Re: [Haskell-cafe] Re: ANNOUNCE: GotoT-transformers version 1.0

11 Sep 2010

      Okay, where that unpost button when I need it...  :-)

So, I hadn't noticed that the script that I copied and pasted didn't 
even compile because I was pressing "up enter" at the console to run it, 
but had forgotten that I was now working with a script with a new name 
and so all I was doing was running a different script over and over 
again!  Doh.  :-)

Anyway, okay, I see your point now.  The following script *does* work.

   import Data.List

   import Control.Monad
   import Control.Monad.IO.Class
   import Control.Monad.Trans.Cont

    goto :: Monad m => ContT r m r -> ContT r m a
    goto (ContT m) = ContT $ \_ -> m return

   myComp :: ContT () IO ()
   myComp = do
     input <- liftIO $ putStrLn "Print something (y/n)?" >> getLine
     unless ("y" `isPrefixOf` input) $ goto exit
     liftIO $ putStrLn "Something."

     input <- liftIO $ putStrLn "Print more (y/n)?" >> getLine
     unless ("y" `isPrefixOf` input) $ goto exit
     liftIO $ putStrLn "More."

     where
       exit = do
         liftIO $ putStrLn "Ok, I'm exiting."
         return ()

    main :: IO ()
    main = runContT myComp return

*HOWEVER*, if we replace main with

     main = runContT myComp (const $ putStrLn "I can't wait to print 
this string!")

Then the program will be eternally disappointed because it will never 
actually get to print that string at the end.

On 9/11/10 6:16 PM, Gregory Crosswhite wrote:
...
...
To recover from my overly complex previous post, here is a much simply
goto based on existing monad transformers:
...
goto :: Monad m =>  ContT r m r ->  ContT r m a
goto (ContT m) = ContT $ \_ ->
                m return
That doesn't actually work, though.  Try running the following script:
import Data.List
import MonadLib
goto :: Monad m => ContT r m r -> ContT r m a
goto (ContT m) = ContT $ \_ -> m return
myComp :: ContT () IO ()
myComp = do
    input <- inBase $ putStrLn "Print something (y/n)?" >> getLine
    unless ("y" `isPrefixOf` input) $ goto exit
    inBase $ putStrLn "Something."
input <- inBase $ putStrLn "Print more (y/n)?" >> getLine
    unless ("y" `isPrefixOf` input) $ goto exit
    inBase $ putStrLn "More."
where
      exit = do
        inBase $ putStrLn "Ok, I'm exiting."
        return ()
main :: IO ()
main = runContT return myComp
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