Here's another approach that more closely models what's going on in the C++
version. I defined an ordNub rather than using nub as nub is O(n^2) as it
only requires Eq.
https://gist.github.com/etrepum/5bfedc8bbe576f89fe09
import qualified Data.Set as S
import Data.List (partition)
import System.Environment (getArgs)
data LWG = LWG { _lion, _wolf, _goat :: {-# UNPACK #-} !Int }
deriving (Show, Ord, Eq)
lionEatGoat, lionEatWolf, wolfEatGoat :: LWG -> LWG
lionEatGoat (LWG l w g) = LWG (l - 1) (w + 1) (g - 1)
lionEatWolf (LWG l w g) = LWG (l - 1) (w - 1) (g + 1)
wolfEatGoat (LWG l w g) = LWG (l + 1) (w - 1) (g - 1)
stableState :: LWG -> Bool
stableState (LWG l w g) = length (filter (==0) [l, w, g]) >= 2
validState :: LWG -> Bool
validState (LWG l w g) = all (>=0) [l, w, g]
possibleMeals :: LWG -> [LWG]
possibleMeals state =
filter validState .
map ($ state) $ [lionEatGoat, lionEatWolf, wolfEatGoat]
ordNub :: Ord a => [a] -> [a]
ordNub = S.toList . S.fromList
endStates :: [LWG] -> [LWG]
endStates states
| not (null stable) = stable
| not (null unstable) = endStates (concatMap possibleMeals unstable)
| otherwise = []
where (stable, unstable) = partition stableState (ordNub states)
main :: IO ()
main = do
[l, w, g] <- map read `fmap` getArgs
mapM_ print . endStates $ [LWG l w g]
On Sat, Jun 7, 2014 at 11:33 PM, Francesco Ariis
On Sat, Jun 07, 2014 at 08:04:09PM -0400, Elric wrote:
Hi,
I came across this article: http://unriskinsight.blogspot.co.at/2014/06/fast-functional-goats-lions-and-... a couple of days ago. This compares performance of solving a problem (which I will get to) using the functional constructs alone in languages like C++11 and Java 8. Since, Haskell is my first foray into FP, I thought I should try solving this in Haskell.
Hello Elric, I gave a go at the problem, managed to get a result (23). I attach the .hs file (not my best Haskell, but hopefully clear enough).
The crucial point in my solution lies in this lines:
carnage :: [Forest] -> [Forest] let wodup = nub aa in -- etc. etc.
Which means after every iteration I call |nub| on my list of possible states; nub is a function from |Data.List| and removes duplicate elements from a list.
If I omit that nub call, the program doesn't reach a solution (as it is computationally quite inefficient). I think that's the problem with your versions.
Let me know if this helps
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