Re: [PATCH v4 6/6] qapi: remove JSON value FIXME

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> With the two major JSON-ish type hierarchies clarified for distinct
> purposes; QAPIExpression for parsed expressions and JSONValue for
> introspection data, remove this FIXME as no longer an action item.
>
> A third JSON-y data type, _ExprValue, is not meant to represent JSON in
> the abstract but rather only the possible legal return values from a
> single function, get_expr(). It isn't appropriate to attempt to merge it
> with either of the above two types.
>
> In theory, it may be possible to define a completely agnostic
> one-size-fits-all JSON type hierarchy that any other user could borrow -
> in practice, it's tough to wrangle the differences between invariant,
> covariant and contravariant types: input and output parameters demand
> different properties of such a structure.
>
> However, QAPIExpression serves to authoritatively type user input to the
> QAPI parser, while JSONValue serves to authoritatively type qapi
> generator *output* to be served back to client users at runtime via
> QMP. The AST for these two types are different and cannot be wholly
> merged into a unified syntax.
>
> They could, in theory, share some JSON primitive definitions. In
> practice, this is currently more trouble than it's worth with mypy's
> current expressive power. As such, declare this "done enough for now".
>
> Signed-off-by: John Snow <jsnow@redhat.com>

Reviewed-by: Markus Armbruster <armbru@redhat.com>