[GHC] #15147: Type checker plugin receives Wanteds that are not completely unflattened

[GHC]

#15147: Type checker plugin receives Wanteds that are not completely unflattened
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        Reporter:  nfrisby           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |             Keywords:  type checker
      Resolution:                    |  plugins
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 adamgundry):

 I'm also seeing this trying to port `uom-plugin` to 8.4. Unfortunately
 I've got the details of how this works in GHC pretty well paged out, and I
 think the code may have evolved a bit since I worked on it.

 The original thinking was to avoid bothering typechecker plugins with the
 complications of flattening and zonking, on the basis that sets (well,
 lists) of constraints are a simpler model to work with. But perhaps in
 practice the plugin interface is so low level that we would be better off
 doing as little as possible before handing constraints over to the plugin.
 Assuming we can make it relatively easy for the plugin to flatten/zonk if
 it wants to, that is...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15147#comment:3>
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