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455 lines
11 KiB
Markdown
455 lines
11 KiB
Markdown
# Templating
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Docblocks allow you to tell Psalm some simple information about how your code works. For example `@return int` in a function return type tells Psalm that a function should return an `int` and `@return MyContainer` tells Psalm that a function should return an instance of a user-defined class `MyContainer`. In either case, Psalm can check that the function actually returns those types _and_ that anything calling that function uses its returned value properly.
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Templated types allow you to tell Psalm even more information about how your code works.
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Let's look at a simple class `MyContainer`:
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```php
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<?php
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class MyContainer {
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private $value;
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public function __construct($value) {
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$this->value = $value;
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}
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public function getValue() {
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return $this->value;
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}
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}
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```
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When Psalm handles the return type of `$my_container->getValue()` it doesn't know what it's getting out, because the value can be arbitrary.
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Templated annotations provide us with a workaround - we can define a generic/templated param `T` that is a placeholder for the value inside `MyContainer`:
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```php
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<?php
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/**
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* @template T
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*/
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class MyContainer {
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/** @var T */
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private $value;
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/** @param T $value */
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public function __construct($value) {
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$this->value = $value;
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}
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/** @return T */
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public function getValue() {
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return $this->value;
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}
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}
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```
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Now we can substitute values for that templated param when we reference `MyContainer` in docblocks e.g. `@return MyContainer<int>`. This tells Psalm to substitute `T` for `int` when evaluating that return type, effectively treating it as a class that looks like
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```php
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<?php
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class One_off_instance_of_MyContainer {
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/** @var int */
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private $value;
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/** @param int $value */
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public function __construct($value) {
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$this->value = $value;
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}
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/** @return int */
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public function getValue() {
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return $this->value;
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}
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}
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```
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This pattern can be used in a large number of different situations like mocking, collections, iterators and loading arbitrary objects. Psalm has a large number of annotations to make it easy to use templated types in your codebase.
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## `@template`, `@psalm-template`
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The `@template`/`@psalm-template` tag allows classes and functions to declare a generic type parameter.
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As a very simple example, this function returns whatever is passed in:
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```php
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<?php
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/**
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* @template T
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* @psalm-param T $t
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* @return T
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*/
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function mirror($t) {
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return $t;
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}
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$a = 5;
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$b = mirror($a); // Psalm knows the result is an int
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$c = "foo";
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$d = mirror($c); // Psalm knows the result is string
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```
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Psalm also uses `@template` annotations in its stubbed versions of PHP array functions e.g.
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```php
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<?php
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/**
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* Takes one array with keys and another with values and combines them
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*
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* @template TKey
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* @template TValue
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*
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* @param array<mixed, TKey> $arr
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* @param array<mixed, TValue> $arr2
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* @return array<TKey, TValue>
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*/
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function array_combine(array $arr, array $arr2) {}
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```
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### Notes
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- `@template` tag order matters for class docblocks, as they dictate the order in which those generic parameters are referenced in docblocks.
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- The names of your templated types (e.g. `TKey`, `TValue`) don't matter outside the scope of the class or function in which they're declared.
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## @param class-string<T>
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Psalm also allows you to parameterize class types
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```php
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<?php
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/**
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* @template T of Foo
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* @psalm-param class-string<T> $class
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* @return T
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*/
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function instantiator(string $class) {
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return new $class();
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}
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class Foo {
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public final function __construct() {}
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}
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class FooChild extends Foo {}
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$r = instantiator(FooChild::class);
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// Psalm knows $r is an object of type FooChild
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```
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## Template inheritance
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Psalm allows you to extend templated classes with `@extends`/`@template-extends`:
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```php
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<?php
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/**
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* @template T
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*/
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class ParentClass {}
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/**
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* @extends ParentClass<int>
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*/
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class ChildClass extends ParentClass {}
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```
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similarly you can implement interfaces with `@implements`/`@template-implements`
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```php
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<?php
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/**
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* @template T
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*/
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interface IFoo {}
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/**
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* @implements IFoo<int>
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*/
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class Foo implements IFoo {}
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```
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and import traits with `@use`/`@template-use`
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```php
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<?php
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/**
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* @template T
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*/
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trait MyTrait {}
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class Foo {
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/**
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* @use MyTrait<int>
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*/
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use MyTrait;
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}
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```
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You can also extend one templated class with another, e.g.
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```php
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<?php
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/**
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* @template T1
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*/
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class ParentClass {}
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/**
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* @template T2
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* @extends ParentClass<T2>
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*/
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class ChildClass extends ParentClass {}
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```
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## Template constraints
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You can use `@template of <type>` to restrict input. For example, to restrict to a given class you can use
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```php
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<?php
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class Foo {}
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class FooChild extends Foo {}
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/**
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* @template T of Foo
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* @psalm-param T $t
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* @return array<int, T>
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*/
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function makeArray($t) {
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return [$t];
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}
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$a = makeArray(new Foo()); // typed as array<int, Foo>
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$b = makeArray(new FooChild()); // typed as array<int, FooChild>
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$c = makeArray(new stdClass()); // type error
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```
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Templated types aren't limited to key-value pairs, and you can re-use templates across multiple arguments of a template-supporting type:
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```php
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<?php
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/**
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* @template T0 of array-key
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*
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* @template-implements IteratorAggregate<T0, int>
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*/
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abstract class Foo implements IteratorAggregate {
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/**
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* @var int
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*/
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protected $rand_min;
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/**
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* @var int
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*/
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protected $rand_max;
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public function __construct(int $rand_min, int $rand_max) {
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$this->rand_min = $rand_min;
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$this->rand_max = $rand_max;
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}
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/**
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* @return Generator<T0, int, mixed, T0>
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*/
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public function getIterator() : Generator {
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$j = random_int($this->rand_min, $this->rand_max);
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for($i = $this->rand_min; $i <= $j; $i += 1) {
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yield $this->getFuzzyType($i) => $i ** $i;
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}
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return $this->getFuzzyType($j);
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}
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/**
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* @return T0
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*/
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abstract protected function getFuzzyType(int $i);
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}
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/**
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* @template-extends Foo<int>
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*/
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class Bar extends Foo {
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protected function getFuzzyType(int $i) : int {
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return $i;
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}
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}
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/**
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* @template-extends Foo<string>
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*/
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class Baz extends Foo {
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protected function getFuzzyType(int $i) : string {
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return static::class . '[' . $i . ']';
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}
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}
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```
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## Template covariance
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Imagine you have code like this:
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```php
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<?php
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class Animal {}
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class Dog extends Animal {}
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class Cat extends Animal {}
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/**
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* @template T
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*/
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class Collection {
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/**
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* @var array<int, T>
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*/
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public array $list;
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/**
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* @param array<int, T> $list
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*/
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public function __construct(array $list) {
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$this->list = $list;
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}
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/**
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* @param T $t
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*/
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public function add($t) : void {
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$this->list[] = $t;
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}
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}
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/**
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* @param Collection<Animal> $collection
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*/
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function addAnimal(Collection $collection) : void {
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$collection->add(new Cat());
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}
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/**
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* @param Collection<Dog> $dog_collection
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*/
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function takesDogList(Collection $dog_collection) : void {
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addAnimal($dog_collection);
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}
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```
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That last call `addAnimal($dog_collection)` breaks the type of the collection – suddenly a collection of dogs becomes a collection of dogs _or_ cats. That is bad.
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To prevent this, Psalm emits an error when calling `addAnimal($dog_collection)` saying "addAnimal expects a `Collection<Animal>`, but `Collection<Dog>` was passed". If you haven't encountered this rule before it's probably confusing to you – any function that accepted an `Animal` would be happy to accept a subtype thereof. But as we see in the example above, doing so can lead to problems.
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But there are also times where it's perfectly safe to pass template param subtypes:
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```php
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<?php
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abstract class Animal {
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abstract public function getNoise() : string;
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}
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class Dog extends Animal {
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public function getNoise() : string { return "woof"; }
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}
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class Cat extends Animal {
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public function getNoise() : string { return "miaow"; }
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}
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/**
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* @template T
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*/
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class Collection {
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/** @var array<int, T> */
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public array $list = [];
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}
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/**
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* @param Collection<Animal> $collection
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*/
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function getNoises(Collection $collection) : void {
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foreach ($collection->list as $animal) {
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echo $animal->getNoise();
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}
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}
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/**
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* @param Collection<Dog> $dog_collection
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*/
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function takesDogList(Collection $dog_collection) : void {
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getNoises($dog_collection);
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}
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```
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Here we're not doing anything bad – we're just iterating over an array of objects. But Psalm still gives that same basic error – "getNoises expects a `Collection<Animal>`, but `Collection<Dog>` was passed".
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We can tell Psalm that it's safe to pass subtypes for the templated param `T` by using the annotation `@template-covariant T` (or `@psalm-template-covariant T`):
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```php
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<?php
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/**
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* @template-covariant T
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*/
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class Collection {
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/** @var array<int, T> */
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public array $list = [];
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}
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```
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Doing this for the above example produces no errors: [https://psalm.dev/r/5254af7a8b](https://psalm.dev/r/5254af7a8b)
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But `@template-covariant` doesn't get rid of _all_ errors – if you add it to the first example, you get a new error – [https://psalm.dev/r/0fcd699231](https://psalm.dev/r/0fcd699231) – complaining that you're attempting to use a covariant template parameter for function input. That’s no good, as it means you're likely altering the collection somehow (which is, again, a violation).
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### But what about immutability?
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Psalm has [comprehensive support for declaring functional immutability](https://psalm.dev/articles/immutability-and-beyond).
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If we make sure that the class is immutable, we can declare a class with an `add` method that still takes a covariant param as input, but which does not modify the collection at all, instead returning a new one:
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```php
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<?php
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/**
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* @template-covariant T
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* @psalm-immutable
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*/
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class Collection {
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/**
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* @var array<int, T>
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*/
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public array $list = [];
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/**
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* @param array<int, T> $list
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*/
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public function __construct(array $list) {
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$this->list = $list;
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}
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/**
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* @param T $t
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* @return Collection<T>
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*/
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public function add($t) : Collection {
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return new Collection(array_merge($this->list, [$t]));
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}
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}
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```
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This is perfectly valid, and Psalm won't complain.
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## Builtin templated classes and interfaces
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Psalm has support for a number of builtin classes and interfaces that you can extend/implement in your own code.
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- `interface Traversable<TKey, TValue>`
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- `interface ArrayAccess<TKey, TValue>`
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- `interface IteratorAggregate<TKey, TValue> extends Traversable<TKey, TValue>`
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- `interface Iterator<TKey, TValue> extends Traversable<TKey, TValue>`
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- `interface SeekableIterator<TKey, TValue> extends Iterator<TKey, TValue>`
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- `class Generator<TKey, TValue, TSend, TReturn> extends Traversable<TKey, TValue>`
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- `class ArrayObject<TKey, TValue> implements IteratorAggregate<TKey, TValue>, ArrayAccess<TKey, TValue>`
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- `class ArrayIterator<TKey of array-key, TValue> implements SeekableIterator<TKey, TValue>, ArrayAccess<TKey, TValue>`
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- `class DOMNodeList<TNode of DOMNode> implements Traversable<int, TNode>`
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- `class SplDoublyLinkedList<TValue> implements Iterator<TKey, TValue>, ArrayAccess<TKey, TValue>`
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- `class SplQueue<TValue> extends SplDoublyLinkedList<TValue>`
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- `abstract class FilterIterator<TKey, TValue, TIterator>`
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