That's slick, but is there some way to use interact twice in the same program?
t10 =
let f = unlines . takeWhile (not . blank) . lines
in do putStrLn "first time"
interact f
putStrLn "second time"
interact f
this results in *** Exception: <stdin>: hGetContents: illegal
operation (handle is closed) -}
I also tried
t15 =
let grabby = unlines . takeWhile (not . blank) . lines
top = ("first time: " ++) . grabby . ("second time: " ++) . grabby
in interact top
but that didn't work either:
thartman@ubuntu:~/haskell-learning/lazy-n-strict>runghc sequencing.hs
a
first time: second time: a
b
b
If someone can explain the subtleties of using interact when you run
out of stdio here, it would be nice to incorporate this into
http://www.haskell.org/haskellwiki/Haskell_IO_for_Imperative_Programmers#IO
where it talks about how using interact is the easy way to approach
these types of problems. Not *that* easy though, as this scenario
suggests.
2009/5/5 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