[GHC] #14691: Replace EvTerm with CoreExpr

[GHC]

#14691: Replace EvTerm with CoreExpr
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.3
  checker)                           |
      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 nomeata):

 Hmpf, no matter how I shake it, it converges back to having many
 constructors for `EvTerm`.

 At first I added only `EvTypeable`, because we have determined that we
 cannot create the Core for that during constraint solving.

 But then I find code like this:
 {{{
 rewriteEvidence ev@(CtWanted { ctev_dest = dest
                              , ctev_loc = loc }) new_pred co
   = do { mb_new_ev <- newWanted loc new_pred
        ; MASSERT( tcCoercionRole co == ctEvRole ev )
        ; setWantedEvTerm dest
                    (mkEvCast (getEvTerm mb_new_ev)
                              (tcDowngradeRole Representational (ctEvRole
 ev) co))
        ; case mb_new_ev of
             Fresh  new_ev -> continueWith new_ev
             Cached _      -> stopWith ev "Cached wanted" }
 }}}
 where an arbitrary `EvTerm`, the result of `getEvTerm` needs to be casted.
 I cannot use Core’s `Cast` for that, because the `EvTerm` may be a
 `EvTypeable`. So seems that I need to add the `EvCast` constructor back to
 `EvTerm` … and bit by bit I am undoing the refactoring that I was hoping
 to do here…

 Maybe the better thing to do is to leave all the existing constructors in
 place, and just add `EvExpr :: CoreExpr -> EvTerm` as an additional leaf
 constructor. This way, the existing code structure can remain.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14691#comment:23>
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