haskell: Phase 9 — tests/errors.sx (14/14, plan ≥10)

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

plans/haskell-completeness.md

@@ -136,7 +136,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 - [x] `hk-test-error` helper in `testlib.sx`:
   `(hk-test-error "desc" thunk expected-substring)` — asserts the thunk raises
   an `hk-error` whose message contains the given substring.
-- [ ] Tests in `lib/haskell/tests/errors.sx` (≥ 10 tests: error message
+- [x] Tests in `lib/haskell/tests/errors.sx` (≥ 10 tests: error message
   content, undefined, head/tail/fromJust on bad input, `hk-test-error` helper).
 - [ ] Conformance programs:
   - `partial.hs` — exercises `head []`, `tail []`, `fromJust Nothing` caught
@@ -293,6 +293,14 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 9 `tests/errors.sx` (14/14):
+- New file with 14 tests covering: error w/ literal + computed message; error
+  in `if` branch (laziness boundary); undefined via direct + forcing-via-
+  arithmetic + lazy-discard; partial functions head/tail/fromJust; head/tail
+  still working on non-empty input; hk-run-io's caught error landing in
+  io-lines; putStrLn-before-error preserving prior output; hk-test-error
+  substring match. Spec called for ≥10.
+
 **2026-05-07** — Phase 9 `hk-test-error` helper in testlib.sx:
 - New 0-arity-thunk-based assertion: `(hk-test-error name thunk substr)` —
   evaluates `(thunk)`, expects an exception, checks `index-of` for the given