Porting some code from GHC 7.8.4 to GHC 7.10.2 and have encountered this rather odd failure:

    Could not deduce (MonadReader

                        (PqColumn m -> m B.ByteString)

                        (ReaderT (PqColumn m -> m B.ByteString) m))

      arising from a use of ‘ask’

    from the context (Monad m)

      bound by the type signature for

                 readFieldByteString :: Monad m =>

                                        PqColumn m -> RowReader m B.ByteString

      at src/EventStore/LibPq.hs:164:24-72

    In a stmt of a 'do' block: readField <- ask

    In the expression:

      do { readField <- ask;

           lift $ readField col }

    In an equation for ‘readFieldByteString’:

        readFieldByteString col

          = do { readField <- ask;

                 lift $ readField col }

I wondered if it was because the environment type (PqColumn m -> m B.ByteString) itself was an instance of MonadReader and the typechecker was getting confused, so I put the environment in a newtype and it was indeed happy.

Unfortunately I can't share enough of this code to get the problematic fragment to compile, and my initial attempts at a small reproduction have been unsuccessful. It's something like the following, but this compiles just fine.

    class MyClass m where

      type MyType m

      defaultValue :: m (MyType m)

    

    l :: Monad m => MyType m -> ReaderT (MyType m -> m Int) m Int

    l n = do

      f <- ask

      lift $ f n

Before I put more effort into reproducing this, does anyone recognise this failure or know what might be going on here?

Cheers,

David