[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     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Can you give some examples of its use in a program?  I still don't get it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9637#comment:4>
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