[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:  osa1
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.6.1
      Resolution:                    |                Keywords:  deriving,
Operating System:  Unknown/Multiple  |  newcomer
 Type of failure:  GHC rejects       |            Architecture:
  valid program                      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:  Phab:D978
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Comment (by jpbernardy):

 I just read the proposed documentation on Phabricator. There is something
 that I think is dissatisfying with the implementation.

 As I understand it, the contract expressed by an empty data type is that
 there should be no value inhabiting it. So, if a piece of program ever
 builds one, the blame should be assigned to that piece of code. However,
 it does not appear that the current implementation does this. Eg:


 (error "oops" :: Foo) `compare` (error "oops" :: Foo)

 should fail with "oops", not ​"compare on empty data type Foo"

 Similarly, a Read instance makes no sense imho.

 Please, consider forcing the arguments in all these instances, before
 failing.

 Finally, I realise that my original report was misleading. I am sorry for
 that.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:28>
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