[GHC] #10924: Template variable unbound in rewrite rule

[GHC]

#10924: Template variable unbound in rewrite rule
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  7.10.3
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:  singletons,
                                     |  templatehaskell
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
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Comment (by crockeea):

 I did figure out a workaround: don't use "as-patterns" at all.
 The following definition of `ppMul` compiles:
 {{{
     ppMul :: PrimePower -> [PrimePower] -> [PrimePower]
     ppMul x [] = [x]
     ppMul (PP(p,e)) (PP (p',e'):pps')
       | p == p' = PP(p,e Num.+ e'):pps'
       | p <<= p' = (PP(p,e)):(PP (p',e'):pps')
       | otherwise = (PP(p',e')):ppMul (PP(p,e)) pps'
 }}}

 Since the point of using singletons is to get type-level functions, it's
 not clear that there is any performance overhead (except perhaps in the
 compiler) by rebuilding `PP(p,e)` and `PP(p,e')`, but it would be nice to
 be able to write this in a more idiomatic manner.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10924#comment:3>
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