parallel longest palindrome checker

pali.hs

import           Control.Parallel.Strategies
import           Data.List                   (maximumBy, subsequences)
import Data.Ord

isPalindrome :: Eq a => [a] -> Bool
isPalindrome xs = xs == reverse xs

-- * note while subsequences is correct, it is asymptotically
-- inefficient due to nested foldr calls

getLongestPalindrome :: Ord a => [a] -> Int
getLongestPalindrome = length . maximum' . filter isPalindrome . subsequences
  where maximum' :: Ord a => [[a]] -> [a]
        maximum' = maximumBy $ comparing length

--- Do it in parallel, in a monad
-- rpar rpar seems to fit your case, according to Simon Marlow's book
main :: IO ()
main = do
  let shorter = [2,3,4,5,4,3,2]
      longer  = [1,2,3,4,5,4,3,2,1]
      result = runEval $ do
                 a <- rpar $ getLongestPalindrome shorter
                 b <- rpar $ getLongestPalindrome longer
                 if a > b
                 then return (a,"a")
                 else return (b,"b")
  print result

-- Don't forget to compile w/ -threaded and use ThreadScope to check
-- performance and evaluation