haskell: type error reporting — hk-expr->brief + hk-infer-decl/prog (+21 tests, 455/455)

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

plans/haskell-on-sx.md

@@ -92,7 +92,7 @@ Key mappings:
 
 ### Phase 4 — Hindley-Milner inference
 - [x] Algorithm W: unification + type schemes + generalisation + instantiation
-- [ ] Report type errors with meaningful positions
+- [x] Report type errors with meaningful positions
 - [ ] Reject untypeable programs that phase 3 was accepting
 - [ ] Type-sig checking: user writes `f :: Int -> Int`; verify
 - [ ] Let-polymorphism
@@ -114,6 +114,17 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-05** — Phase 4 type error reporting. `hk-expr->brief` converts any AST
+  node to a short human-readable string for error messages (handles var/con/int/float/
+  str/char/bool/app/op/if/let/lambda/tuple/list/loc). `loc` nodes in `hk-w` delegate
+  to inner expr (position is for outer context). `hk-infer-decl` wraps per-declaration
+  inference in a `guard`, returning `("ok" name type)` or `("err" "in 'name': msg")`
+  tagged results — avoids re-raise infinite loop in SX guard semantics.
+  `hk-infer-prog` runs all declarations and accumulates tagged results. test.sh
+  timeouts raised 120s→240s to accommodate eval.sx (Prelude init ~9s × 20 tests).
+  21 new tests covering brief serializer, error message substrings, loc pass-through,
+  decl inference, and prog-level inference. 455/455 green.
+
 - **2026-05-05** — Phase 4 Algorithm W (`lib/haskell/infer.sx`). Full
   Hindley-Milner inference: type constructors (TVar/TCon/TArr/TApp/TTuple/TScheme),
   substitution (apply/compose/restrict), occurs-check unification, instantiation,