Psalm supports PHP 7.4

Thursday, November 28th 2019 is a big day for PHP: version 7.4 will be released to the world.

There are five major new language enhancements coming in PHP 7.4:

Typed properties

Arrow functions

Return type covariance (and param type contravariance)

Null coalescing assignment

Spread operator in arrays

Psalm is ready for them.

Typed properties

Psalm has best-in-class support for verifying docblock-provided property types, and that verification is even more important with PHP 7.4’s new explicit property types.

Psalm provides a number of property checks that other packages do not:

Prevent property access before initialisation

In PHP 7.4, accessing a property with an explicit type causes a fatal error in PHP if that property has not yet been defined. Psalm detects those errors for you:

<?php class A { private string $s; public function __construct() { echo $this->s; // causes fatal error $this->s = "hello"; } }

Warn about properties not initialised within constructors

Psalm will also warn you about properties that weren’t assigned a value inside the constructor:

<?php class A { private string $string_set_in_method; public int $unitialised_int; public function __construct() { // Psalm understands that $this->string_set_in_method // is set here $this->setString(); } private function setString() : void { $this->string_set_in_method = "hello"; } } echo (new A)->unitialised_int; // causes fatal error

Arrow functions (aka short closures)

Arrow functions are good for a few reasons, but perhaps their main benefit is that they save space. You may be used to passing closures to array_map like this:

<?php /** * @param array<int> $ints * @return array<string> */ function formatInts(array $ints) : array { return array_map( function(int $i) : string { return number_format($i, 3); }, $ints ); }

With arrow functions the same closure can be expressed on a single line:

<?php /** * @param array<int> $ints * @return array<string> */ function formatInts(array $ints) : array { return array_map( fn($i) => number_format($i, 3), $ints ); }

Note that we’ve dropped the param type and return type declarations for that closure – they’re optional, and Psalm can infer them in most situations.

Psalm also detects when the given array_map return type doesn’t match up with the expected one:

<?php /** * @param array<int> $ints * @return array<object> */ function formatInts(array $ints) : array { return array_map( fn($i) => number_format($i, 3), $ints ); }

Return type covariance & param type contravariance

In PHP 7.3 this causes a fatal error:

class A { public function getInstance(): self { return new self(); } } class AChild extends A { public function getInstance(): self { return new self(); } }

In PHP 7.4 the above behaviour is now allowed.

Psalm detects which version of PHP you’re running, and will allow this behaviour if it detects PHP 7.4. Users of PHP 7.3 and below will still see an error reported.

Null coalescing assignment

PHP now supports the ??= null coalescing assignment operator.

$foo ??= $bar is equivalent to $foo = $foo ?? $bar , and Psalm will treat it as such.

Psalm will also warn you when the null coalescing assignment is unnecessary:

<?php function takesString(string $s) : void { $s ??= "hello"; echo $s; }

Spread operator in arrays

Psalm can understand the new array spread syntax.

<?php $arrayA = [1, 2, 3]; $arrayB = [4, 5]; $result = [0, ...$arrayA, ...$arrayB, 6 ,7]; echo $result[3]; echo $result[7]; echo $result[9]; // error

Enjoy PHP 7.4!

PHP 7.4 is a massive step forward for the language, and Psalm allows you to use its new features in a type-safe manner.

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