Here's my contrived example that threw the error.
If you go into ghci, and do a `:t (foo' "blah" myDoohickey)`, you will get
the type signature "IO ()".
Doing the same for myOtherDoohickey returns "IO True"
So you would think that you'd be able to uncomment the code that makes IO an
instance of Toplevel. foo' is a function that allows IO to run monadic
values of type Doohickey. But it doesn't work.
---
import IO
import Control.Monad.Reader
class (Monad n) => Doohickey n where
putRecord :: String -> n ()
class (Monad m) => Toplevel m where
foo :: (Doohickey n) => FilePath -> n a -> m a
newtype IOToplevelT a = IOToplevelT { runIOToplevelT :: ReaderT Handle IO a
} deriving (Monad, MonadReader Handle, MonadIO)
instance Doohickey IOToplevelT where
putRecord = liftIO . putStrLn
foo' s k = do
f <- liftIO $ openFile s AppendMode
runReaderT (runIOToplevelT k) f
--instance Toplevel IO where
-- foo = foo'
myDoohickey = do
putRecord "foo"
putRecord "bar"
myOtherDoohickey = do
putRecord "hello"
putRecord "world"
return True
On Mon, Apr 13, 2009 at 7:55 PM, Jason Dusek
Copypasting and loading your code doesn't throw an error. Please, pastebin an example that demonstrates the error.
-- Jason Dusek