Re: [Haskell-cafe] applicative challenge

half-assed state

for a real state solution, there's follow up here: http://groups.google.com/group/haskell-cafe/browse_thread/thread/d6143504c0e... 2009/5/5 Thomas Hartman :

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interact (\s -> let (first,second) = span (not . null) (lines s)               in unlines ("first":first++"second":takeWhile (not.null) second))

So, that didn't quite do the right thing, and it seemed like using span/break wouldn't scale well for more than two iterations. Here's another attempt, which is a little closer I think, except that it seems to be using some sort of half-assed state without being explicit about it:

module Main where

t17 = interact f17 f17 s = let (first,rest) = grabby s            (second,_) = grabby rest        in "first\n" ++ first ++ "second\n" ++ second

grabby :: String -> (String,String) grabby s =  let (beg,end) = break null . lines $ s  in (unlines beg, (unlines . drop 2 $ end))

2009/5/5 Ketil Malde :

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Thomas Hartman writes:

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That's slick, but is there some way to use interact twice in the same program?

No :-)

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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) -}

Yes. Interacting uses hGetContents, and hGetContents semi-closes (or fully-closes) the handle.  If you do it from GHCi, you only get to run your program once.

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I also tried

t15 =   let grabby = unlines . takeWhile (not . blank) . lines       top = ("first time: " ++) . grabby . ("second time: " ++) . grabby   in  interact top

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but that didn't work either: thartman@ubuntu:~/haskell-learning/lazy-n-strict>runghc sequencing.hs a first time: second time: a b b

Well - the input to the leftmost grabby is "second time" prepended to the input from the first, and then you prepend "first time" - so this makes sense.

Something like this, perhaps:

interact (\s -> let (first,second) = span (not . null) (lines s)                in unlines ("first":first++"second":takeWhile (not.null) second))

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If someone can explain the subtleties of using interact when you run out of stdio here, it would be nice to incorporate this into

hGetContents - there can only be one.

-k -- If I haven't seen further, it is by standing in the footprints of giants