On Wed, Feb 25, 2009 at 02:32, Patrick LeBoutillier
I'm having problems implementing the equivalent of this function in haskell. Inside a do block, is there a way to terminate the function immediately and return a result ("return" in the imperative sense, not the Haskell sense)? If not, must one really use deeply nested if/then/else statements to treat these special cases? All I could come up was this, which I find quite ugly:
For complex control flow continuation monad can be quite useful. But one must be careful not to abuse it. Code with heavy use of continuations can be very hard to follow and hard to debug as well. Here is an example: module Main where import Control.Monad.Cont checkErrors :: Int -> Maybe String checkErrors ident = (`runCont` id) $ do response <- callCC $ \exit -> do when (ident == 1) (exit . Just $ "Error! 1!") when (ident == 2) (exit . Just $ "Error! 2!") when (ident == 3) (exit . Just $ "Error! 3!") when (ident == 4) (exit . Just $ "Error! 4!") when (ident == 5) (exit . Just $ "Error! 5!") return Nothing return response main = forever $ getLine >>= \n -> print (checkErrors (read n)) It runs : $ ./callcc 0 Nothing 1 Just "Error! 1!" 5 Just "Error! 5!" 3 Just "Error! 3!" 2 Just "Error! 2!" 1 Just "Error! 1!" 9 Nothing 8 Nothing ^C Please read documentation on Control.Monad.Cont. There are more elaborate explanations there. All best Christopher Skrzętnicki