Generics

IonPath supports parameterized types for reusable data structures. Built-in generics include Maybe<T>, Array<T>, and Partial<T>.

Syntax

Generic types are declared with angle-bracket type parameters:

msg Response<T> {
    data: T;
    timestamp: datetime;
    success: bool;
}

msg Page<T> {
    items: T[];
    total: u4;
    page: u4;
}

Instantiation

Use generic types by providing concrete type arguments:

msg User { id: u4; name: string; }

service UserService() {
    GetUser(id: u4): Response<User>;
    ListUsers(page: u4): Page<User>;
}

Built-in Generics

The standard library (std feature) provides these built-in generic types:

Type Description

Maybe<T> Optional wrapper. Shorthand: T?. C#: IonMaybe<T>

Array<T> Dynamic array. Shorthand: T[]. C#: IonArray<T>

Partial<T> Sparse update tracker. Shorthand: ~T. C#: IonPartial<T>

Maybe<T> (Optional)

Maybe<T> represents a value that may or may not be present. In C# it generates IonMaybe<T> — a Rust-style Option monad with implicit conversions:

// C# generated code
IonMaybe<string> email = "user@example.com";  // implicit wrap
IonMaybe<string> empty = default;               // no value

if (email.HasValue)
    Console.WriteLine(email.Value);             // "user@example.com"

Partial<T> (Sparse Update)

Partial<T> tracks which fields have been modified. Only modified fields are serialized on the wire — ideal for PATCH-like updates:

// C# usage
var patch = new IonPartial<User>();
patch.Set(u => u.name, "New Name");  // only 'name' will be sent on the wire