[GHC] #15033: ScopedTypeVariable and RankNTypes don't scope throughout function body

13 Apr 2018

      #15033: ScopedTypeVariable and RankNTypes don't scope throughout function body
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           Reporter:  parsonsmatt    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           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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 I'm writing a DSL that uses `TypeApplications`, and I noticed the
 following weird behavior with `ScopedTypeVariables` and
 `AllowAmbiguousTypes`.

 You can define a function that takes the `TypeApplication` after a
 parameter:

 {{{#!hs
 lol :: String -> forall a. IO ()
 lol str = putStrLn str

 main = lol "hello" @Int
 }}}

 This works! Unfortunately, it is impossible to *refer* to that `a` type
 variable in the body of the function. It is as though
 `ScopedTypeVariables` were not turned on.

 A reproduction (tested on 8.2.2 and 8.4.1):

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE RankNTypes          #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 import Data.Typeable
 import Data.Proxy

 no :: String -> forall x. Typeable x => IO ()
 no _ = print (typeRep (Proxy :: Proxy x))

 yes :: forall x. Typeable x => String -> IO ()
 yes _ = print (typeRep (Proxy :: Proxy x))
 }}}

 `no` fails to compile, saying that `x` type variable is not in scope.
 `yes` works just fine.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15033
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