[GHC] #7411: Exceptions are optimized away in certain situations

[GHC]

#7411: Exceptions are optimized away in certain situations
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        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:                    |
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Comment (by tdammers):

 OK, boiled it down to a simpler test case that doesn't even involve
 `deepseq`:

 {{{
 import Control.Exception

 dslist :: [a] -> b -> b
 dslist xs b = go xs `seq` b
   where
     go [] = ()
     go (x:xs) = x `seq` go xs
     -- the following variation also reproduces the problem:
     -- go (x:xs) = go (x `seq` xs)

 main = evaluate (('a':undefined) `dslist` return () :: IO ())
 }}}

 This is essentially what `deepseq` does on lists, with all the typeclass
 instances and type variables manually unrolled.

 Note that in order to trigger the bug, we actually do need the `go`
 function; if instead we pass the `b` around and recurse directly, the
 problem goes away, like so:

 {{{
 import Control.Exception

 dslist :: [a] -> b -> b
 dslist xs b = go xs `seq` b
   where
     dslist [] a = a
     dslist (x:xs) a = x `seq` dslist xs a

 main = evaluate (('a':undefined) `dslist` return () :: IO ())
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7411#comment:22>
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