haskell: Phase 9 — undefined in prelude + lazy CAFs (+2 tests, 59/59)

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

lib/haskell/tests/eval.sx

@@ -247,6 +247,18 @@
       false))
   true)
 
+(hk-test
+  "undefined: raises hk-error with Prelude.undefined message"
+  (guard
+    (e (true (>= (index-of e "hk-error: Prelude.undefined") 0)))
+    (begin (hk-deep-force (hk-run "main = undefined")) false))
+  true)
+
+(hk-test
+  "undefined: lazy — only fires when forced"
+  (hk-deep-force (hk-run "main = if True then 42 else undefined"))
+  42)
+
 (hk-test
   "second arg never forced"
   (hk-eval-expr-source "(\\x y -> x) 1 (error \"never\")")
@@ -257,30 +269,30 @@
   (hk-eval-expr-source "(\\x y -> y) (error \"never\") 99")
   99)
 
+;; ── not / id built-ins ──
 (hk-test
   "constructor argument is lazy under wildcard pattern"
   (hk-eval-expr-source
     "case Just (error \"deeply\") of Just _ -> 7 ; Nothing -> 0")
   7)
-
 (hk-test
   "lazy: const drops its second argument"
   (hk-prog-val "const x y = x\nresult = const 5 (error \"boom\")" "result")
   5)
-
-;; ── not / id built-ins ──
 (hk-test
   "lazy: head ignores tail"
   (hk-prog-val
     "myHead (x:_) = x\nresult = myHead (1 : (error \"tail\") : [])"
     "result")
   1)
+
 (hk-test
   "lazy: Just on undefined evaluates only on force"
   (hk-prog-val
     "wrapped = Just (error \"oh no\")\nresult = case wrapped of Just _ -> True ; Nothing -> False"
     "result")
   (list "True"))
+
 (hk-test "not True" (hk-eval-expr-source "not True") (list "False"))
 
 (hk-test "not False" (hk-eval-expr-source "not False") (list "True"))