[GHC] #15512: Rewrite rules should be able to produce custom compiler errors

13 Aug 2018

      #15512: Rewrite rules should be able to produce custom compiler errors
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           Reporter:  ChaiTRex       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It would be nice if rewrite rules could produce compiler errors with
 custom messages when a certain combination of functions can only produce
 bottom or some other suitably undesirable outcome.

 For example, `length . cycle` is either never going to terminate or is
 going to give an error message (''e''.''g''., if the input list is empty).
 It would be nice to be able to tell the programmer that at compile time.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15512
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