GHCi type inference error

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.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	hvr
Operating system
Architecture