Hi, I had this crazy idea a couple of nights ago. I didn't dare to ask in the haskell-cafe because I fear it could be too stupid. The xmonad community is smaller (even though the greatest haskeller I know about read and write xmonad code: so posting here could be even worse...;-) so I'm posting here. It is vaguely related to XMonad after all. XMobar, the status bar I'm writing for XMonad, now runs monitors internally. You set monitors and external commands in the configuration file. I'm thinking of writing also command line options, but the possibility of using a readable data type for reading a configuration file can make users' life easier, I think, even though there are no debugging messages and that can be confusing (but you can test the configuration with ghc!). Now, in the configuration there is this commands option, a list of tuples: the data type for commands and an Int, the interval for each successive runs. The data type for the commands was suggested by Spencer. Very simply: data Command = Exec ProgramName Args | Network Interface Args etc., where Network is a constructor to be used for running the network interface monitor. Then there is a class (Exec - not much fantasy, I know) for which a method (run :: e -> IO String) must be defined. So there are all the needed instances for running, showing, etc, etc. Now, I though that writing a plugin should just mean creating a type and making it an instance of Exec. XMobar takes the list of the commands to be run from the configuration "commands" option, so that's easy: just a polymorphic list of commands to be run and I don't need anything else for supporting plugins...;-) Cool I though, it's just a matter of creating the needed polymorphic list. With one more constrain: Config, that has that list inside, must be readable! Ok, this is not an easy task, since existentially qualified types are not derivable, and reading them seemed not even possible.[1] Writing a too complicated parser for a Read instance is not an option. Now, thanks to the help of Claus Reinke I came up with a solution for that too.[2] The code[3] works! You can feed readConfig with a file like this: MD {rec1 = "Ciao", rec2 = "Ciao Ciao", rec3 = [MT (T3a,1),MT (T2b,3),MT (T1b,3),MT (T1a,3)]} and it reads it! Now I would like to ask you if such an approach could be tried or is just insane. I mean, would you run a status bar that, to configure with you favorite plugin (type HelloWorld = HelloWorld [] []), requires you to write in Config.hs something like: hidden = undefined :: (Commands,(HelloWorld,())) I don't know if I would. Let me know your thoughts please, in exchange I'll chase and try to fix as many XMobar bugs as I possibly can ...;-) All the best Andrea [1] According to this thread: http://www.haskell.org/pipermail/haskell-cafe/2004-April/006092.html [2] Look here: http://www.haskell.org/pipermail/haskell-cafe/2007-July/028205.html http://www.haskell.org/pipermail/haskell-cafe/2007-July/028227.html [3] the code {-# OPTIONS -fglasgow-exts #-} module Test where import Control.Monad import Text.Read import Text.ParserCombinators.ReadPrec data MyData = MD { rec1 :: String , rec2 :: String , rec3 :: [MyType] } deriving (Read,Show) data MyType = forall e . (MyClass e,Show e, Read e) => MT (e,Int) instance Show MyType where show (MT a) = "MT " ++ show a class MyClass c where myShow :: c -> String data TipoA = T1a | T2a | T3a deriving(Show,Read,Eq) instance MyClass TipoA where myShow T1a = "t1a" myShow T2a = "t2a" myShow T3a = "t3a" data TipoB = T1b | T2b | T3b deriving(Show,Read,Eq) instance MyClass TipoB where myShow T1b = "t1b" myShow T2b = "t2b" myShow T3b = "t3b" -- tricky part starts here -- the solution comes from Claus Reinke class ReadAsAnyOf ts ex where -- read an existential as any of hidden types ts readAsAnyOf :: ts -> ReadPrec ex instance ReadAsAnyOf () ex where readAsAnyOf ~() = mzero instance (Read t, Show t, MyClass t, ReadAsAnyOf ts MyType) => ReadAsAnyOf (t,ts) MyType where readAsAnyOf ~(t,ts) = r t `mplus` readAsAnyOf ts where r t = do { m <- readPrec; return (MT (m `asTypeOf` (t,0))) } instance Read MyType where readPrec = readMT readMT :: ReadPrec MyType readMT = prec 10 $ do Ident "MT" <- lexP parens $ readAsAnyOf hidden -- r T1a `mplus` r T1b hidden = undefined :: (TipoA,(TipoB,())) readConfig :: FilePath -> IO MyData readConfig f = do s <- readFile f case reads s of [(config,_)] -> return config [] -> error ("Corrupt config file: " ++ f) _ -> error ("Some problem occured. Aborting...")