Re: [GHC] #11052: Standalone derived Typeable instance for promoted lists is not found

2 Nov 2015

      #11052: Standalone derived Typeable instance for promoted lists is not found
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        Reporter:  liamoc            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 The error is correct, though the error message is bad (especially the
 second one).

 With `-fprint-explicit-kinds` the error is better:
 {{{
 Test2.hs:10:28:
     No instance for (Typeable [k1] ('[] k1))
       arising from a use of ‘typeRep’
     In the first argument of ‘typeRepTyCon’, namely
       ‘(typeRep (Proxy :: Proxy []))’
     In the expression: typeRepTyCon (typeRep (Proxy :: Proxy []))
     In an equation for ‘nilTyCon’:
         nilTyCon = typeRepTyCon (typeRep (Proxy :: Proxy []))

 Test2.hs:11:28:
     No instance for (Typeable (k0 -> [k0] -> [k0]) ((':) k0))
       (maybe you haven't applied enough arguments to a function?)
       arising from a use of ‘typeRep’
     In the first argument of ‘typeRepTyCon’, namely
       ‘(typeRep (Proxy :: Proxy (:)))’
     In the expression: typeRepTyCon (typeRep (Proxy :: Proxy (:)))
     In an equation for ‘consTyCon’:
         consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy (:)))
 }}}
 The type `'[]` can have any kind `[k1]`, and these must have different
 Typeable instances. So, the (implicit, and not even mentioned in the
 original error message) kind variable is really ambiguous. You could
 choose a particular kind with a signature:
 {{{
 nilStarTyCon  = typeRepTyCon (typeRep (Proxy :: Proxy ('[] :: [*])))
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052#comment:2
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