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The following code:

module Foo where

import Data.Typeable

tyConOf :: Typeable a => Proxy a -> TyCon
tyConOf = typeRepTyCon . typeRep

tcList :: TyCon
tcList = tyConOf (Proxy :: Proxy [])

fails because -XPolyKinds is not enabled. But the error message that you get is quite different on GHC 7.10 and 8.0.

On GHC 7.10.3:

[1 of 1] Compiling Foo              ( Foo.hs, Foo.o )

Foo.hs:9:19:
    Couldn't match kind ‘* -> *’ with ‘*’
    Expected type: Proxy a0
      Actual type: Proxy []
    In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’
    In the expression: tyConOf (Proxy :: Proxy [])
    In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])

But on GHC 8.0.1-rc1:

Foo.hs:9:19: error:                                                                                       
    • Expected kind ‘Proxy []’,                                                                           
        but ‘Data.Proxy.Proxy :: Proxy []’ has kind ‘Proxy []’                                            
    • In the first argument of ‘tyConOf’, namely ‘(Proxy :: Proxy [])’                                    
      In the expression: tyConOf (Proxy :: Proxy [])
      In an equation for ‘tcList’: tcList = tyConOf (Proxy :: Proxy [])

Trac metadata

Trac field	Value
Version	8.0.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture