mirror of
https://github.com/logos-co/logos-cpp-sdk.git
synced 2026-08-31 17:51:07 +00:00
Dario Gabriel Lipicar
4d2bff1730
No generator in any language read the optional flag — it had never been
implemented. `?T` was a HARD REJECT on the cdylib backend ("module not
cdylib-eligible"), `std::optional<T>` in an impl header fell through to the
opaque `any` with no diagnostic, and a `? name: T` field was emitted as a
required `T`. Three real contracts in the workspace already declare optionals
and were silently getting one of those three answers.
`?T` is TWO-state: a value of T, or empty. Never three — "one LIDL type <-> one
type per language" leaves nowhere for a third state, because every target has
exactly one empty inhabitant.
ONE MEANING, TWO SPELLINGS. `? name: T` (the field flag) and `name: ?T` (the
type kind) are the same declaration. Backends no longer answer that themselves:
logos-lidl's fieldIsOptional/fieldValueType are re-exported from lidl_compat.h
and every site THIS COMMIT TOUCHES reads them, so the two spellings emit
byte-identical code on the cdylib and client backends. That
caught a live drift on the way in — lidlRecordCollidesWithBytesTag read `f.type`
and so refused `? _bytes: tstr` while letting `_bytes: ?tstr` straight through,
one declaration with two answers.
THE WIRE RULE DEPENDS ON THE SLOT. Absent and explicit null are the SAME state
on decode and DIFFERENT on encode:
- decode is liberal, by exactly one inhabitant: in an optional slot absent and
null both mean empty; in a required slot both stay errors. A present value
goes through the decoder a required T would get, so a wrong type still fails
at the same path — optional widens the domain, it does not switch checking
off. `?bstr` therefore keeps the LENIENT bytes decode a bare `bstr` gets,
rather than silently becoming stricter in the optional slot.
- encode has one canonical form: empty OMITS the key where the slot is NAMED
(a record field) and is spelled null where it is POSITIONAL (an argument, a
return, an event parameter — no key to omit, and arity must not change). Key
omission lives in the record emitter because a Codec only ever sees a value,
never the slot it sits in. A round trip therefore canonicalises.
- `?any` collapses onto `any`: nlohmann::json already carries null, so
std::optional<LogosMap> would give the slot two spellings of empty.
The dispatch gate now admits a missing trailing optional argument and
materialises it as null, exactly the way a missing record field already was. A
method with no optional parameter emits the byte-identical gate it always did.
Header-first: `std::optional<T>` <-> `?T`, composing with records and
containers. `std::optional<std::optional<T>>` has NO LIDL type (three C++ states
over a two-state wire), so it maps down to `?T` — which makes the author's own
declaration stop compiling against the generated codec, deliberately — and says
so at derivation time instead of leaving a conversion error in generated code.
The Qt/Lp consumer surface is NOT fixed and does not pretend to be. The wrappers
real modules get come from legacy/main.cpp, where the AST is flattened to a
single Qt type-name string per slot before optionality could be seen; Qt has no
optional metatype, so `?T` lands on QVariant — the right shape (an invalid
QVariant is Qt's empty inhabitant) with no type. The generator now prints a Note
naming every flattened slot so an affected build is never silent, and
docs/project.md records exactly what a Qt consumer will still do with an
optional field.
Verified by output equivalence, not by a green build: the generator was built
before and after and run over every .lidl in the workspace plus the impl-header
fixtures, in cdylib, consumer-qt, consumer-lp, client and header-first modes.
428 of 465 artefacts are byte-identical; all 37 that differ belong to one of the
four contracts that declare an optional (the 38th path is the manifest). The
harness's sensitivity is pinned by a negative control: qt vs lp output differs
in 45 files. The emitted codec was additionally compiled under -Wall -Wextra and
run against the rules above — omission, absent==null, required-still-rejects,
present-but-wrong-still-fails, and canonicalising round trip.
Tests: 199 pass, 0 fail (180 before, 19 new).
Requires logos-lidl's optionality accessors and logos-protocol's
Codec<std::optional<T>>.
NOT FIXED, AND IT IS THE PATH THAT MATTERS MOST. The legacy interface-wrapper
path is untouched, and it is the one every real module builds through
(buildPlugin.nix:145 -> logos-cpp-generator --general-only). There the two
spellings still diverge:
? maybe: tstr -> QString maybe{}; __m.value("maybe").toString()
maybe: ?tstr -> QVariant maybe{}; __m.value("maybe")
and --api-style lp diverges too, neither side being std::optional. So R3 holds
on the backends below and NOT on the Qt consumer a shipping module actually
gets. logos-chat-module -- the contract that prompted this work -- uses the
field-flag spelling, so it lands on the branch that silently defaults.
The cause is upstream of codegen: legacy/main.cpp's moduleRecordsToJson and
moduleMethodsToJson flatten every TypeExpr to a single Qt TYPE-NAME STRING, so
optionality (along with nesting, map key types and descriptions) is gone before
generator_lib.cpp sees it. Widening that interface is a larger change and is
deliberately not attempted here. The only R3 test on a Qt surface covers
lidl_gen_client.cpp, which is on no live build path.