT_MAX is not exactly representable by a float).

This could be relaxed for semi-representable values. So 1.5 could be allowed for an int parameter (casted to 1). But float(99999999999999999999) would not, because it would lose a lot of information in the transfer (would be casted to PHP_INT_MAX).

I believe the current behavior (error on non-exactly-representable) is the correct one. However, this could be changed to an E_NOTICE instead indicating that partial data was lost.

We could also change the E_RECOVERABLE_ERROR on data-loss to an E_WARNING. That would allow data-loss to continue. The value passed in would still be cast according to the normal casting rules. So passing “foo” to an int parameter would result in int(1) and an E_WARNING.

int_hint.php

<?php
function foo(int $a) {
    var_dump($a); 
}
foo(1); // int(1)
foo("1"); // int(1)
foo(1.0); // int(1)
foo("1a"); // Notice: A non well formed numeric value encountered int(1)
foo("a"); // E_RECOVERABLE_ERROR
foo(999999999999999999999999999999999999); // E_RECOVERABLE_ERROR (since it's not exactly representable by an int)
foo(1.5); // E_RECOVERABLE_ERROR
foo(array()); // E_RECOVERABLE_ERROR
foo(new StdClass); // E_RECOVERABLE_ERROR
?>

float_hint.php

<?php
function foo(float $a) {
    var_dump($a); 
}
foo(1); // float(1)
foo("1"); // float(1)
foo(1.0); // float(1)
foo("1a"); // Notice: A non well formed numeric value encountered float(1)
foo("a"); // E_RECOVERABLE_ERROR
foo(1.5); // float(1.5)
foo(array()); // E_RECOVERABLE_ERROR
foo(new StdClass); // E_RECOVERABLE_ERROR
?>

string_hint.php

<?php
function foo(string $a) {
    var_dump($a); 
}
foo(1); // string "1"
foo("1"); // string "1"
foo(1.0); // string "1"
foo("1a"); // string "1a"
foo("a"); // string "a"
foo(1.5); // string "1.5"
foo(array()); // E_RECOVERABLE_ERROR
foo(new StdClass); // E_RECOVERABLE_ERROR
?>

boolean_hint.php

<?php
function foo(boolean $a) {
    var_dump($a); 
}
foo(1); // bool(true)
foo("1"); // bool(true)
foo(1.0); // bool(true)
foo(0); // bool(false)
foo("0"); // bool(false)
foo("1a"); // bool(true)
foo("a"); // bool(true)
foo(1.5); // bool(true)
foo(array()); // E_RECOVERABLE_ERROR
foo(new StdClass); // E_RECOVERABLE_ERROR
?>

• Return value and parameter type hint by Felipe
• Strict and weak parameter type checking by Lukas and Zeev
• Optional Strict Type Checking for Scalars by Paul (Ilia's proposal)
• Parser and Reflection-API only Type Hints by Derick (Implemented)
• Parameter Type Casting Hints by Anthony

• 0.1 - Initial Draft