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

[GHC]

#15512: Rewrite rules should be able to produce custom compiler errors
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        Reporter:  ChaiTRex          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      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 simonpj):

 You could rewrite
 {{{
 forall x. length (cycle x) = error "length of cycle error"
 }}}
 but it would still be a run-time error, not a compile-time error.  It's
 not clear how to achieve the latter.  e.g.
 {{{
 let v = length (cycle x)
 in if ... then ...v... else True
 }}}
 then `v` might never be evaluated.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15512#comment:4>
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