#7411: Exceptions are optimized away in certain situations -------------------------------------+------------------------------------- Reporter: SimonHengel | Owner: tdammers Type: bug | Status: new Priority: high | Milestone: 8.6.1 Component: Compiler | Version: 7.6.1 Resolution: | Keywords: seq, deepseq, | evaluate, exceptions Operating System: Linux | Architecture: x86_64 | (amd64) Type of failure: Incorrect result | Test Case: at runtime | simplCore/should_fail/T7411 Blocked By: | Blocking: Related Tickets: #5129 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by tdammers): Another data point: this only happens with lists, not with tuples - e.g., the following works as expected: {{{ import Control.Exception import Control.DeepSeq main = evaluate (('a', undefined :: [Char]) `deepseq` return () :: IO ()) }}} Which is baffling in that the `NFData` instances for tuples and lists are strikingly similar and *should* do the same thing. It gets even more baffling when I implement a custom (but isomorphic) drop-in replacement for `[]`, like so: {{{ import Control.Exception import Control.DeepSeq data List a = Nil | Cons a (List a) instance NFData a => NFData (List a) where rnf v = case v of Nil -> () (Cons x (Cons y xs)) -> rnf x `seq` rnf y `seq` rnf xs main = evaluate ((Cons 'a' $ Cons undefined $ Nil) `deepseq` return () :: IO ()) }}} This example reproduces the problem; however, changing the instance like this behaves: {{{ instance NFData a => NFData (List a) where rnf v = case v of Nil -> () (Cons x (Cons y xs)) -> rnf x `seq` rnf y }}} In other words, if the `NFData` instance says *not* to look beyond the second list element, then the failure occurs, but if does look beyond, then it gets optimized away. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7411#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler