import abc
from typing import Generic, TypeVar

__all__ = ('Nullable',)

NullableType = TypeVar('NullableType')


class Nullable(Generic[NullableType], abc.ABC):
    def null(self) -> bool:
        """must never change"""
        raise NotImplementedError

    def _resolve(self) -> NullableType:
        """if .null(), throw"""
        raise NotImplementedError

    def resolve(self) -> NullableType:
        assert not self.null()
        return self._resolve()