from typing import Generic, TypeVar, Optional, Callable

T = TypeVar("T")


class Thunk(Generic[T]):
    value: Optional[T]

    def __init__(self, func: Callable[[], T]):
        self.func = func
        self.value = None

    def get(self) -> T:
        if self.value is None:
            self.value = self.func()
        return self.value


class Node:
    value: Thunk[int]


class Add(Node):
    def __init__(self, left: Node, right: Node):
        self.value = Thunk(lambda: left.value.get() + right.value.get())


class Mul(Node):
    def __init__(self, left: Node, right: Node):
        self.value = Thunk(lambda: left.value.get() * right.value.get())


class Lit(Node):
    def __init__(self, num: int):
        self.num = num
        self.value = Thunk(lambda: num)


if __name__ == "__main__":
    tree = Add(Mul(Lit(3), Lit(4)), Lit(5))
    print(tree.value.get())