[GHC] #8646: Distinguish between update frames in rts/Printer.c

[GHC]

#8646: Distinguish between update frames in rts/Printer.c
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        Reporter:  Tarrasch          |            Owner:  simonmar
            Type:  feature request   |           Status:  patch
        Priority:  lowest            |        Milestone:  _|_
       Component:  Runtime System    |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by ezyang):

 Your function has an invariant, "must be an UPDATE_FRAME". So logically,
 it should check if that invariant is violated. I think your check in (1)
 is the right one but I haven't double-checked.

 If an invariant is violated, barfing seems to be the right thing to do.
 Honestly, I would also barf when you expected an update frame but didn't
 get one.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8646#comment:4>
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