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feat(codecs): add empty_or_null codec for nullable typed codes across Rust and Go - #382
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This adds a new field codec that decodes a required wire key holding a typed code whose absent value the provider sends as `""` or JSON null, per ADR 0033 (#368). Both spellings decode to `None`, while any other value is parsed with the type's `FromStr`, so whitespace-only or control-bearing input remains a decode error that never echoes the value. The key stays required, matching `required_option`, and `None` re-encodes as null through serde's default serialization. **New codec:** * Added the `empty_or_null` module in `crates/libfmp/src/codecs.rs`, applied via `deserialize_with = "crate::codecs::empty_or_null::deserialize"` (the exact path gen_go matches). * Added a test covering empty/null-to-`None` mapping, round-trip serialization, missing-field errors, and that decode errors never contain the offending value. **Documentation:** * Added ADR 0033 describing nullable response members. * Extended ADR 0001 to cover observed wire evidence where a typed code or date seen as `""` or null decodes to `None`. * Updated ADR 0030 to document the fifth Go field codec introduced by ADR 0033.
This pull request adds support for the `empty_or_null` typed-code codec in the Go model generator, so Rust fields using `crate::codecs::empty_or_null::deserialize` on an `Option` of a string-backed type map to a nil-able Go pointer. **Codec support:** * Added the `EmptyOrNullString` variant and its `EMPTY_OR_NULL_CODEC` path, mapping an `Option` of a string-backed scalar to a `*string` public type backed by a `jsontext.Value` shadow, and rejecting any other shape with an explanatory error. * Treated the key as required and decoded both `""` and null to nil through the raw shadow, per ADR 0033. **Testing:** * Added a test that decodes empty and null values to nil through the raw shadow, checks the rendered public and shadow tags, the required-key handling, and the generated unmarshal body, and asserts that non-string and bare shapes fail with an `empty_or_null` error.
Link ADR 0033 in the design index, covering nullable response members and how `""` or null on typed codes and dates decode to `None` / `nil` based on observed wire evidence.
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September 30, 2026 12:14
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Providers sometimes send nullable typed codes as an empty string or JSON null on a key that is otherwise required. To handle these consistently and safely, this change introduces a dedicated codec that maps both spellings to an absent value while keeping decode errors from leaking the offending input, as specified by ADR 0033.
What changed:
empty_or_nullfield codec in libfmp, decoding""and null toNone, parsing other values viaFromStr, keeping the key required, and re-encodingNoneas null.EmptyOrNullStringvariant that maps an optional string-backed scalar to a nil-able*stringbacked by ajsontext.Valueshadow, rejecting unsupported shapes with a clear error.