haskell: class/instance declarations — parse + instance dict eval (+11 tests, 503/503)

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

plans/haskell-on-sx.md

@@ -99,7 +99,7 @@ Key mappings:
 - [x] Unit tests: inference for 50+ expressions
 
 ### Phase 5 — typeclasses (dictionary passing)
-- [ ] `class` / `instance` declarations
+- [x] `class` / `instance` declarations
 - [ ] Dictionary-passing elaborator: inserts dict args at call sites
 - [ ] Standard classes: `Eq`, `Ord`, `Show`, `Num`, `Functor`, `Monad`, `Applicative`
 - [ ] `deriving (Eq, Show)` for ADTs
@@ -114,6 +114,17 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-06** — Phase 5 class/instance declarations. Parser: `hk-parse-class`
+  and `hk-parse-instance` added to the parser closure; `hk-parse-decl` gains
+  arms for `"class"` and `"instance"` reserved words (tokenizer already marks
+  them reserved). `class Eq a where { ... }` → `("class-decl" name tvar decls)`;
+  `instance Eq Int where { ... }` → `("instance-decl" name inst-type decls)`.
+  Eval: `hk-type-ast-str` converts type AST to a string key. `hk-bind-decls!`
+  gains arms for `class-decl` (registers `__class__Name` marker) and
+  `instance-decl` (builds method dict, binds as `dictClassName_TypeStr` in env).
+  11 new tests in `tests/class.sx` covering AST shapes + runtime dict
+  construction. 503/503 green.
+
 - **2026-05-05** — Phase 4 inference unit tests (50+ expressions). Added 16 new
   `hk-t` expression tests to `tests/infer.sx`: nested application (`not(not True)`,
   `negate(negate 1)`), bool/mixed lambdas (`\\x->\\y->x&&y`, `\\x->x==1`),