[GHC] #12549: Panic on ":t datatypeName"

[GHC]

#12549: Panic on ":t datatypeName"
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        Reporter:  johnleo       |                Owner:  johnleo
            Type:  bug           |               Status:  new
        Priority:  normal        |            Milestone:
       Component:  Compiler      |              Version:  8.1
      Resolution:                |             Keywords:
Operating System:  MacOS X       |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:                |  Differential Rev(s):
       Wiki Page:                |
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Comment (by johnleo):

 Replying to [comment:13 goldfire]:
 > But you don't have `+v` in 8.0. The type that you're seeing has been
 instantiated and then regeneralized, so all the variable ordering has been
 forgotten. I still claim that the internal types assigned to `f` are the
 same. You just can't ask GHCi for this type in 8.0. You could, though do a
 `pprTrace` or `traceTc` from `TcExpr.tcInferId` which can print out the
 uninstantiated type.
 >
 > In your last sentence: that `k` really ought to be in scope throughout
 the type. If it's not, there's your bug!

 OK, I understand.  Perhaps it is a bug in 8.0 as well then.  The code
 seems to be instantiating variables in independent groups, in this case
 the group before the class constraint and then separately the group after
 the class constraint.  If you know a reason for this let me know.  Options
 for fixing it would include doing all the variables together whether they
 are before or after the constraint, or saving variables from early groups
 to be in scope for later groups (perhaps the two are essentially
 equivalent).  I'll explore further.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12549#comment:14>
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