[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:  monoidal
            Type:  feature request            |           Status:  new
        Priority:  normal                     |        Milestone:  7.8.1
       Component:  Compiler                   |          Version:  7.6.1
      Resolution:                             |         Keywords:  deriving
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by simonpj):

 There was some follow-up email on ghc-devs about this. (But let's
 accumulate comments here.)  My summary

  * If we have `EmptyDataDecls` then surely
 {{{
 data T deriving( Eq )
 }}}
  should work, just as well as standalone deriving
 {{{
 data T
 deriving Eq T
 }}}
  The two must behave the same.

  * Surely all the built-in classes (Eq, Show, Ord etc) should derive for
 with `EmptyDataDecls` too.

  * Haskell 2010 doesn't have `EmptyDataDecls` (eg the
 [http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2
 data type chapter] specifies n>0 constructors).  But oddly the
 [http://www.haskell.org/onlinereport/haskell2010/haskellch11.html#x18-18200011
 deriving appendix] clause 6 says that nothing is derived if there are no
 constructors.  I think we should just ignore clause 6, since H2010 doesn't
 have empty data types.

  * It would be better if the `error "Void Eq"` mentioned the data type.
 More like `error "Eq on empty data type T"`.

 Might you do those things, Omer?

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7401#comment:13>
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