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psalm/docs/annotating_code/adding_assertions.md

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# Adding assertions
Psalm has three docblock annotations that allow you to specify that a function verifies facts about variables and properties:
- `@psalm-assert` (used when throwing an exception)
- `@psalm-assert-if-true`/`@psalm-assert-if-false` (used when returning a `bool`)
A list of acceptable assertions [can be found here](assertion_syntax.md).
## Examples
If you have a class that verified its input is an array of strings, you can make that clear to Psalm:
```php
<?php
/** @psalm-assert string[] $arr */
function validateStringArray(array $arr) : void {
foreach ($arr as $s) {
if (!is_string($s)) {
throw new UnexpectedValueException('Invalid value ' . gettype($s));
}
}
}
```
This enables you to call the `validateStringArray` function on some data and have Psalm understand that the given data *must* be an array of strings:
```php
<?php
function takesString(string $s) : void {}
function takesInt(int $s) : void {}
function takesArray(array $arr) : void {
takesInt($arr[0]); // this is fine
validateStringArray($arr);
takesInt($arr[0]); // this is an error
foreach ($arr as $a) {
takesString($a); // this is fine
}
}
```
Similarly, `@psalm-assert-if-true` and `@psalm-assert-if-false` will filter input if the function/method returns `true` and `false` respectively:
```php
<?php
class A {
public function isValid() : bool {
return (bool) rand(0, 1);
}
}
class B extends A {
public function bar() : void {}
}
/**
* @psalm-assert-if-true B $a
*/
function isValidB(A $a) : bool {
return $a instanceof B && $a->isValid();
}
/**
* @psalm-assert-if-false B $a
*/
function isInvalidB(A $a) : bool {
return $a instanceof B || !$a->isValid();
}
function takesA(A $a) : void {
if (isValidB($a)) {
$a->bar();
}
if (isInvalidB($a)) {
// do something
} else {
$a->bar();
}
$a->bar(); //error
}
```
As well as getting Psalm to understand that the given data must be a certain type, you can also show that a variable must be not null:
```php
<?php
/**
* @psalm-assert !null $value
*/
function assertNotNull($value): void {
// Some check that will mean the method will only complete if $value is not null.
}
```
And you can check on null values:
```php
<?php
/**
* @psalm-assert-if-true null $value
*/
function isNull($value): bool {
return ($value === null);
}
```