[GHC] #9111: base should export Typeable instances of its promoted data constructors

[GHC]

#9111: base should export Typeable instances of its promoted data constructors
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        Reporter:  goldfire          |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 Deriving `Typeable` automatically is exactly what `AutoDeriveTypeable` is
 for (see
 [http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html
 #auto-derive-typeable user manual])

 Moreover the manual claims that `AutoDeriveTypeable` will also derive
 `Typeable` for the promoted data constructors.

 I think we should probably switch it on for the entire `base` package, and
 the chair of the core libraries committee seems to agree.  That would be
 simpler than a lot of ad-hoc instances.

 (Indeed I kind of wonder '''whether it is ever useful NOT to derive
 `Typeable`'''?  If not, we could just derive `Typeable` automatically for
 everything.  In implementation terms it would mean we could get rid of all
 the instance declarations in the interface file, since they would all be
 there by implication.)

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9111#comment:8>
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