[GHC] #15731: Add sortOn/coerce rule

[GHC]

#15731: Add sortOn/coerce rule
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Core           |           Version:  8.6.1
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When the argument to `sortOn` is very cheap, we're better off ''not''
 decorating the list. Detecting this is not easy in general, but it's
 trivial for coercions. We should probably offer something like

 {{{#!hs
 {-# RULES
 "sortOn/coerce" forall b xs.
     sortOn @_ @b coerce xs = sortWith (coerce @_ @b) xs
  #-}
 }}}

 As far as I can tell, we can't use type application directly like that,
 but we can go via `Proxy#`.

 `PrelRules` could do better, of course, catching selector functions, but I
 don't think we want to go that far.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15731>
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