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Tue, 4 Aug 2020 05:33:29 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 805071132FD2; Tue, 4 Aug 2020 07:33:28 +0200 (CEST) From: Markus Armbruster To: John Snow Subject: Re: qapi-schema esotera References: <5bfa3895-304d-8372-c0db-fda4c1a1ba59@redhat.com> <64792de9-6719-3987-a66b-aed8cca61572@redhat.com> Date: Tue, 04 Aug 2020 07:33:28 +0200 In-Reply-To: <64792de9-6719-3987-a66b-aed8cca61572@redhat.com> (John Snow's message of "Mon, 3 Aug 2020 13:51:10 -0400") Message-ID: <875z9zgel3.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=205.139.110.120; envelope-from=armbru@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/04 01:28:23 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" John Snow writes: > On 8/3/20 1:25 PM, Eric Blake wrote: >> On 8/3/20 11:49 AM, John Snow wrote: >>> UNION is split into two primary forms: >>> >>> 1. Simple (No discriminator nor base) >>> 2. Flat (Discriminator and base) >>> >>> In expr.py, I notice that we modify the perceived type of the >>> 'type' expression based on the two union forms. >>> >>> 1a. Simple unions allow Array[T] >>> 1b. Flat unions disallow Array[T] >> >> Rather, branches in a simple unions are syntactic sugar for a >> wrapper struct that contains a single member 'data'; because of that >> extra nesting, the type of that single member is unconstrained.=C2=A0 In >> flat unionw, the type MUST be a QAPI struct, because its members >> will be used inline; as currently coded, this prevents the use of an >> intrinsic type ('int', 'str') or an array type. >> > > I meant syntactically here, to be clear. I'm looking at expr.py -- if > there are deeper constraints on the semantics of the information > provided, that happens later. > > Specifically, check_union's use of check_type() changes depending on > the form of the union. One allows a string, the other allows a List of > strings, provided the list is precisely one element long. > >> If you need to use an array type in a flat union, you can't do: >> >> { 'union' ... >> =C2=A0 'data': { 'foo': [ 'MyBranch' ] } } >> >> but you can provide a wrapper type yourself: >> >> { 'struct': 'MyBranch', 'data': { 'array': [ 'MyType' ] } } >> { 'union' ... >> =C2=A0 'data': { 'foo': 'MyBranch' } } >> >>> >>> =C2=A0From the docs: >>> >>> Syntax: >>> =C2=A0=C2=A0=C2=A0=C2=A0 UNION =3D { 'union': STRING, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 'data': BRANCHES, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 '*if': COND, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 '*features': FEATURES } >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | { 'union= ': STRING, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 'data': BRANCHES, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 'base': ( MEMBERS | STRING ), >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 'discriminator': STRING, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 '*if': COND, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0 '*features': FEATURES } >>> =C2=A0=C2=A0=C2=A0=C2=A0 BRANCHES =3D { BRANCH, ... } >>> =C2=A0=C2=A0=C2=A0=C2=A0 BRANCH =3D STRING : TYPE-REF >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 | ST= RING : { 'type': TYPE-REF, '*if': COND } >>> >>> Both arms use the same "BRANCHES" grammar production, which both >>> use TYPE-REF. >>> >>> =C2=A0=C2=A0=C2=A0=C2=A0 TYPE-REF =3D STRING | ARRAY-TYPE >>> =C2=A0=C2=A0=C2=A0=C2=A0 ARRAY-TYPE =3D [ STRING ] >>> >>> Implying that List[T] should be allowed for both productions. >>> Can I ask for a ruling from the judges? >> >> As you found, the docs are a bit misleading; the semantic constraint >> on flat union branches being a struct (because they will be inlined) >> prevents the use of type-refs that are valid in simple unions (where >> those simple types will be wrapped in an implicit struct).=C2=A0 A patch >> to improve the docs would be a reasonable idea. >> > > Yes. I was working on a YAML prototype and I am trying to follow the > existing parser as closely as possible. In some cases, this highlights > differences between the grammar as advertised and what the parser > actually does. Please report all such differences, so we can fix them. > If we are to keep the current state of things, splitting UNION into > two separate productions might be nice. It *is* two productions, joined with |. The work unions really, really need is: * Eliminate the simple union sugar. * Make flat unions less cumbersome to write. I'd like to fuse struct and union into a single object type, like introspect.json already does. The former is a matter of massaging the schema and simplifying code. The latter requires actual thought. No big deal, just takes time, and time is always in short supply.