[GHC] #8778: Typeable TypeNats

[GHC]

#8778: Typeable TypeNats
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        Reporter:  dmcclean                 |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:  ⊥
       Component:  Compiler (Type checker)  |          Version:  7.8.1-rc1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  4385
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Comment (by diatchki):

 Aha, thanks for the Phabricator tip!

 I redid the instances to be of the form `\_ -> blah`.

 About the `Typeable` instance:  in normal modules we only allow machine-
 generated instances.  The module `Data.Typeable.Internal` is an exception
 though---it allows manual instances to accommodate custom instances for
 some types.

 One note about the type-literal instances:   they have a much higher
 chance of collision than a typical kind, as both `Nat` and `Symbol` have
 infinitely many type-constructors, which need to fit into the fingerprint.
 I am not sure how much of a realistic problem this is, but it is something
 to keep in mind.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8778#comment:14>
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