Merge loops/haskell into architecture: Phase 17 — import decls, type annotations, typecheck 15/15

plans/haskell-completeness.md

@@ -316,11 +316,11 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 Real Haskell programs use these on every page; closing the gaps unblocks
 larger conformance programs and removes one-line workarounds in test sources.
 
-- [ ] Type annotations in expressions: `(x :: Int)`, `f (1 :: Int)`,
+- [x] Type annotations in expressions: `(x :: Int)`, `f (1 :: Int)`,
   `return (42 :: Int)`. Parser currently rejects `::` in `aexp` position;
   desugar should drop the annotation (we have no inference at this layer
   yet, so it's a parse-only pass-through).
-- [ ] `import` declarations anywhere at the start of a module — currently
+- [x] `import` declarations anywhere at the start of a module — currently
   only the very-top-of-file form is recognised. Real test programs that
   mix prelude code with `import qualified Data.IORef` need this.
 - [ ] Multi-line top-level `where` blocks (`where { ... }` with explicit
@@ -359,10 +359,100 @@ 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._
 
+**2026-05-10** — Phase 17 second box: `import` declarations anywhere among
+top-level decls. `hk-collect-module-body` previously ran a fixed
+import-loop at the start, then a separate decl-loop; merged into a single
+`hk-body-step` dispatcher that routes `import` to the imports list and
+everything else to `hk-parse-decl`. Each call site (initial step + post-
+semicolon loop) now uses the dispatcher. Imports collected mid-stream
+still feed into `hk-bind-decls!` correctly because the eval side reads
+them via the imports list, not by AST position. tests/parse-extras.sx
+12 → 17 covering very-top, mid-stream, post-main, two-imports-different-
+positions, and unqualified mid-file. Regression: eval 66/0, exceptions
+14/0, typecheck 15/0, records 14/0, ioref 13/0, map 26/0, set 17/0.
+
+**2026-05-08** — Phase 17 first box: expression type annotations `(x :: Int)`,
+`f (1 :: Int)`, `(\x -> x+1) :: Int -> Int`. Parser's `hk-parse-parens`
+gains a `::` arm after the first inner expression: consume `::`, parse a
+type via the existing `hk-parse-type`, expect `)`, emit `(:type-ann EXPR
+TYPE)`. Desugar drops the annotation — `:type-ann E _ → (hk-desugar E)` —
+since the existing eval path has no type-directed dispatch; Phase 20 will
+let inference consume the annotation. tests/parse-extras.sx 12/12; eval,
+exceptions, typecheck, records, ioref still clean.
+
+**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