[GHC] #15192: Refactor of Coercion

[GHC]

#15192: Refactor of Coercion
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        Reporter:  ningning          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4747
       Wiki Page:                    |
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Comment (by simonpj):

 Can you elaborate?

 * What are the advantages of the new form?  (It looks more symmetrical;
 but it has more arguments.)   Can you say what the "several places" where
 is is useful are?  Are there any disadvanatages?

 * Can we use the new form in `OptCoercion` to simplify coercions?

 * I see no constraints on `r` in the rule. So you could fix on `N` and use
 `SubCo` to downgrade to `R`?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15192#comment:4>
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