haskell: Phase 11 — Data.Map transforming (foldlWithKey/foldrWithKey/mapWithKey/filterWithKey)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -186,7 +186,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   `member`, `size`, `null`.
 - [x] Bulk operations: `fromList`, `toList`, `toAscList`, `keys`, `elems`.
 - [x] Combining: `unionWith`, `intersectionWith`, `difference`.
-- [ ] Transforming: `foldlWithKey`, `foldrWithKey`, `mapWithKey`, `filterWithKey`.
+- [x] Transforming: `foldlWithKey`, `foldrWithKey`, `mapWithKey`, `filterWithKey`.
 - [ ] Updating: `adjust`, `insertWith`, `insertWithKey`, `alter`.
 - [ ] Module wiring: `import Data.Map` and `import qualified Data.Map as Map`
   resolve to the `map.sx` namespace dict in the eval import handler.
@@ -304,6 +304,16 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 11 transforming (foldlWithKey/foldrWithKey/mapWithKey/filterWithKey):
+- Folds traverse in-order. `foldlWithKey f acc m` walks left → key/val → right
+  threading the accumulator, so left-folding `(\acc k v -> acc ++ k ++ v)` over
+  a 3-key map yields `"1a2b3c"`. `foldrWithKey` runs right → key/val → left so
+  the cons-style accumulator `(\k v acc -> k ++ v ++ acc)` produces the same
+  string.
+- `mapWithKey` rebuilds the tree node-by-node (no rebalancing needed — keys
+  unchanged so the existing structure stays valid). `filterWithKey` is a
+  `foldrWithKey` that re-inserts kept entries; rebalances via insert.
+
 **2026-05-07** — Phase 11 combining (unionWith/intersectionWith/difference):
 - All three implemented via `reduce` over the smaller map's `to-asc-list`,
   inserting / skipping into the result. Verified: