Re: [Haskell-beginners] How would you run a monad within another monad?

14 Apr 2009

      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