haskell: do-notation desugar + stub IO monad (return/>>=/>>) (+14 tests, 382/382)

plans/haskell-on-sx.md

@@ -80,7 +80,7 @@ Key mappings:
 - [x] Pattern match forces scrutinee before matching
 - [x] Infinite structures: `repeat x`, `iterate f x`, `[1..]`, Fibonacci stream (sieve deferred — needs lazy `++` and is exercised under `Classic programs`)
 - [x] `seq`, `deepseq` from Prelude
-- [ ] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
+- [x] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
 - [ ] Classic programs in `lib/haskell/tests/programs/`:
   - [ ] `fib.hs` — infinite Fibonacci stream
   - [ ] `sieve.hs` — lazy sieve of Eratosthenes
@@ -114,6 +114,26 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-25** — Phase 3 do-notation + stub IO monad. Added a
+  `hk-desugar-do` pass that follows Haskell 98 §3.14 verbatim:
+  `do { e } = e`, `do { e ; ss } = e >> do { ss }`,
+  `do { p <- e ; ss } = e >>= \p -> do { ss }`, and
+  `do { let ds ; ss } = let ds in do { ss }`. The desugarer's
+  `:do` branch now invokes this pass directly so the surface
+  AST forms (`:do-expr`, `:do-bind`, `:do-let`) never reach the
+  evaluator. IO is represented as a tagged value
+  `("IO" payload)` — `return` (lazy builtin) wraps; `>>=` (lazy
+  builtin) forces the action, unwraps, and calls the bound
+  function on the payload; `>>` (lazy builtin) forces the
+  action and returns the second one. All three are non-strict
+  in their action arguments so deeply nested do-blocks don't
+  walk the whole chain at construction time. 14 new tests in
+  `lib/haskell/tests/do-io.sx` cover single-stmt do, single
+  and multi-bind, `>>` sequencing (last action wins), do-let
+  (single, multi, interleaved with bind), bind-to-`Just`,
+  bind-to-tuple, do inside a top-level fun, nested do, and
+  using `(>>=)`/`(>>)` directly as functions. 382/382 green.
+
 - **2026-04-25** — Phase 3 `seq` + `deepseq`. Built-ins were strict
   in all args by default (every collected thunk forced before
   invoking the underlying SX fn) — that defeats `seq`'s purpose,