Attribute Binding

Introduction

The proposed attribute binding feature allows a simplified way to call functions of attributes on actual values.
This enables ergonomic, method-like APIs for attributes without duplicating call the receiver.

Motivation

Zirric attributes are not only used for metadata but to design contracts.
In practice this leads to unnecessarily complex calls of attribute functions.

Attribute(value).attrFunc(value, otherArgs)

Proposed Solution

Introduce a @Bound() attribute for functions declared inside attributes.
This allows the compiler to bind the target value automatically as the first argument
when calling the function through the attribute instance accessor.

attr Error {
  @Bound()
  @Returns(String)
  debug(err)
}

@Error({ err -> err.message })
data MessageError {
  @String message
}

MessageError("msg")[@Error].debug() // `err` is passed automatically

Detailed Design

• @Bound() can be applied to functions declared inside an attribute.
• A bound function is invoked through the attribute instance accessor
  (value[@Attribute].method()), and the target value is passed as the first
  argument.
• The function signature remains explicit; tooling can still infer the argument
  types from the attribute body.
• @Bound() can only be applied to functions with at least one argument.
• @Bound() can only be applied to functions inside attributes.

Changes to the Standard Library

• Add a @Bound attribute in prelude to mark bound functions.
• Bindings are opt-in and do not change existing attributes.

Alternatives Considered

• Manual receiver arguments in every attribute function, which is verbose.
• Special syntax for attribute methods, which would add new language surface
  area without clear benefits over an attribute-based marker.

Acknowledgements

• Inspired by method receivers in languages like Go and extension methods in C#.