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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