Using DeepSeq for exception ordering

[Albert Y. C. Lai]

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")