longestpalindrome.hs

#! /usr/bin/env nix-shell
#! nix-shell -p "haskellPackages.ghcWithPackages ( p: [p.aeson p.parsec])" 
#! nix-shell -p ghcid 
#! nix-shell -i runhaskell


{- problem: return the longest palindrome
-}

import System.Environment (getArgs)
import Control.Monad (forever)

isPalindrome :: String -> Bool 
isPalindrome s = s == reverse s

windows :: Int -> String -> [String]
windows n s | n > length s = []
            | n == length s = [s]
            | otherwise = take n s : windows n (tail s)

allWindows :: String -> [String]
allWindows s = concat windowsLists where 
    windowsLists = map (flip windows s) (reverse [1..length s]) 


longestPalindrome :: String -> String 
longestPalindrome = head . filter isPalindrome . allWindows 


-- IO functions

runOne :: (String -> String) -> IO ()
runOne f = do
    putStr "give me a string: " 
    str <- getLine
    putStrLn $ "longest palindrome: \"" ++ f str ++ "\""

runForever :: (String -> String) -> IO ()
runForever = forever . runOne

runWithArgs :: (String -> String) -> IO ()
runWithArgs f = do
    args <- getArgs 
    let pals = map f args 
    mapM_ putStrLn pals

-- runInteractive :: IO ()
runInteractive :: (String -> String) -> IO ()
runInteractive f = interact (unlines . map (show . f) . lines)

main :: IO ()
main = runInteractive longestPalindrome
-- main = runForever longestPalindrome``
-- main = runWithArgs longestPalindrome