RUNTIME VALUE CONVERSIONS

Mutable classes and methods

Convert basic classes with explicitly declared fields, a constructor, and ordinary instance methods. Runtime value conversion preserves shared instance identity.

Declare fields and methods

class Counter:
    name: str
    value: int

    def __init__(self, name: str, value: int) -> None:
        self.name = name
        self.value = value

    def add(self, amount: int) -> int:
        self.value += amount
        return self.value

def advance(counter: Counter, amount: int) -> Counter:
    counter.add(amount)
    return counter

def main() -> int:
    counter = Counter("jobs", 2)
    same = advance(counter, 3)
    add_more = same.add
    print(add_more(1), counter.value, counter is same)
    return 0

The program prints 6 6 True. The function parameter, result, and local aliases refer to the same instance. The stored bound method retains its receiver.

Field declarations and writes

Declare each field in the class body as name: type, without a class-level initializer. Assign instance values through self.name in __init__, and initialize every field before it is read. Fields can hold supported scalar, union, list, tuple, object, or declared-class values.

Field assignment checks the declared type. An assignment such as counter.value = "three" fails the integer field check. Reading an uninitialized field, creating an undeclared attribute, or accessing an unknown method reports a runtime error.

A container field checks its current elements when the field is assigned. Mutating the container itself is separate from replacing the field; container annotations do not permanently constrain writes through aliases.

Calls and bound methods

Use self as the first positional method parameter. Remaining parameters support defaults, keyword calls, positional-only markers, and keyword-only markers under the function binding rules. Methods can read or update fields, call other supported functions, and return admitted values. Store or pass an ordinary bound method just like a function value.

Bound methods compare equal when they refer to the same instance and the same underlying method. Different receivers or methods remain distinct. Instances can be passed to functions, returned, and stored in supported containers.

Class forms and limits

• Use one or more declared fields, with at most 64 fields and 64 method definitions.
• Construct objects by calling the class name with the arguments accepted by __init__, including keywords and defaults.
• Use ordinary methods and __init__; properties, static/class methods, operator protocols, and other special methods are unsupported.
• Inheritance, decorators, metaclasses, dynamic attributes, and class-level executable assignments are unsupported.
• Objects and their bound methods remain valid until their owning C context is destroyed.

For immutable records with direct typed conversion, use Simple immutable records.