[GHC] #8499: Template Haskell: newName not new enough

[GHC]

#8499: Template Haskell: newName not new enough
------------------------------------+-------------------------------------
       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Template Haskell  |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 When I compile this:

 {{{
 {-# LANGUAGE TemplateHaskell, PolyKinds, DataKinds #-}
 {-# OPTIONS_GHC -Wall #-}

 import Language.Haskell.TH

 $( do TyConI (DataD _ _ [PlainTV tvb_a] _ _) <- reify ''Maybe
       my_a <- newName "a"
       return [TySynD (mkName "SMaybe") [KindedTV my_a (AppT (ConT ''Maybe)
 (VarT tvb_a))]
               (TupleT 0)] )
 }}}

 I get this:

 {{{
 /Users/rae/temp/Bug.hs:6:4: Warning:
     This binding for ‛a’ shadows the existing binding
       bound at /Users/rae/temp/Bug.hs:6:4
 }}}

 The problem is that, in the library definition for `Maybe`, the name of
 its type variable is `a`. In my Template Haskell code, I read in `Maybe`'s
 definition (using `reify`) to extract the name of `Maybe`'s type variable.
 I then create a new name, also seeded with `"a"`. Then, I create a type
 synonym definition

 {{{
 type SMaybe (a_newName :: Maybe a_Maybe) = ()
 }}}

 where `a_newName` is the variable I `newName`d, and `a_Maybe` is the name
 I slurped from `Maybe`'s definition. These variables print with different
 uniques, but the warning comes up anyway.

 This problem actually came up in real code (singletons library), where I'm
 trying to get the generated code to be warning-free.

 This was tested on 7.7.20131031.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8499>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler