haskell: desugar guards/where/list-comp → core AST (+15 tests, 226/226)

plans/haskell-on-sx.md

@@ -68,7 +68,7 @@ Key mappings:
 - [x] Unit tests in `lib/haskell/tests/parse.sx` (43 tokenizer tests, all green)
 
 ### Phase 2 — desugar + eager-ish eval + ADTs (untyped)
-- [ ] Desugar: guards → nested `if`s; `where` → `let`; list comp → `concatMap`-based; do-notation stays for now (desugared in phase 3)
+- [x] Desugar: guards → nested `if`s; `where` → `let`; list comp → `concatMap`-based; do-notation stays for now (desugared in phase 3)
 - [ ] `data` declarations register constructors in runtime
 - [ ] Pattern match (tag-based, value-level): atoms, vars, wildcards, constructor patterns, `as` patterns, nested
 - [ ] Evaluator (still strict internally — laziness in phase 3): `let`, `lambda`, application, `case`, literals, constructors
@@ -114,6 +114,27 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 2 kicks off with `lib/haskell/desugar.sx` — a
+  tree-walking rewriter that eliminates the three surface-only forms
+  produced by the parser, leaving a smaller core AST for the evaluator:
+  - `:where BODY DECLS` → `:let DECLS BODY`
+  - `:guarded ((:guard C1 E1) (:guard C2 E2) …)` → right-folded
+    `(:if C1 E1 (:if C2 E2 … (:app (:var "error") (:string "…"))))`
+  - `:list-comp E QUALS` → Haskell 98 §3.11 translation:
+    empty quals → `(:list (E))`, `:q-guard` → `(:if … (:list (E)) (:list ()))`,
+    `:q-gen PAT SRC` → `(concatMap (\PAT -> …) SRC)`, `:q-let BINDS` →
+    `(:let BINDS …)`. Nested generators compile to nested concatMap.
+  Every other expression, decl, pattern, and type node is recursed
+  into and passed through unchanged. Public entries `hk-desugar`,
+  `hk-core` (tokenize → layout → parse → desugar on a module), and
+  `hk-core-expr` (the same for an expression). 15 new tests in
+  `lib/haskell/tests/desugar.sx` cover two- and three-way guards,
+  case-alt guards, single/multi-binding `where`, guards + `where`
+  combined, the four list-comprehension cases (single-gen, gen +
+  filter, gen + let, nested gens), and pass-through for literals,
+  lambdas, simple fun-clauses, `data` decls, and a module header
+  wrapping a guarded function. 226/226 green.
+
 - **2026-04-24** — Phase 1 parser is now complete. This iteration adds
   operator sections and list comprehensions, the two remaining
   aexp-level forms, plus ticks the “AST design” item (the keyword-