[GHC] #13961: TH.reify can be ambiguous when giving a name that's in multiple namespaces

11 Jul 2017

      #13961: TH.reify can be ambiguous when giving a name that's in multiple namespaces
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           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           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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 What should this program do?

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Language.Haskell.TH

 (***) :: Bool -> Bool -> Bool
 (***) _ _ = True

 type (***) a b = Bool

 $(return [])

 main :: IO ()
 main = putStrLn $(reify (mkName "***") >>= stringE . show)
 }}}

 The issue is that `reify (mkName "***")` could conceivable look up the
 `Info` of two different things: the value-level `(***)` or the type-level
 `(***)`. In this case, it happens to pick the value-level one:

 {{{
 $ /opt/ghc/8.2.1/bin/runghc Bug.hs
 VarI Bug.*** (AppT (AppT ArrowT (ConT GHC.Types.Bool)) (AppT (AppT ArrowT
 (ConT GHC.Types.Bool)) (ConT GHC.Types.Bool))) Nothing
 }}}

 So if you want to look up the type-level `(***)`'s info in this way,
 you're hosed.

 If the above scenario seems contrived, and you find yourself thinking "but
 RyanGlScott, why don't you just use `'(***)` and `''(***)`?", keep in mind
 that there are times I need to look up //unqualified// names, and in those
 situations, `mkName` is all I have at my disposal for creating the `Name`
 to look up.

 There has to be a better way to go about business here. Perhaps we should
 introduce another function

 {{{#!hs
 reifyAll :: Name -> [Info]
 }}}

 That finds all possible `Info`s in all the namespaces the argument `Name`
 can be found in?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13961
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