On Thu, Nov 8, 2012 at 12:01 PM, Nicolas Frisby <nicolas.frisby@gmail.com> wrote:

And the important observation is: all of them throw A if interpreted in ghci or compiled without -O, right?

On Thu, Nov 8, 2012 at 11:24 AM, Albert Y. C. Lai <trebla@vex.net> wrote:

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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