haskell: Phase 16 — exception handling (catch/try/throwIO/evaluate/handle/throw)

hk-bind-exceptions! in eval.sx registers throwIO, throw, evaluate, catch,
try, handle, displayException. SomeException constructor pre-registered
in runtime.sx (arity 1, type SomeException).

throwIO and the existing error primitive both raise via SX `raise` with a
uniform "hk-error: msg" string. catch/try/handle parse it back into a
SomeException via hk-exception-of, which strips nested
'Unhandled exception: "..."' host wraps (CEK's host_error formatter) and
the "hk-error: " prefix.

catch and handle evaluate the handler outside the guard scope (build an
"ok"/"exn" outcome tag inside guard, then dispatch outside) so that a
re-throw from the handler propagates past this catch — matching Haskell
semantics rather than infinite-looping in the same guard.

14 unit tests in tests/exceptions.sx (catch success, catch error, try
Right/Left, handle, throwIO + catch/try, evaluate, nested catch, do-bind
through catch, branch on try result, IORef-mutating handler).

Conformance: safediv.hs (8/8) and trycatch.hs (8/8). Scoreboard now
285/285 tests, 36/36 programs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/haskell/tests/program-trycatch.sx

@@ -0,0 +1,95 @@
+;; trycatch.hs — try pattern: branch on Left/Right (Phase 16 conformance).
+
+(define
+  hk-trycatch-source
+  "parseInt :: String -> IO Int
+parseInt \"zero\" = return 0
+parseInt \"one\"  = return 1
+parseInt \"two\"  = return 2
+parseInt s = throwIO (SomeException (\"unknown: \" ++ s))
+
+describe :: Either SomeException Int -> String
+describe (Right v) = \"got \" ++ show v
+describe (Left (SomeException m)) = \"err: \" ++ m
+
+trial :: String -> IO String
+trial s = do
+  r <- try (parseInt s)
+  return (describe r)
+
+run3 :: String -> String -> String -> IO [String]
+run3 a b c = do
+  ra <- trial a
+  rb <- trial b
+  rc <- trial c
+  return [ra, rb, rc]
+
+")
+
+(hk-test
+  "trycatch.hs — Right branch"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source "main = trial \"one\"")))
+  (list "IO" "got 1"))
+
+(hk-test
+  "trycatch.hs — Left branch with message"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source "main = trial \"banana\"")))
+  (list "IO" "err: unknown: banana"))
+
+(hk-test
+  "trycatch.hs — chain over three inputs, all good"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source "main = run3 \"zero\" \"one\" \"two\"")))
+  (list "IO"
+    (list ":" "got 0"
+      (list ":" "got 1"
+        (list ":" "got 2"
+          (list "[]"))))))
+
+(hk-test
+  "trycatch.hs — chain over three inputs, mixed"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source "main = run3 \"zero\" \"qux\" \"two\"")))
+  (list "IO"
+    (list ":" "got 0"
+      (list ":" "err: unknown: qux"
+        (list ":" "got 2"
+          (list "[]"))))))
+
+(hk-test
+  "trycatch.hs — Left from throwIO carries message"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source
+        "main = do { r <- try (throwIO (SomeException \"explicit\")); return (describe r) }")))
+  (list "IO" "err: explicit"))
+
+(hk-test
+  "trycatch.hs — Right preserves the int"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source
+        "main = do { r <- try (return 42); return (describe r) }")))
+  (list "IO" "got 42"))
+
+(hk-test
+  "trycatch.hs — pattern-bind on Right inside do"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source
+        "main = do { Right v <- try (parseInt \"two\"); return (v + 100) }")))
+  (list "IO" 102))
+
+(hk-test
+  "trycatch.hs — handle alias on parseInt failure"
+  (hk-deep-force
+    (hk-run
+      (str hk-trycatch-source
+        "main = handle (\\(SomeException m) -> return (\"caught: \" ++ m)) (parseInt \"nope\" >>= (\\v -> return (show v)))")))
+  (list "IO" "caught: unknown: nope"))