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

[Gregory Crosswhite]

> 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