plans: tick nqueens.hs, progress log 2026-04-25

plans/haskell-on-sx.md

@@ -85,7 +85,7 @@ Key mappings:
   - [x] `fib.hs` — infinite Fibonacci stream
   - [x] `sieve.hs` — lazy sieve of Eratosthenes
   - [x] `quicksort.hs` — naive QS
-  - [ ] `nqueens.hs`
+  - [x] `nqueens.hs`
   - [ ] `calculator.hs` — parser combinator style expression evaluator
 - [ ] `lib/haskell/conformance.sh` + runner; `scoreboard.json` + `scoreboard.md`
 - [ ] Target: 5/5 classic programs passing
@@ -114,6 +114,16 @@ Key mappings:
 
 _Newest first._
 
+- **2026-04-25** — Classic program `nqueens.hs`: backtracking n-queens via list
+  comprehension and multi-clause `where`. Three fixes needed: (1) `hk-eval-let`
+  now delegates to `hk-bind-decls!` so multi-clause `where`/`let` bindings
+  (e.g., `go 0 = [[]]; go k = [...]`) are grouped as multifuns; (2) added
+  `concatMap`, `concat`, `abs`, `negate` to `hk-prelude-src` (list comprehensions
+  desugar to `concatMap`); (3) cached the Prelude env in `hk-env0` so
+  `hk-eval-expr-source` copies it instead of re-parsing. Tests: `queens 4 = 2`,
+  `queens 5 = 10`. n=8 (92 solutions) is too slow at ~50s/n — omitted.
+  397/397 green.
+
 - **2026-04-25** — Classic program `quicksort.hs`: naive functional quicksort.
   `qsort (x:xs) = qsort smaller ++ [x] ++ qsort larger where smaller = filter (< x) xs; larger = filter (>= x) xs`.
   No new runtime additions needed — right sections, `filter`, `++` all worked out of the box.