Interesting whoCreated output (stack trace related)

[Simon Marlow]

On 25/06/2014 15:28, Ömer Sinan Ağacan wrote:
> I'm running this program:
>
>    {-# LANGUAGE DeriveDataTypeable, ScopedTypeVariables #-}
>    import GHC.Stack
>    import Control.Exception.Base
>    import Data.Typeable
>
>    data MyException = MyException deriving (Show, Typeable)
>
>    instance Exception MyException
>
>    err1 :: IO Integer
>    err1 = {-# SCC error1 #-} err2 >>= return
>
>    err2 :: IO Integer
>    err2 = {-# SCC error2 #-} throw MyException
>
>    main = print =<< whoCreated =<< catch err1 =<< (\(_ :: MyException)
> -> return (42 :: Integer))
>
> I'd expect `whoCreated` to return something like [Main.CAF, Main.main,
> Main.main.\] but instead this is the output:
>
>    ["Main.CAF (<entire-module>)","Main.err2
> (ioerr.hs:14:1-43)","Main.error2 (ioerr.hs:14:27-43)","Main.main
> (ioerr.hs:16:1-103)","Main.main.\\ (ioerr.hs:16:81-102)"]
>
> This output has two things that look somewhat weird to me. First, I'd
> expect cost-centre stack to be restored when exception is catched and
> then program would produce shorter stack trace like I mentioned
> above.(without err1 or err2 calls)
>
> Second, when no cost-centre restoring is done, I'd expect stack trace
> to include `Main.err1` and `Main.error1`.
>
> So can anyone explain my why stack trace contains err2 calls but not
> err1 calls? I think none of error1 and error2 should have been
> included in the stack trace or both of them should have been included.

I have no idea, but I just want to point out that whoCreated is 
inherently fragile.  If its argument is a thunk (as it might well be) 
then you'll see the CCS of the thunk, and not the CCS of the value. 
Optimisation flags can easily change the output you get here.  To get 
more predictable output you could try forcing the value before passing 
it to whoCreated.

Cheers,
Simon