haskell: Phase 13 — Eq default verification (+5 tests, class-defaults.sx 5/5)

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

lib/haskell/tests/class-defaults.sx

@@ -0,0 +1,38 @@
+;; class-defaults.sx — Phase 13: class default method implementations.
+
+;; ── Eq default: myNeq derived from myEq via `not (myEq x y)` ──
+(define
+  hk-myeq-source
+  "class MyEq a where\n  myEq :: a -> a -> Bool\n  myNeq :: a -> a -> Bool\n  myNeq x y = not (myEq x y)\ninstance MyEq Int where\n  myEq x y = x == y\n")
+
+(hk-test
+  "Eq default: myNeq 3 5 = True (no explicit myNeq in instance)"
+  (hk-deep-force (hk-run (str hk-myeq-source "main = myNeq 3 5\n")))
+  (list "True"))
+
+(hk-test
+  "Eq default: myNeq 3 3 = False"
+  (hk-deep-force (hk-run (str hk-myeq-source "main = myNeq 3 3\n")))
+  (list "False"))
+
+(hk-test
+  "Eq default: myEq still works in same instance"
+  (hk-deep-force (hk-run (str hk-myeq-source "main = myEq 7 7\n")))
+  (list "True"))
+
+;; ── Override path: instance can still provide the method explicitly. ──
+(hk-test
+  "Default override: instance-provided beats class default"
+  (hk-deep-force
+    (hk-run
+      "class Hi a where\n  greet :: a -> String\n  greet x = \"default\"\ninstance Hi Bool where\n  greet x = \"override\"\nmain = greet True"))
+  "override")
+
+(hk-test
+  "Default fallback: empty instance picks default"
+  (hk-deep-force
+    (hk-run
+      "class Hi a where\n  greet :: a -> String\n  greet x = \"default\"\ninstance Hi Bool where\nmain = greet True"))
+  "default")
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}