[GHC] #11167: Fixity of field-deconstructors incorrect

4 Dec 2015


      #11167: Fixity of field-deconstructors incorrect
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.2
           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:
-------------------------------------+-------------------------------------
 The example below

 {{{#!hs
 module Foo where

 data SomeException

 newtype ContT r m a = ContT {runContT :: (a -> m r) -> m r}

 runContT' :: ContT r m a -> (a -> m r) -> m r
 runContT' = runContT

 catch_ :: IO a -> (SomeException -> IO a) -> IO a
 catch_ = undefined

 -- has type error
 foo :: IO ()
 foo = (undefined :: ContT () IO a) `runContT` (undefined :: a -> IO ())
 `catch_` (undefined :: SomeException -> IO ())

 -- typechecks
 foo' :: IO ()
 foo' = (undefined :: ContT () IO a) `runContT'` (undefined :: a -> IO ())
 `catch_` (undefined :: SomeException -> IO ())
 }}}

 works with GHC 7.10 but breaks with GHC HEAD currently with:

 {{{
 foo.hs:15:47: error:
     • Couldn't match expected type ‘a0 -> IO ()’
                   with actual type ‘IO ()’
     • In the second argument of ‘runContT’, namely
         ‘(undefined :: a -> IO ())
          `catch_` (undefined :: SomeException -> IO ())’
       In the expression:
         runContT
           (undefined :: ContT () IO a)
           (undefined :: a -> IO ())
           `catch_` (undefined :: SomeException -> IO ())
       In an equation for ‘foo’:
           foo
             = runContT
                 (undefined :: ContT () IO a)
                 (undefined :: a -> IO ())
                 `catch_` (undefined :: SomeException -> IO ())

 foo.hs:15:48: error:
     • Couldn't match expected type ‘IO ()’
                   with actual type ‘a1 -> IO ()’
     • In the first argument of ‘catch_’, namely
         ‘(undefined :: a -> IO ())’
       In the second argument of ‘runContT’, namely
         ‘(undefined :: a -> IO ())
          `catch_` (undefined :: SomeException -> IO ())’
       In the expression:
         runContT
           (undefined :: ContT () IO a)
           (undefined :: a -> IO ())
           `catch_` (undefined :: SomeException -> IO ())
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11167
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