[GHC] #9637: Type level programming needs an error function

[GHC]

#9637: Type level programming needs an error function
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        Reporter:  augustss          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9180, #9636      |  Differential Revisions:  Phab:D1236
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Comment (by simonpj):

 On the implementation front, I have some suggestions, which I've added
 under `SLPJ alternative design` on the wiki page [wiki:CustomTypeErros]

 (The title of the wiki page is mis-spelled.  Rename?)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9637#comment:9>
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