haskell: Phase 9 — hk-test-error helper in testlib.sx (+2 tests, 66/66)

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

lib/haskell/testlib.sx

@@ -56,3 +56,21 @@
         (append!
           hk-test-fails
           {:actual actual :expected expected :name name})))))
+
+(define
+  hk-test-error
+  (fn
+    (name thunk expected-substring)
+    (let
+      ((caught (guard (e (true (if (string? e) e (str e)))) (begin (thunk) nil))))
+      (cond
+        ((nil? caught)
+          (do
+            (set! hk-test-fail (+ hk-test-fail 1))
+            (append! hk-test-fails {:actual "no error raised" :expected (str "error containing: " expected-substring) :name name})))
+        ((>= (index-of caught expected-substring) 0)
+          (set! hk-test-pass (+ hk-test-pass 1)))
+        (:else
+          (do
+            (set! hk-test-fail (+ hk-test-fail 1))
+            (append! hk-test-fails {:actual caught :expected (str "error containing: " expected-substring) :name name})))))))

lib/haskell/tests/eval.sx

@@ -289,6 +289,16 @@
   (hk-deep-force (hk-run "main = head [42]"))
   42)
 
+(hk-test-error
+  "hk-test-error helper: catches matching error"
+  (fn () (hk-deep-force (hk-run "main = error \"boom\"")))
+  "hk-error: boom")
+
+(hk-test-error
+  "hk-test-error helper: catches head [] error"
+  (fn () (hk-deep-force (hk-run "main = head []")))
+  "Prelude.head: empty list")
+
 (hk-test
   "second arg never forced"
   (hk-eval-expr-source "(\\x y -> x) 1 (error \"never\")")