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>

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!`