JSON library — parser + serializer (with a roundtrip test app)

• Type: feature
• Status: done — implemented + green on pinned v33 (commit 13c74d2)
• Owner: —
• Opened: 2026-06-19
• Relation: one of the demo-eligible-as-library items from idea-demo-app-candidates (was survivor #2). Reusable RTL unit whose acceptance test is the demo. Sibling to feature-hashing-library / feature-compression-library / feature-bignum-library / feature-sat-solver-library. Per [[project_own_rtl_from_scratch]]: write our OWN unit with FPC-ish naming, do NOT port FPC fpjson.

Goal

A JSON unit: parse a UTF-8 JSON string into a value tree, navigate/mutate it, serialize back (compact + pretty). FPC-flavored surface so existing code style carries, but our own elegant implementation.

Surface (sketch — finalize when picked up)

• value tree: object / array / string / number / bool / null (a tagged union or small class hierarchy — exercises variant or class/VMT lane)
• Parse(s): TJSONValue (raises on malformed — exceptions lane)
• typed access: AsString / AsInteger / AsBoolean, Items[i], Values[key]
• ToString(pretty: Boolean) serializer

Coverage

managed strings (heavy) · dynamic arrays (array nodes) · collections/hashing (object members) · recursion (nested parse/emit) · variant or class hierarchy · exceptions (parse errors). Hand-written JSON is normal practice, so this does not undermine the "use a library" story.

Acceptance / oracle

• Roundtrip identity: Parse(s) -> ToString -> Parse yields an equal tree; canonical re-emit is byte-stable.
• A fixed sample-document set parses + re-emits byte-identical across all targets.
• Demo: examples/json/ round-trips a bundled set, prints canonical form.

Constraints

Own .pas unit (not .inc); FPC-ish naming; no fpjson port; must not regress self-host / cross-bootstrap.

Log

• 2026-06-19 — Opened from the demo/library organization pass.
• 2026-06-22 — Implemented (track B), commit 3050ec5: lib/rtl/json.pas (TJSONValue tree + TJSONReader recursive-descent parser + canonical compact/ pretty serializer; numbers kept as raw lexeme for byte-stable re-emit; EJSONError on malformed input; \uXXXX -> UTF-8). Oracle examples/json/jsondemo.pas covers roundtrip identity, canonical stability, escapes/unicode, typed access, and parse-error exceptions; ends ALL OK. Wired into make lib-test + make demos. Verified green against a freshly self-hosted compiler (HEAD). Blocked: the pinned stable is v32, which predates obj.Free (562eb95) and bare Copy (dd706ff) that the lib needs. Re-pinning past v32 is currently unsafe because of bug-impl-prescan-codegen-regression (silent miscompile in unit impl sections, introduced by 7ba91bf). Once that bug is fixed and the stable re-pinned, make lib-test goes green and this closes. Code is committed but NOT pushed (gate red until then).
• 2026-06-22 (later) — DONE. v33 was re-pinned (includes obj.Free + bare Copy), and examples/json/jsondemo.pas runs ALL OK on pinned v33. The json lib-test lines pass; the make lib-test umbrella is still red, but only because of the unrelated png regression (bug-impl-prescan-codegen-regression), not json. Closing json on its own merits.