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Attributes

Attributes are annotations that attach metadata to types, fields, and methods. IonPath supports both built-in and custom attributes.

Declaring Custom Attributes

Declare a custom attribute with the attribute keyword:

attribute @deprecated(version: string, reason: string);
attribute @description(text: string);
attribute @maxLength(n: i4);

Attribute arguments can only be scalar types — string, i4, bool, etc. Using a non-scalar type triggers ION0004.

Applying Attributes

Apply attributes with the @ prefix before a declaration:

@deprecated("2.0", "Use UserV2 instead")
msg User {
    @description("The unique user identifier")
    id: u4;
    name: string;
}

@deprecated("3.0", "Use NewService")
service OldService() {
    DoThing(): void;
}

Built-in Attributes

These attributes are provided by the standard library and used internally by the compiler:

Attribute Description
@builtin Marks a type as part of the standard library. Internal use only.
@scalar Marks a type as scalar (no indirection). Used for primitives.
@tag(n) CBOR tag annotation — wraps the encoded value with a CBOR tag number.
@deadline(ms) Request timeout in milliseconds. Applied to service methods.
@deprecated Marks a type, field, or method as deprecated.
@internal Marks a declaration as internal/hidden.
@bits(n) Specifies the bit count for numeric types.
@union Automatically applied to union types.
@unionCase Automatically applied to union case types.

Validation

  • ION0004 — Using a non-scalar type in attribute arguments (e.g., a message type)
  • ION0005 — Referencing an attribute that is not declared or imported