[GHC] #7401: Can't derive instance for Eq when datatype has no constructor, while it is trivial do do so.

[GHC]

#7401: Can't derive instance for Eq when datatype has no constructor, while it is
trivial do do so.
---------------------------------+------------------------------------------
    Reporter:  jpbernardy        |       Owner:                           
        Type:  feature request   |      Status:  infoneeded               
    Priority:  normal            |   Milestone:  7.8.1                    
   Component:  Compiler          |     Version:  7.6.1                    
    Keywords:  deriving          |          Os:  Unknown/Multiple         
Architecture:  Unknown/Multiple  |     Failure:  GHC rejects valid program
  Difficulty:  Unknown           |    Testcase:                           
   Blockedby:                    |    Blocking:                           
     Related:                    |  
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Comment(by jpbernardy):

 Basically what kosmikus said. This occurs often when you do generic
 programming. ie, when you encode the type A as A + 0 where 0 is a type
 with no constructor and + is an either-like type.

 So: yes, I have seen this in real programs. It's not too hard to write an
 instance manually, but the error message is confusing. I suspect that a
 proper fix (allowing to derive an instance) should be very easy, if it is
 not the case, a better error message (hinting at the user to write an
 instance with 'undefined') would also be an acceptable fix.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7401#comment:6>
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