atdj: add <json repr="object"> for sum types (externally-tagged encoding)#491
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mjambon merged 1 commit intoahrefs:masterfrom Apr 11, 2026
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Sum types annotated with <json repr="object"> now encode tagged variants
as single-key JSON objects {"Constructor": payload} instead of the default
two-element array ["Constructor", payload].
Unit variants (no payload) are always encoded as plain strings regardless
of the repr annotation, consistent with all other backends.
This matches the Rust/Serde default externally-tagged encoding and also
maps naturally to YAML as a single-key mapping:
# array encoding (default):
- - Circle
- 3.14
# object encoding (<json repr="object">):
- Circle: 3.14
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].Util.extractTag()to also handleJSONObjectvalues (in addition toStringandJSONArray).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:
Decoding uses
((JSONObject)o).getDouble("Circle")(key-based access) instead of((JSONArray)o).getDouble(1)(index-based). Encoding wraps the payload with{\"Circle\":…}instead of[\"Circle\",…].Test plan
dune runtest atdj/test— codegen diff test passes (Java runtime tests require javac, not available in CI)