haskell: typecheck.sx 10/15→15/15 + plan Phases 20-22 (HM gaps, classes, integration)

Five "typed ok: …" tests in tests/typecheck.sx compared an unforced thunk
against an integer/list. The untyped-path convention is hk-deep-force on
the result; hk-run-typed follows the same shape but the tests omitted
that wrap. Added hk-deep-force around hk-run-typed in those five tests.
typecheck.sx now 15/15; infer.sx still 75/75.

Plan adds three phases capturing the remaining type-system work:
- Phase 20: Algorithm W gaps (case, do, record accessors, expression
  annotations).
- Phase 21: type classes with qualified types ([Eq a] => …) and
  constraint propagation, integrated with the existing dict-passing
  evaluator.
- Phase 22: typecheck-then-run as the default conformance path, with a
  ≥ 30/36 typechecking threshold before swap.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/haskell/tests/typecheck.sx

@@ -16,15 +16,18 @@
       true)))
 
 ;; ─── Valid programs pass through ─────────────────────────────────────────────
-(hk-test "typed ok: simple arithmetic" (hk-run-typed "main = 1 + 2") 3)
+(hk-test "typed ok: simple arithmetic"
+  (hk-deep-force (hk-run-typed "main = 1 + 2")) 3)
 
-(hk-test "typed ok: boolean" (hk-run-typed "main = True") (list "True"))
+(hk-test "typed ok: boolean"
+  (hk-deep-force (hk-run-typed "main = True")) (list "True"))
 
-(hk-test "typed ok: let binding" (hk-run-typed "main = let x = 1 in x + 2") 3)
+(hk-test "typed ok: let binding"
+  (hk-deep-force (hk-run-typed "main = let x = 1 in x + 2")) 3)
 
 (hk-test
   "typed ok: two independent fns"
-  (hk-run-typed "f x = x + 1\nmain = f 5")
+  (hk-deep-force (hk-run-typed "f x = x + 1\nmain = f 5"))
   6)
 
 ;; ─── Untypeable programs are rejected ────────────────────────────────────────
@@ -76,7 +79,7 @@
 
 (hk-test
   "run-typed sig ok: Int declared matches"
-  (hk-run-typed "main :: Int\nmain = 1 + 2")
+  (hk-deep-force (hk-run-typed "main :: Int\nmain = 1 + 2"))
   3)
 
 {:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

plans/haskell-completeness.md

@@ -359,10 +359,79 @@ that to single-digit minutes.
 - [ ] Verify the scoreboard output is byte-identical to the old per-process
   driver, then keep the per-process path as `--isolated` for debugging.
 
+### Phase 20 — Close Algorithm W gaps
+
+`lib/haskell/infer.sx` already implements core HM (TVar/TCon/TArr/TApp/TTuple/
+TScheme, substitution, occurs-check unification, instantiate/generalize, let-
+polymorphism). 75 inference unit tests + 15 typecheck integration tests pass.
+The remaining gaps that block typing real programs:
+
+- [ ] `case` expressions in `hk-w`. Needs to infer scrutinee type, then for
+  each `(:alt pat body)` infer the pattern's binding env (extending
+  `hk-w-pat`) and unify body types across alts.
+- [ ] `do` notation: extend `hk-type-env0` with `return :: a -> IO a`,
+  `(>>=) :: IO a -> (a -> IO b) -> IO b`, `(>>) :: IO a -> IO b -> IO b`,
+  and primitive IO actions (`putStrLn :: String -> IO ()`,
+  `getLine :: IO String`, etc.). May need a `TApp (TCon "IO") a` shape.
+- [ ] Record-accessor desugaring leaves `__rec_field` placeholder visible to
+  inference. Either skip generated accessor clauses during `hk-infer-prog`
+  or rewrite the desugar to produce a typed shape.
+- [ ] Type annotations in expressions `(x :: Int)` (parser also needed; see
+  Phase 17). Infer should unify the inferred type with the annotation.
+- [ ] Tests in `lib/haskell/tests/infer-extras.sx` (≥ 10) covering the
+  above shapes.
+
+### Phase 21 — Type classes (Eq, Ord, Num, Show)
+
+The evaluator already implements typeclass dispatch via dict-passing
+(`__default__ClassName_method` + per-instance dicts). The type system
+ignores `class` and `instance` decls. Closing this means HM with
+constraints (qualified types `[ClassName var] => type`).
+
+- [ ] Extend the type representation: `(TQual CONSTRAINTS TYPE)` where
+  `CONSTRAINTS = [(class-name . type-arg), …]`.
+- [ ] Generalize → `forall vars. preds => type`; instantiate → fresh-rename
+  vars in both preds and type.
+- [ ] During inference, when a primitive operator that needs a class is
+  used (e.g. `+`), emit a constraint `(Num t)`; collect constraints in
+  the substitution-threading.
+- [ ] At let-generalization, simplify constraints (defaulting for `Num`
+  literals → `Int`; entailment via known instances).
+- [ ] `class` declarations register members with their qualified type;
+  `instance` declarations register a witness.
+- [ ] At top-level, if any unsolvable constraint remains → type error
+  ("No instance for X").
+- [ ] Tests in `lib/haskell/tests/typeclasses.sx` (≥ 12 covering Eq, Ord,
+  Num overloading, show on instances, instance ambiguity rejection).
+
+### Phase 22 — Typecheck-then-run as the default
+
+- [ ] Replace `hk-run` with a typecheck-first variant in the conformance
+  driver, or run conformance twice (once typed, once untyped) and report
+  both pass-rates in `scoreboard.md`.
+- [ ] Investigate which existing 36 programs are untypeable due to gaps
+  closed in Phase 20-21 vs genuinely dynamically-typed; aim for ≥ 30/36
+  programs typechecking before committing to the swap.
+- [ ] If swap is committed, retire `hk-run` callsites in tests in favour
+  of `hk-run-typed`; keep the untyped path available for parser/eval
+  development against in-progress features.
+
 ## Progress log
 
 _Newest first._
 
+**2026-05-08** — Plan extends with Phases 20-22 (HM type system). Discovered
+during planning that `lib/haskell/infer.sx` already lands core Algorithm W
+(75 inference unit tests pass; let-polymorphism, sig checking, error
+reporting via `hk-expr->brief`). Fixed five regressing tests in
+`lib/haskell/tests/typecheck.sx` that compared an unforced thunk against
+the expected value — added `hk-deep-force` around `hk-run-typed` to match
+the existing untyped-path convention. typecheck.sx now 15/15.
+Phase 20 captures the remaining Algorithm W gaps (case, do, record
+accessors, expression annotations); Phase 21 captures type classes with
+qualified types; Phase 22 captures the integration step (typecheck-then-run
+across conformance).
+
 **2026-05-08** — Phase 16 Exception handling complete (6 ops + module wiring +
 14 unit tests + 2 conformance programs). `hk-bind-exceptions!` in `eval.sx`
 registers `throwIO`, `throw`, `evaluate`, `catch`, `try`, `handle`, and