amp/docs/structs.md

Structs

A struct is a generic computer science term for an object composed of public properties. The \Amp\Stuct trait is intended to make using public properties a little safer by throwing an \Error when undefined properties are attempted to be written or read.

PHP allows for dynamic creation of public properties. This can lead to insidious bugs created by typos related to writing to and reading from public properties. One common solution to this problem is to make all properties private and provide public setter and getter methods which control access to the underlying properties. However effective this solution may be, it requires that additional code be written and subsequently tested for the setter and getter methods.

Let's try some examples with anonymous classes to demonstrate the advantages of using the \Amp\Struct trait. Running the following code will not error; although, the typo will likely create some unexpected behavior:

$obj = new class {
    public $foo = null;
};

$obj->fooo = "bar";

If you were to access the $foo property of the $obj object after the above code, you might expect the value to be "bar" when it would actually be NULL.

When a class uses the \Amp\Struct trait, an \Error will be thrown when attempting to access a property not defined in the class definition. For example, the code below will throw an \Error with some context that attempts to help diagnose the issue.

$obj = new class {
    use Amp\Struct;

    public $foo = null;
};

$obj->fooo = "bar";

The message for the thrown \Error will be similar to: Uncaught Error: class@anonymous@php shell code0x10ee8005b property "fooo" does not exist ... did you mean "foo?"

Although, an \Error being thrown in your code may cause some havoc, it will not allow for unpredictable behavior caused by the use of properties which are not part of the class definition.