Re: [GHC] #15832: returnIO/bindIO destroys runtime explicit stack information in some cases

1 Nov 2018

      #15832: returnIO/bindIO destroys runtime explicit stack information in some cases
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        Reporter:  infinity0         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by infinity0):

 Here is an even more realistic example, which hopefully convinces you that
 this is a Very Important bug (at least with stack-trace printing), and
 closer to my real-world code:

 Good code:

 {{{#!hs
 data X = X Int | XX deriving (Eq, Show)

 mkX = X

 doInnocentThings a = case a of
   X i -> putStr ""
   XX -> putStr "XX"

 doBadThing a = case a of
   X i -> print i
   XX -> print "XX"

 main :: IO ()
 main = do
   return undefined
   let a = mkX (error "XXX")
   doInnocentThings a
   doBadThing a
 }}}

 Good stack trace:

 {{{
 *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
   Main.main.a,
   called from Main.main.a,
   called from Main.CAF:sat_s2O2
   --> evaluated by: Main.doBadThing,
   called from Main.main,
   called from Main.CAF:sat_s2O2
   --> evaluated by: Main.doBadThing,
   called from Main.main
 Test-good: XXX
 CallStack (from HasCallStack):
   error, called at Test-good.hs:16:16 in main:Main
 CallStack (from -prof):
   Main.main.a (Test-good.hs:16:16-26)
   Main.main.a (Test-good.hs:16:11-27)
   Main.CAF:sat_s2O2 (<no location info>)
 }}}

 Here the stack trace mentions "doBadThing", it is obvious which piece of
 code is at fault. Now:

 Bad code:

 (change `return undefined` to `_ <- return undefined` in the good code)

 Bad stack trace:

 {{{
 *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace:
   Main.main.a,
   called from Main.main.a
 Test-bad: XXX
 CallStack (from HasCallStack):
   error, called at Test-bad.hs:16:16 in main:Main
 CallStack (from -prof):
   Main.main.a (Test-bad.hs:16:16-26)
   Main.main.a (Test-bad.hs:16:11-27)
 }}}

 There is no "evaluated by" entry, the only thing I see is the `let a =`
 statement being mentioned, which is perfectly fine logic-wise. There is no
 way to debug this, except by guessing based on knowledge of the whole
 codebase.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15832#comment:6
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