Table of Contents

PHP RFC: Nullable Casting

Introduction

PHP supports expression casting to primitive type (like int) by using “( type ) expression”, but it currently doesn't allow to use a nullable type as introduced by PHP 7.1 (e.g. ?int).

Due to the lack of support for nullable casting, it is necessary to write additional code to preserve a possible null value through the type conversion. This feature would also bring more consistency and completeness to the existing language.

Motivating Example

In strict type-checking mode (<?php declare(strict_types=1);), given two functions like the following (disclaimer: dummy implementation): 

function getInt(): int
{
    return mt_rand();
}
 
function processString(string $s): void
{
    printf("process a string of %d byte(s)\n", strlen($s));
}

then the following call: 

processString(getInt());

will throw a TypeError (“Argument 1 passed to processString() must be of the type string, int given”), but here we can use a cast (int to string conversion is always safe): 

processString((string) getInt());

(which will print something like “process a string of 9 byte(s)”).

Now given two functions like the following (with nullable type declarations): 

function getIntOrNull(): ?int
{
    $r = mt_rand();
    return $r % 2 === 0 ? $r : null;
}
 
function processStringOrNull(?string $s): void
{
    if ($s === null) {
        printf("process null\n");
    } else {
        printf("process a string of %d byte(s)\n", strlen($s));
    }
}

then the following call: 

processStringOrNull(getIntOrNull());

will sometimes work (print “process null”) and sometimes throw a TypeError (“Argument 1 passed to processStringOrNull() must be of the type string or null, int given”), so we would want to use a “nullable cast”: 

processStringOrNull((?string) getIntOrNull());

but currently this syntax is not supported (“Parse error: syntax error, unexpected '?'”) and we must resort to something more verbose (and error-prone) like: 

processStringOrNull(($tmp = getIntOrNull()) === null ? null : (string) $tmp);
unset($tmp);

or writing custom casting functions.

(Note that in weak type-checking mode, there is never a TypeError, the ?int is automatically converted to ?string, correctly preserving a null value. But we can prefer strict typing, to catch unintended conversions.)

settype()

When the desired type is not known before runtime, we cannot use a cast operator, but we can use the settype() function, for example: 

function getIntOrNullAs(string $type)
{
    $x = getIntOrNull();
    settype($x, $type);
    return $x;
}

but currently $type cannot contain a nullable type like "?string" (“Warning: settype(): Invalid type”, $x not converted).

Proposal

The proposal is to add support of nullable types to the current casting feature. Basically, (int) is the “plain” int cast, and (?int) will be a nullable int cast. Generally speaking, what changes is the possibility to use a leading question mark symbol (?) before the type of a cast, turning it into a nullable cast.

The only difference of nullable casting compared to plain casting is that if the expression value is null, it will be kept as null instead of being forced to the destination plain type:

type: int bool float string array object
(type)null 0 false 0.0 "" [] {}
(?type)null null null null null null null

Notes:

If the expression value is not null, nullable casting will give the same result as plain casting: e.g. (?int)false will give 0, (?array)"" will give [""].

Additional proposal for settype()

Additionally, it was requested on the mailing list to consider adding support of nullable types to the settype() function, e.g. settype($variable, "?int"), which here would be the same as $variable = (?int)$variable; and return true (but in general "?int" could be a dynamic string).

In short, for a currently valid $type argument to settype($variable, $type), it would enable to use '?'.$type to preserve nullability.

"?null"

In "?null", the “?” is redundant (“nullable null”), but it could happen in dynamic code, e.g. settype($x, '?' . gettype($y)) when $y is null.

Possible options:

  1. Allow it as equivalent to plain "null" silently.
  2. Allow it as equivalent to plain "null" but emit a specific Notice.
  3. Disallow it and emit a specific Warning (like the existing “Cannot convert to resource type”).
  4. Disallow it and emit the existing generic Warning “Invalid type”.

For demonstration, the current patch uses option 2.

Backward Incompatible Changes

None.

Proposed PHP Version(s)

Next PHP 7.x (7.4 now).

RFC Impact

To SAPIs

:?: Help needed

To Existing Extensions

:?: Help needed

To Opcache

:?: Help needed

Unaffected PHP Functionality

Proposed Voting Choices

(Each child vote result will be considered only if its parent vote passes.)

(The voting period would be two weeks)

Patches and Tests

Discussion

"Not 100% needed"

Current alternatives:

"A cast where you can't be sure of what you'll get back"

“I understand the use-case for when you want to pass something to a nullable parameter, but if you think about this cast in isolation, it hardly makes sense.”

"What about e.g. nullable_intval()?"

But we're missing “arrayval()” and “objectval()”... And we might use short closure fn($x) => (?int)$x

Fallible Casting

One might expect to also have e.g. (?int)"foo" and (?int)"" give null rather than 0, (?string)[42] give null rather than "Array"... and to be able to use (?int)$value ?? $default, (?string)$_GET["input"] ?? ""...

Alternative syntax

E.g. “(null|int) $x”

References