Re: [Haskell-beginners] lazy mapM

I'm not sure I understand what you mean by "I know you have the best intentions in writing this, but there are pitfalls.". Anyway, here's the code which doesn't work apparently because mapM is waiting for the whole list before it goes further. prompt = ">> " commands :: [IO String] commands = readCommand : commands where readCommand = putStr prompt >> getLine display :: Show a => [ a ] -> IO () display = mapM_ $ putStr . show executeCommand :: String -> String executeCommand = printf "Command not implemented: '%s'" processCommands :: [IO String] -> IO [ String ] processCommands = mapM processOneCommand where processOneCommand cmd = cmd >>= (return . executeCommand ) main = hSetBuffering stdout NoBuffering >> processCommands commands >>= display This is just for learning purposes and I'm looking for the "haskell way to do it". My intention is to write the function processCommands such that it takes the decision to either fetch the next command from the command list (i.e. console) or to exit the application. Regarding your comment "Just know that at some point you should learn to use conduits or pipes for a much better approach to modeling things like this.". Can you point me to some documentation? Thanks! On Mon, Apr 1, 2013 at 3:53 AM, David McBride wrote:

I know you have the best intentions in writing this, but there are pitfalls. Unexpected things happen when you interleave IO in this manner, but nonetheless, here's how you would do it.

myGetLine = do x <- getLine if (x == "exit") then return [] else do xs <- unsafeInterleaveIO myGetLine return (x:xs)

main = do x <- myGetLine print x

Just know that at some point you should learn to use conduits or pipes for a much better approach to modeling things like this.

On Sun, Mar 31, 2013 at 7:26 PM, Ovidiu D wrote:

...

Hi again,

Given the following code:

g :: IO String -> IO String

f :: [IO String] -> IO [ String ] f = mapM g

The implementation of f is wrong because I would like to: 1. Make f behave lazy Its input list is made of lines read from stdin and I want it to process lines one by one as they are entered by the user.

2. Implement f such that it stops consuming items from the input list when the input item meets some condition. For example: isExit item = ("exit" == item)

I tried to implement my own custom iteration by recursion but I got stuck in the combination of IO and list monads.

Any help is appreciated.

Thanks!

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