On 12-11-08 07:12 AM, Simon Hengel wrote:

I was just going to say that I can give at least one counterexample
where this does not hold:

   evaluate (('a' : undefined) `deepseq` return () :: IO ())
   throwIO exceptionB

But then I realized that here exceptionA is optimized away altogether.
For me this smells like a bug.  Is this related to [1]?

[1] http://hackage.haskell.org/trac/ghc/ticket/2273

Interesting. A few more tests (all GHC 7.4.2, linux, x86 32-bit, use "ghc -O" to compile):

The following cases throw A:

import Control.DeepSeq
import Control.Exception

main = do
  evaluate (('a' : error "A") `deepseq` return () :: Maybe ())
  throwIO (userError "B")

main = do
  evaluate (('a' : error "A") `deepseq` ())
  throwIO (userError "B")

main = do
  evaluate (('a' : error "A") `deepseq` True)
  throwIO (userError "B")

main = do
  x <- evaluate (('a' : error "A") `deepseq` putStrLn "hi")
  x
  throwIO (userError "B")

The following cases throw B:

main = do
  evaluate (('a' : error "A") `deepseq` return () :: IO ())
  throwIO (userError "B")

main = do
  evaluate (('a' : error "A") `deepseq` putStrLn "hi")
  throwIO (userError "B")

main = do
  evaluate (('a' : error "A") `deepseq` getLine)
  throwIO (userError "B")

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