[GHC] #9299: GHCi type inference error

11 Jul 2014


      #9299: GHCi type inference error
------------------------------------+-------------------------------------
       Reporter:  erikd             |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  GHCi              |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  Other
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 This code (required conduit-1.1.6):

 {{{
 {- Bolled down from
 https://github.com/snoyberg/conduit/blob/process/conduit-
 extra/Data/Conduit/Process.hs
 -}

 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE RankNTypes #-}

 import System.Process
 import Control.Monad.IO.Class (MonadIO, liftIO)
 import System.IO (Handle, hClose)
 import Data.Conduit
 import Data.ByteString (ByteString)
 import Data.Maybe (fromMaybe)
 import Control.Applicative ((<$>), (<*>))

 data UseProvidedHandle = UseProvidedHandle
 data Inherited = Inherited
 data ClosedStream = ClosedStream

 class InputSource a where
     isStdStream :: (Maybe Handle -> IO a, Maybe StdStream)

 instance InputSource Handle where
     isStdStream = (\(Just h) -> return h, Just CreatePipe)
 instance InputSource ClosedStream where
     isStdStream = (\(Just h) -> hClose h >> return ClosedStream, Just
 CreatePipe)
 instance (r ~ (), MonadIO m, i ~ ByteString) => InputSource (ConduitM i o
 m r) where
     isStdStream = (\(Just h) -> return $ sinkHandle h, Just CreatePipe)
 instance (r ~ (), r' ~ (), MonadIO m, MonadIO n, i ~ ByteString) =>
 InputSource (ConduitM i o m r, n r') where
     isStdStream = (\(Just h) -> return (sinkHandle h, liftIO $ hClose h),
 Just CreatePipe)
 instance InputSource Inherited where
     isStdStream = (\Nothing -> return Inherited, Just Inherit)
 instance InputSource UseProvidedHandle where
     isStdStream = (\Nothing -> return UseProvidedHandle, Nothing)

 sinkHandle :: MonadIO m => Handle -> Consumer ByteString m ()
 sinkHandle = error "sinkHandle"


 conduitProcess :: (MonadIO m, InputSource stdin)
                => CreateProcess
                -> m (stdin, ProcessHandle)
 conduitProcess cp = liftIO $ do
     let (getStdin, stdinStream) = isStdStream

     (stdinH, _, _, ph) <- createProcess cp
                     { std_in = fromMaybe (std_in cp) stdinStream
                     }
     (,) <$> getStdin stdinH <*> return ph


 main :: IO ()
 main = putStrLn "Hello"
 }}}

 Compiles fine with ghc, and runs correctly with `runghc`, but fails to
 load into ghci with the following error:

 {{{
 GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Main             ( test.hs, interpreted )

 test.hs:45:9:
     No instance for (InputSource a0)
       arising from the ambiguity check for ‘stdinStream’
     The type variable ‘a0’ is ambiguous
     When checking that ‘stdinStream’
       has the inferred type ‘Maybe StdStream’
     Probable cause: the inferred type is ambiguous
     In the second argument of ‘($)’, namely
       ‘do { let (getStdin, stdinStream) = isStdStream;
             (stdinH, _, _, ph) <- createProcess
                                     (cp {std_in = fromMaybe (std_in cp)
 stdinStream});
             (,) <$> getStdin stdinH <*> return ph }’
     In the expression:
       liftIO
       $ do { let (getStdin, stdinStream) = isStdStream;
              (stdinH, _, _, ph) <- createProcess
                                      (cp {std_in = fromMaybe (std_in cp)
 stdinStream});
              (,) <$> getStdin stdinH <*> return ph }
 Failed, modules loaded: none.
 }}}

 Same problem in 7.6.3, 7.8.2 and 7.8.3.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9299
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