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Narrowing Types

Narrowing a type is usually needed when we have a union type like bool|int and we want to work only with one of the types in a branch. Other use case for narrowing is when we we want to execute code only for a specific subtype, for example when we’re interested only in InvalidArgumentException but we’ve got Exception in a variable.

PHPStan supports several ways of narrowing types.

Strict comparison operators #

In case of scalar values type can be narrowed using === and !== operators:

if ($stringOrBool === true) {
    // $stringOrBool is true here
}

These operators can either be used in conditions, or in an assert() call:

assert($stringOrBool === true);

// $intOrString is true after the assert() call

Type-checking functions #

• is_array()
• is_bool()
• is_callable()
• is_float() / is_double() / is_real()
• is_int() / is_integer() / is_long()
• is_numeric()
• is_iterable()
• is_null()
• is_object()
• is_resource()
• is_scalar()
• is_string()
• is_subclass_of()
• is_a

These functions can either be used in conditions, or in an assert() call.

instanceof operator #

// $exception is Exception
if ($exception instanceof \InvalidArgumentException) {
    // $exception is \InvalidArgumentException
}

The instanceof operator can either be used in conditions, or in an assert() call.

Custom type-checking functions and methods #

Narrowing types can also be performed by assertion libraries like webmozart/assert and beberlei/assert. You can install PHPStan extensions for these libraries so that PHPStan can understand how these custom function calls narrow down types.

Let’s consider a custom type-checking function like this:

public function foo(object $object): void
{
    $this->checkType($object);
    $object->doSomething(); // Call to an undefined method object::doSomething().
}

public function checkType(object $object): void
{
    if (!$object instanceof \BarService) {
        throw new WrongObjectTypeException();
    }
}

During the analysis of the foo() method, PHPStan doesn’t understand that the type of $object was narrowed to BarService because it doesn’t descend to called functions and symbols, it just reads their typehints and PHPDocs.

Recommended ways of solving this problem are:

1. Switching to inlined type check - having instanceof right in the foo() method.
2. Switching to an assertion library like webmozart/assert or beberlei/assert and using it directly in the foo() method.
3. Writing a custom type-specifying extension for the checkType() method.

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