[GHC] #12389: Limit duplicate export warnings for datatypes

[GHC]

#12389: Limit duplicate export warnings for datatypes
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        Reporter:  dfeuer            |                Owner:  parsonsmatt
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11959            |  Differential Rev(s):  Phab:D4134
       Wiki Page:                    |
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Comment (by simonpj):

 > I'm going to attempt a patch for this.

 Thanks!  But what exactly is "this"?

 I think you mean something like "allow leading and trailing commas in the
 sub-export lists".  As I said above, that's ok with me.  but

 * It should be consistent with exports lists themselves.  Do they allow
 leading commas?  If not, it'd make sense to add them.  Thus
 {{{
 module M( , f, g, ) where ...
 }}}

 * Do we allow multiple leading or trailing commas?  What about repeated
 commas in the middle of a list?

 * What about import lists?  Should they not be consistent?

 * Should we require a language extension flag?

 So although this is a small and superficial change, it ''is'' a user-
 facing one, and so should really go through the [https://github.com/ghc-
 proposals/ghc-proposals GHC proposals process].  Because it's small, it'll
 be quick!  But the debate is always helpful.

 I'm conscious that this may seem obstructive, but I've learned that it
 really is worth writing down the specification and debating it before
 implementing it.  Doing so materially increases quality by giving a way
 for more people to contribute.

 Thanks for helping!

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12389#comment:14>
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