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 wrote:

Copypasting and loading your code doesn't throw an error. Please, pastebin an example that demonstrates the error.

-- Jason Dusek

I wonder how you would do this with type familes... On Tue, Apr 14, 2009 at 2:39 PM, Arthur Chan wrote:

I seem to have finally solved my own problem, via something I learned from RWH. The solution is to use functional dependencies...

The problem was that the compiler needed to know the relationship between Doohickeys and Toplevels, and I couldn't figure out how to tell it that...

{-# LANGUAGE GeneralizedNewtypeDeriving, NoMonomorphismRestriction, FunctionalDependencies, MultiParamTypeClasses #-}

import IO import Control.Monad.Reader

class (Monad n) => Doohickey n where putRecord :: String -> n ()

class (Monad m, Doohickey n) => Toplevel m n | m -> n where foo :: FilePath -> n a -> m a

newtype IODoohickey a = IODoohickey { runIODoohickey :: ReaderT Handle IO a } deriving (Monad, MonadReader Handle, MonadIO)

instance Doohickey IODoohickey where putRecord = liftIO . putStrLn

instance Toplevel IO IODoohickey where foo s k = do f <- liftIO $ openFile s AppendMode runReaderT (runIODoohickey k) f

myDoohickey = do putRecord "foo" putRecord "bar"

myOtherDoohickey = do putRecord "hello" putRecord "world" return True

On Tue, Apr 14, 2009 at 2:05 PM, Arthur Chan wrote:

...

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 wrote:

...

Copypasting and loading your code doesn't throw an error. Please, pastebin an example that demonstrates the error.

-- Jason Dusek