[GHC] #8778: Typeable TypeNats

[GHC]

#8778: Typeable TypeNats
--------------------------------------------+------------------------------
        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 simonpj):

 The issue in #9203 is that a `Typeable` instance should look like
 {{{
 instance Typeable ... where
   typeRep# = \_ -> rep
     where
       rep = ...blah...
 }}}
 and NOT like
 {{{
 instance Typeable ... where
   typeRep# p = ...blah...
 }}}
 to ensure that the computation of `rep` is shared among all invocations of
 `typeRep#`.

 So Sjoerd is quite right, I think.

 (I'm a bit confused because I thought we only allowed machine-generated
 instances of `Typeable`.)

 BTW if you have a Github account you automatically have a Phabricator
 account.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8778#comment:12>
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