haskell: guards + where clauses (+11 tests, 173/173)

plans/haskell-on-sx.md

@@ -61,7 +61,7 @@ Key mappings:
   - [x] `case … of` and `do`-notation expressions (plus minimal patterns needed for arms/binds: var, wildcard, literal, 0-arity and applied constructor, tuple, list)
   - [x] Patterns — full: `as` patterns, nested, negative literal, `~` lazy, infix constructor (`:` / consym), extend lambdas/let with non-var patterns
   - [x] Top-level decls: function clauses (simple — no guards/where yet), pattern bindings, multi-name type signatures, `data` with type vars and recursive constructors, `type` synonyms, `newtype`, fixity (`infix`/`infixl`/`infixr` with optional precedence, comma-separated ops, backtick names). Types: vars / constructors / application / `->` (right-assoc) / tuples / lists. `hk-parse-top` entry.
-  - [ ] `where` clauses + guards
+  - [x] `where` clauses + guards (on fun-clauses, case alts, and let/do-let bindings — with the let funclause shorthand `let f x = …` now supported)
   - [ ] Module header + imports (stub)
   - [ ] List comprehensions + operator sections
 - [ ] AST design modelled on GHC's HsSyn at a surface level
@@ -114,6 +114,28 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-24** — Phase 1: guards + where clauses. Factored a single
+  `hk-parse-rhs sep` that all body-producing sites now share: it reads
+  a plain `sep expr` body or a chain of `| cond sep expr` guards, then
+  — regardless of which form — looks for an optional `where` block and
+  wraps accordingly. AST additions:
+  - `:guarded GUARDS` where each GUARD is `:guard COND EXPR`
+  - `:where BODY DECLS` where BODY is a plain expr or a `:guarded`
+  Both can nest (guards inside where). `hk-parse-alt` now routes through
+  `hk-parse-rhs "->"`, `hk-parse-fun-clause` and `hk-parse-bind` through
+  `hk-parse-rhs "="`. `hk-parse-where-decls` reuses `hk-parse-decl` so
+  where-blocks accept any decl form (signatures, fixity, nested funs).
+  As a side effect, `hk-parse-bind` now also picks up the Haskell-native
+  `let f x = …` funclause shorthand: a varid followed by one or more
+  apats produces `(:fun-clause NAME APATS BODY)` instead of a
+  `(:bind (:p-var …) …)` — keeping the simple `let x = e` shape
+  unchanged for existing tests. 11 new tests in
+  `lib/haskell/tests/parser-guards-where.sx` cover two- and three-way
+  guards, mixed guarded + equality clauses, single- and multi-binding
+  where blocks, guards plus where, case-alt guards, case-alt where,
+  let with funclause shorthand, let with guards, and a where containing
+  a type signature alongside a fun-clause. 173/173 green.
+
 - **2026-04-24** — Phase 1: top-level decls. Refactored `hk-parse-expr` into a
   `hk-parser tokens mode` with `:expr` / `:module` dispatch so the big lexical
   state is shared (peek/advance/pat/expr helpers all reachable); added public