haskell: Phase 8 deriving Show — verify nested-paren behavior (+4 tests, 15/15)

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

lib/haskell/tests/deriving.sx

@@ -30,6 +30,31 @@
 
 ;; ─── Eq ──────────────────────────────────────────────────────────────────────
 
+(hk-test
+  "deriving Show: nested ADT wraps inner constructor in parens"
+  (hk-deep-force
+    (hk-run
+      "data Tree = Leaf | Node Int Tree Tree deriving (Show)\nmain = show (Node 1 Leaf (Node 2 Leaf Leaf))"))
+  "Node 1 Leaf (Node 2 Leaf Leaf)")
+
+(hk-test
+  "deriving Show: Maybe Maybe wraps inner Just"
+  (hk-deep-force (hk-run "main = show (Just (Just 3))"))
+  "Just (Just 3)")
+
+(hk-test
+  "deriving Show: negative argument wrapped in parens"
+  (hk-deep-force (hk-run "main = show (Just (negate 3))"))
+  "Just (-3)")
+
+(hk-test
+  "deriving Show: list element does not need parens"
+  (hk-deep-force
+    (hk-run "data Box = Box [Int] deriving (Show)\nmain = show (Box [1,2,3])"))
+  "Box [1,2,3]")
+
+;; ─── combined Eq + Show ───────────────────────────────────────────────────────
+
 (hk-test
   "deriving Eq: same constructor"
   (hk-deep-force
@@ -58,8 +83,6 @@
       "data Color = Red | Green | Blue deriving (Eq)\nmain = show (Red /= Blue)"))
   "True")
 
-;; ─── combined Eq + Show ───────────────────────────────────────────────────────
-
 (hk-test
   "deriving Eq Show: combined"
   (hk-deep-force

plans/haskell-completeness.md

@@ -103,9 +103,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   (needs Char tagging — currently Char = Int by representation, ambiguous in
   show); `\n`/`\t` escape inside Strings.
 - [x] `show` Prelude binding calls `hk-show-val`; `print x = putStrLn (show x)`.
-- [ ] `deriving Show` auto-generates proper show for record-style and
+- [x] `deriving Show` auto-generates proper show for record-style and
   multi-constructor ADTs. Nested application arguments wrapped in parens:
-  if `show arg` contains a space, emit `"(" ++ show arg ++ ")"`.
+  if `show arg` contains a space, emit `"(" ++ show arg ++ ")"`. _Records
+  deferred — Phase 14._
 - [ ] `showsPrec` / `showParen` stubs so hand-written Show instances compile.
 - [ ] `Read` class stub — just enough for `reads :: String -> [(a,String)]` to
   type-check; no real parser needed yet.
@@ -286,6 +287,16 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-06** — Phase 8 `deriving Show` nested constructor parens verified:
+- The Phase 8 audit's precedence-based `hk-show-prec` already does the right
+  thing for `deriving Show`: each constructor arg is shown at prec 11, so any
+  inner constructor with args (or any negative number) gets parenthesised, while
+  nullary constructors and lists/tuples (whose own bracketing is unambiguous)
+  do not. Multi-constructor ADTs (e.g. `Tree = Leaf | Node …`) handled.
+  Records deferred to Phase 14.
+- 4 new tests in `tests/deriving.sx` exercising nested ADT + Maybe-Maybe +
+  negative-arg + list-arg cases; suite is 15/15.
+
 **2026-05-06** — Phase 8 `print` is `putStrLn (show x)` in prelude:
 - Added `print x = putStrLn (show x)` to `hk-prelude-src` and removed the
   standalone `print` builtin. `print` now resolves through the Haskell-level