haskell: Phase 14 — record update r { field = v } (parser + desugar + eval)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -254,9 +254,11 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 - [x] Record creation `Foo { bar = 1, baz = "x" }` parsed as
   `(:rec-create CON [(FNAME EXPR) …])`. Eval builds the same tagged list as
   positional construction (field order from the data decl).
-- [ ] Record update `r { field = v }` parsed as `(:rec-update EXPR [(FNAME EXPR)])`.
+- [x] Record update `r { field = v }` parsed as `(:rec-update EXPR [(FNAME EXPR)])`.
   Eval forces the record, replaces the relevant positional slot, returns a new
   tagged list. Field → index mapping stored in `hk-constructors` at registration.
+  _Field map lives in `hk-record-fields` (desugar.sx) for load-order reasons,
+  not `hk-constructors`._
 - [ ] Exhaustive record patterns: `Foo { bar = b }` in case binds `b`,
   wildcards remaining fields.
 - [ ] Tests in `lib/haskell/tests/records.sx` (≥ 12 tests: creation, accessor,
@@ -313,6 +315,20 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 14 record-update syntax `r { field = v }`:
+- Parser: `varid {` after a primary expression now triggers
+  `hk-parse-rec-update` returning `(:rec-update record-expr [(fname expr) …])`.
+  (Generalising to arbitrary base expressions is future work — `var` covers
+  the common case.)
+- Desugar: a `:rec-update` node passes through with both record-expr and
+  field-expr children desugared.
+- Eval: forces the record, walks its positional args alongside the field
+  list (from `hk-record-fields`) to find which slots are being overridden,
+  builds a fresh tagged-list value with new thunks for the changed fields
+  and the original args otherwise. Multi-field update works. Verified end-
+  to-end on `alice { age = 31 }` (only age changes; name preserved). No
+  regressions in eval / match / desugar suites.
+
 **2026-05-07** — Phase 14 record-creation syntax `Foo { f = e, … }`:
 - Parser: post-`conid` peek for `{` triggers `hk-parse-rec-create`, returning
   `(:rec-create cname [(fname expr) …])`.