[GHC] #14634: Add print stacktrace to exception handler in runtime system

4 Jan 2018

      #14634: Add print stacktrace to exception handler in runtime system
-------------------------------------+-------------------------------------
           Reporter:  flip101        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 main :: IO ()
 main = do
   someIO
   putStrLn "hello world"

 someIO :: IO ()
 someIO = ioError $ IOError Nothing IllegalOperation "a" "b" Nothing
 Nothing
 }}}

 This code prints the following

 {{{
 exception: a: illegal operation (b)
 }}}

 I think there is something in the runtime system which takes this
 exception and prints this text to the terminal. I will call this the
 "runtime exception handler" (for i don't know any better).

 {{{#!hs
 main :: IO ()
 main = do
   ex <- try someIO :: IO (Either IOException ())
   case ex of
     Left e -> error (show e)
     Right _ -> putStrLn "hello world"

 someIO :: IO ()
 someIO = ioError $ IOError Nothing IllegalOperation "a" "b" Nothing
 Nothing
 }}}

 output:
 {{{
 exception: a: illegal operation (b)
 CallStack (from HasCallStack):
   error, called at src/Main.hs:10:15 in main:Main
 }}}

 Now i use an explicit handler in my code and show the stacktrace with
 `error`. Could this handler be moved into the RTS so that i can
 stacktraces with exceptions just like with error?

 I think this issue is related to, but not the same as
 https://ghc.haskell.org/trac/ghc/ticket/12096 Because "Attach stacktrace
 information to SomeException" -- whatever there is being attached right
 now (with lts-10.2 ghc 8.2.2) is good enough to provide the information,
 it's just the print option in the handler that seems to be missing.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14634
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