class ReadOnlyError(Exception):
    """Raised when a write operation is attempted on a read-only store."""

    pass


class ConfigMismatchError(Exception):
    """Raised when stored config doesn't match current config."""

    pass


class MigrationRequiredError(Exception):
    """Database requires migration. Run 'haiku-rag migrate' to upgrade."""

    pass


class AmbiguousDatabaseError(Exception):
    """An operation that works on one database was asked of a configured set.

    Raised by the CLI for a command that cannot tell which database to use, and
    by the client for a method that has no meaning across several.
    """


class UnknownDatabaseError(KeyError):
    """A name that does not belong to the databases in question.

    A `KeyError`, since selecting by name is a lookup, and one type wherever a
    name is checked: at construction, per query, and when placing evidence.
    """

    def __str__(self) -> str:
        # KeyError quotes its argument, which reads as a missing dict key rather
        # than the sentence this carries.
        return str(self.args[0]) if self.args else ""


class AmbiguousCitationError(Exception):
    """A cited chunk id names a chunk in more than one database.

    A citation records the id alone, so nothing downstream can say which
    database it came from.
    """


class SourceUnavailableError(Exception):
    """A configured database could not be opened.

    Carries the configured name and never the location: a path or URI in an
    error message travels into logs and into whatever a consumer renders, and
    the point of naming databases is that locations stay in the configuration.
    """