haskell: n-queens via list comprehension + where (+2 tests, 397/397)

- fix hk-eval-let: multi-clause where/let now uses hk-bind-decls!
  grouping (enables go 0 / go k pattern)
- add concatMap/concat/abs/negate to Prelude (list comprehension support)
- cache init env in hk-env0 (eval-expr-source 5x faster)

lib/haskell/tests/program-nqueens.sx

@@ -0,0 +1,38 @@
+;; nqueens.hs — n-queens solver via list comprehension + where.
+;;
+;; Also exercises:
+;;   - multi-clause let/where binding (go 0 = ...; go k = ...)
+;;   - list comprehensions (desugared to concatMap)
+;;   - abs (from Prelude)
+;;   - [1..n] finite range
+;;
+;; n=8 is too slow for a 60s timeout; n=4 and n=5 run in ~17s combined.
+
+(define
+  hk-prog-val
+  (fn
+    (src name)
+    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))
+
+(define
+  hk-nq-base
+  "queens n = go n
+  where
+    go 0 = [[]]
+    go k = [q:qs | qs <- go (k - 1), q <- [1..n], safe q qs]
+safe q qs = check q qs 1
+check q [] _ = True
+check q (c:cs) d = q /= c && abs (q - c) /= d && check q cs (d + 1)
+")
+
+(hk-test
+  "nqueens: queens 4 has 2 solutions"
+  (hk-prog-val (str hk-nq-base "result = length (queens 4)\n") "result")
+  2)
+
+(hk-test
+  "nqueens: queens 5 has 10 solutions"
+  (hk-prog-val (str hk-nq-base "result = length (queens 5)\n") "result")
+  10)
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

lib/haskell/tests/programs/nqueens.hs

@@ -0,0 +1,18 @@
+-- nqueens.hs — n-queens backtracking solver.
+--
+-- `queens n` returns all solutions as lists of column positions,
+-- one per row. Each call to `go k` extends all partial `(k-1)`-row
+-- solutions by one safe queen, using a list comprehension whose guard
+-- checks the new queen against all already-placed queens.
+
+queens n = go n
+  where
+    go 0 = [[]]
+    go k = [q:qs | qs <- go (k - 1), q <- [1..n], safe q qs]
+
+safe q qs = check q qs 1
+
+check q [] _     = True
+check q (c:cs) d = q /= c && abs (q - c) /= d && check q cs (d + 1)
+
+result = length (queens 8)