[GHC] #13859: Bad error message when compacting a Compact#

[GHC]

#13859: Bad error message when compacting a Compact#
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        Reporter:  andrewthad        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      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 bgamari):

 There are a whole lot of cases to which this objection applies; see
 `Compact.cmm` around line 120. Perhaps we should just replace the word
 "mutable" in the error message with the closure type name? It's perhaps a
 bit more cryptic but less misleading, in general I think it would be nice
 if we could just make the type checking a bit more precise, turning most
 of these errors into more comprehensible compile-time errors.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13859#comment:1>
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