[GHC] #7989: "No constructor has all these fields" message can be improved

[GHC]

#7989: "No constructor has all these fields" message can be improved
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    Reporter:  akio              |       Owner:                  
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.7             
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  Other           
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by simonpj):

 OK.... It's not easy to understand that code!  Maybe it would be worth a
 few more named intermediate values, with types, and a comment?

 You could produce a very big set when a very small set exists, couldn't
 you?

 Could you ameliorate that as follows?  Use `T` to mean the data type of
 the first field in `rbinds`.
  * Sort the `membership` list in order of increasing number of `True`
 fields.  So the earlier fields `f` in the list have few `T` constructors
 that have field `f`.
  * Now a field with a different type will be right at the front of the
 list.
  * And the `(&&)` process will terminate more quickly because of all the
 `False` items.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7989#comment:7>
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