[GHC] #11663: GHC 8.0 behavior WRT scoped type variables changed

1 Mar 2016

      #11663: GHC 8.0 behavior WRT scoped type variables changed
-------------------------------------+-------------------------------------
           Reporter:  bitemyapp      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc2
           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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 Not sure if this was intentional or not.

 This compiles under GHC 8.0.0.20160221

 {{{
 module Main where

 import Control.Exception
 import System.IO

 main =
   writeFile "zzz" "hi"
   `catch` (\(e :: SomeException) -> return ())
 }}}

 Run with an unwritable file named "zzz" in the directory, it ends without
 complaint.

 Run under GHC 7.10.3, it throws an error at compilation time:

 {{{
 code/printPls.hs:8:19:
     Illegal type signature: ‘SomeException’
       Perhaps you intended to use ScopedTypeVariables
     In a pattern type-signature
 }}}

 Did something intentionally change to make this compile out of the box?

 When I add:

 {{{
 {-# LANGUAGE ScopedTypeVariables #-}
 }}}

 Both versions compile. This distinction holds in GHCi as well.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11663
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