How to exit a loop in Haskell?

Question

The current version of the Pipes manual uses the following two functions in one example:

stdout :: () -> Consumer String IO r stdout () = forever $ do str <- request () lift $ putStrLn str stdin :: () -> Producer String IO () stdin () = loop where loop = do eof <- lift $ IO.hIsEOF IO.stdin unless eof $ do str <- lift getLine respond str loop 

As indicated in the tutorial itself, P.stdin is a bit more complicated due to the need to check the end of input.

Are there any good ways to rewrite P.stdin so as not to require a manual tail recursive loop and use larger order flow controllers like P.stdout? In an imperative language, I would use a structured while or break loop to do the same:

 while(not IO.isEOF(IO.stdin) ){ str <- getLine() respond(str) } forever(){ if(IO.isEOF(IO.stdin) ){ break } str <- getLine() respond(str) } 

Answer 1

Looks like whileM_ :

 stdin () = whileM_ (lift . fmap not $ IO.hIsEOF IO.stdin) (lift getLine >>= respond) 

or using do-notation similarly to the original example:

 stdin () = whileM_ (lift . fmap not $ IO.hIsEOF IO.stdin) $ do str <- lift getLine respond str 

The monad-loops package also offers whileM , which returns a list of intermediate results instead of ignoring re-action results and other useful combinators.

Answer 2

I prefer the following:

 import Control.Monad import Control.Monad.Trans.Either loop :: (Monad m) => EitherT ema -> me loop = liftM (either id id) . runEitherT . forever -- I'd prefer 'break', but that in the Prelude quit :: (Monad m) => e -> EitherT emr quit = left 

You use it as follows:

 import Pipes import qualified System.IO as IO stdin :: () -> Producer String IO () stdin () = loop $ do eof <- lift $ lift $ IO.hIsEOF IO.stdin if eof then quit () else do str <- lift $ lift getLine lift $ respond str 

See this blog post for an explanation of this technique.

The only reason I don’t use this in a tutorial is because I find it less convenient for beginners.

Answer 3

Since there is no implicit stream, there is no such thing as a “break”. In addition, your sample is already a small block that will be used in more complex code.

If you want to stop "line creation" this should be supported by your abstraction. That is, some “control” of “pipes” using a special monad in Consumer and / or other monads related to this.

Answer 4

You can simply import System.Exit and use exitWith ExitSuccess

Eg. if (input == 'q') then exitWith ExitSuccess else print 5 (nothing)