atdcpp: add <json repr="object"> for sum types (externally-tagged encoding)#489
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mjambon merged 1 commit intoahrefs:masterfrom Apr 11, 2026
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Tagged variants are encoded as single-key JSON objects
{"Constructor": payload} instead of the default two-element array
["Constructor", payload]. This matches the default Rust/Serde
externally-tagged encoding. It also reads naturally in YAML as a
single-key mapping, which is one motivation for the feature.
Unit variants (no payload) are always encoded as plain strings
regardless of the repr annotation.
The RapidJSON writer uses StartObject/Key/EndObject instead of
StartArray/String/EndArray. The reader checks x.IsObject() and
extracts the sole member name via MemberBegin()->name.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
mjambon
requested review from
Khady,
cyberhuman,
rgrinberg and
zindel
as code owners
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Summary
<json repr="object">now encode/decode tagged variants as single-key JSON objects{"Constructor": payload}instead of the default two-element array["Constructor", payload].shapetype to the test suite withCircle,Square, andPointvariants.Motivation
This encoding matches the Rust/Serde default externally-tagged representation and was already implemented for OCaml in atdml (commit 0fc67a5). It also maps naturally to YAML as a single-key mapping:
Uses RapidJSON's
StartObject/Key/EndObjectAPI for writing, andIsObject()/MemberBegin()->namefor reading — symmetric with the existingStartArray/String/EndArraypath.Test plan
dune runtest atdcpp/test— codegen diff test passes (C++ runtime tests require g++ and RapidJSON, not available in CI)