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

plans/haskell-completeness.md

@@ -128,7 +128,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
   the tag has to live in a string prefix rather than as the head of a list.
   Catchers use `(index-of e "hk-error: ")` to detect.
 - [x] `undefined :: a` = `error "Prelude.undefined"`.
-- [ ] Partial functions emit proper error messages: `head []` →
+- [x] Partial functions emit proper error messages: `head []` →
   `"Prelude.head: empty list"`, `tail []` → `"Prelude.tail: empty list"`,
   `fromJust Nothing` → `"Maybe.fromJust: Nothing"`.
 - [ ] Top-level `hk-run-io` catches `hk-error` tag and returns it as a tagged
@@ -293,6 +293,15 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 9 partial functions emit proper error messages:
+- Added empty-list catch clauses to `head`, `tail` in the prelude. Added
+  `fromJust`, `fromMaybe`, `isJust`, `isNothing` (the last three were missing).
+  `fromJust Nothing` raises `"Maybe.fromJust: Nothing"`. Multi-clause dispatch
+  tries the constructor pattern first, then falls through to the empty-list /
+  Nothing error clause.
+- 5 new tests in `tests/eval.sx`. Suite is 64/64. Verified no regressions in
+  match, stdlib, fib, quicksort, program-maybe.
+
 **2026-05-07** — Phase 9 `undefined = error "Prelude.undefined"` + lazy CAFs:
 - Added `undefined = error "Prelude.undefined"` to `hk-prelude-src`. Without
   any other change this raised at prelude-load time because `hk-bind-decls!`