[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       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 On GHC 7.10.2, if I try and compile the following short program:

 {{{#!hs
 {-# LANGUAGE DataKinds, PolyKinds, DeriveDataTypeable, StandaloneDeriving
 #-}
 module Test where

 import Data.Typeable

 deriving instance Typeable '[]
 deriving instance Typeable '(:)

 nilTyCon  = typeRepTyCon (typeRep (Proxy :: Proxy '[]))
 consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy '(:) ))
 }}}

 Then GHC reports the following errors:

 {{{
 Test.hs:9:27:
     No instance for (Typeable '[]) 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 '[]))

 Test.hs:10:27:
     No instance for (Typeable (':))
       (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 (:)))
 }}}

 It seems a bug to me that the very instances I asked to be derived (and
 apparently were without complaint from GHC) are now not found by the
 typechecker in the very next line.

 I think that if the instance cannot be derived, GHC should say so, or if
 it can, then GHC should find it. I can't really see what's going on with
 the dump flags, because the typechecker fails before the desugarer runs.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052
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