haskell: Algorithm W type inference + 32 tests (434/434)

Full HM inference in lib/haskell/infer.sx: unification, substitution,
occurs check, instantiation, generalisation, let-polymorphism.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-on-sx.md

@@ -91,7 +91,7 @@ Key mappings:
 - [x] Target: 5/5 classic programs passing
 
 ### Phase 4 — Hindley-Milner inference
-- [ ] Algorithm W: unification + type schemes + generalisation + instantiation
+- [x] Algorithm W: unification + type schemes + generalisation + instantiation
 - [ ] Report type errors with meaningful positions
 - [ ] Reject untypeable programs that phase 3 was accepting
 - [ ] Type-sig checking: user writes `f :: Int -> Int`; verify
@@ -114,6 +114,18 @@ Key mappings:
 
 _Newest first._
 
+- **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,
+  generalisation (let-polymorphism). Algorithm W covers literals, var, con, lambda,
+  multi-param lambda, application, let (simple bind + fun-clause), if, binary ops
+  (desugared to double application), tuples, and list literals. Initial type
+  environment provides monomorphic arithmetic/comparison/boolean ops plus
+  polymorphic list functions (`head`/`tail`/`null`/`length`/`reverse`/`:`).
+  `hk-infer-type` is the public entry point. test.sh updated to load infer.sx.
+  32 new tests in `lib/haskell/tests/infer.sx` cover all node types + let-
+  polymorphism. 434/434 green.
+
 - **2026-04-25** — `conformance.sh` runner + `scoreboard.json` + `scoreboard.md`.
   Script runs each classic program's test suite, prints per-program pass/fail,
   and writes both files. `--check` mode skips writing for CI use.