Why Function Calls Are Not Allowed as Attribute Default Values in PHP?

Why PHP Attributes Don't Allow Functions

In PHP, attribute default values cannot include function calls within class declarations. This restriction has sparked inquiries into its reasoning.

Design Perspective

The PHP compiler's design plays a crucial role in this limitation. When parsing class variable declarations, the grammar expects static scalars or arrays of static scalars as valid values.

Although modifying the grammar to accommodate function calls within attributes might eliminate syntax errors, it would introduce a new issue during compilation. The compiler operates by generating an array of opcodes during class declaration. Non-static values, such as the result of a function call, would disrupt this process by changing the expected opcode sequence.

Structural Limitations

To rectify this, the compiler would require a new opcode array scoped solely to class variable declarations. However, determining the appropriate时机 to evaluate this scoped array presents another challenge:

• File Loading: The function might not be available at this point.
• Property Access: Delaying evaluation until property access would hinder object creation.
• Object Construction: Evaluating at construction is more sensible but still presents issues with cyclical references.

Implementation Challenges

While other dynamic languages have overcome these obstacles, doing so in PHP would require significant restructuring. The language designers have thus far deemed this a low priority, as it would require a considerable amount of effort without a clear benefit.

Conclusion

The restriction on function calls in PHP attribute default values is a consequence of the compiler's design and the practical difficulty of implementing this functionality within existing structures.

The above is the detailed content of Why Function Calls Are Not Allowed as Attribute Default Values in PHP?. For more information, please follow other related articles on the PHP Chinese website!