[Haskell-cafe] applicative challenge

[Thomas Davie]

On 4 May 2009, at 23:15, Thomas Hartman wrote:

> {-# LANGUAGE NoMonomorphismRestriction #-}
> import Data.List
> import Control.Monad
> import Control.Applicative
>
> -- Can the function below be tweaked to quit on blank input,
> provisioned in the applicative style?
> -- which function(s) needs to be rewritten to make it so?
> -- Can you tell/guess which function(s) is the problem just by looking
> at the code below?
> -- If so, can you explain what the strategy for doing so is?
> notQuiteRight = takeWhile (not . blank) <$> ( sequence . repeat $  
> echo )
>
> echo = do
>          l <- getLine
>          putStrLn l
>          return l
>
>
> -- this seems to work... is there a way to make it work Applicatively,
> with lifted takeWhile?
> seemsToWork = sequenceWhile_ (not . blank) (repeat echo)
>
> sequenceWhile_ p [] = return ()
> sequenceWhile_ p (mx:mxs) = do
>  x <- mx
>  if p x
>    then do sequenceWhile_ p mxs
>    else return ()

Conor's already give you a comprehensive explanation of why  
Applicative can't be used to do this, but that doesn't mean you can't  
use applicative style!

How about...

echo = unlines . takeWhile (not . blank) . lines

seemsToWork = interact echo

Bob