[GHC] #9687: Missing Typeable instances for built-in types

[GHC]

#9687: Missing Typeable instances for built-in types
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              Reporter:  selinger    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by selinger):

 Simon,

 what you suggest seems like the logical solution, but doesn't currently
 work. Test.hs:

 {{{
 import Data.Typeable

 instance (Typeable a, Typeable b, Typeable c, Typeable d, Typeable e,
 Typeable f, Typeable g, Typeable h) => Typeable (a,b,c,d,e,f,g,h)
 }}}

 And here's what happens when you compile it:

 {{{
 selinger at firefly-virtual:/tmp$ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.8.3
 selinger at firefly-virtual:/tmp$ ghc Test
 [1 of 1] Compiling Main             ( Test.hs, Test.o )

 Test.hs:1:1:
     Typeable instances can only be derived; replace the following
 instance:
       instance (Typeable a, Typeable b, Typeable c, Typeable d,
                 Typeable e, Typeable f, Typeable g, Typeable h) =>
                Typeable (a, b, c, d, e, f, g, h)
         -- Defined at Test.hs:3:10
 selinger at firefly-virtual:/tmp$
 }}}

 By the way, this bug doesn't just apply in the case of built-in types such
 as 8-tuples. It would also apply to any type that is defined by a library,
 where the library author neglected to derive a Typeable instance. It seems
 impossible for user code to provide the requisite instances.

 -- Peter

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9687#comment:2>
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