haskell: Phase 9 — partial fns proper error messages (head []/tail []/fromJust Nothing) (+5 tests, 64/64)

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

lib/haskell/tests/eval.sx

@@ -259,6 +259,36 @@
   (hk-deep-force (hk-run "main = if True then 42 else undefined"))
   42)
 
+(hk-test
+  "head []: raises Prelude.head: empty list"
+  (guard
+    (e (true (>= (index-of e "Prelude.head: empty list") 0)))
+    (begin (hk-deep-force (hk-run "main = head []")) false))
+  true)
+
+(hk-test
+  "tail []: raises Prelude.tail: empty list"
+  (guard
+    (e (true (>= (index-of e "Prelude.tail: empty list") 0)))
+    (begin (hk-deep-force (hk-run "main = tail []")) false))
+  true)
+
+;; ── not / id built-ins ──
+(hk-test
+  "fromJust Nothing: raises Maybe.fromJust: Nothing"
+  (guard
+    (e (true (>= (index-of e "Maybe.fromJust: Nothing") 0)))
+    (begin (hk-deep-force (hk-run "main = fromJust Nothing")) false))
+  true)
+(hk-test
+  "fromJust (Just 5) = 5"
+  (hk-deep-force (hk-run "main = fromJust (Just 5)"))
+  5)
+(hk-test
+  "head [42] = 42 (still works for non-empty)"
+  (hk-deep-force (hk-run "main = head [42]"))
+  42)
+
 (hk-test
   "second arg never forced"
   (hk-eval-expr-source "(\\x y -> x) 1 (error \"never\")")
@@ -269,16 +299,17 @@
   (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)
+
 (hk-test
   "lazy: head ignores tail"
   (hk-prog-val