import System.Environment
import System.Directory
import System.IO
import Data.List

type EnvKey = String
type EnvValue = String

data EnvFile =
    End
    | Entry EnvKey EnvValue EnvFile
    deriving (Show,Eq,Read)

main = do
    let f = (Entry "foo" "bar" End)
    putStrLn (show f)

setEntry :: EnvFile -> EnvKey -> EnvValue -> EnvFile
setEntry env key value = let env'=(removeEntry env key) in
                            (Entry key value env')

removeEntry :: EnvFile -> EnvKey -> EnvFile
removeEntry End _ = End
removeEntry (Entry entryKey entryValue rest) key = if entryKey==key
                                                    then rest
                                                    else (Entry entryKey entryValue (removeEntry rest key))

getEntry :: EnvFile -> EnvKey -> Maybe EnvValue
getEntry End _ = Nothing
getEntry (Entry entryKey entryValue rest) key = if entryKey==key
                                                    then return entryValue
                                                    else (getEntry rest key)

replaceEnvFile :: EnvFile -> IO ()
replaceEnvFile env = do
    path <- getEnvFilePath
    (tempName, tempHandle) <- openTempFile "/tmp" ".fenv"
    hPutStr tempHandle (show env)
    hClose tempHandle
    removeFile path
    renameFile tempName path

getEnvFile :: IO EnvFile
getEnvFile = do
    path <- getEnvFilePath
    contents <- readFile path
    return $ read contents

getEnvFilePath :: IO FilePath
getEnvFilePath = do
    home <- getHomeDirectory
    return (home ++ "/.fenv")