import System.Environment
import System.Directory
import System.IO
import System.Exit (exitSuccess,exitFailure)
import Data.List

type EnvKey = String
type EnvValue = String

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

main = do
    args <- getArgs
    cliRun args

cliRun :: [String] -> IO ()
cliRun ("get":args) = cliGet args
cliRun ("set":args) = cliSet args
cliRun ("unset":args) = cliUnset args
cliRun ("list":_) = cliList
cliRun _ = cliFail "USAGE: fenv <get|set|unset> [...args]"

cliGet :: [String] -> IO ()
cliGet (name:_) = do
    env <- getEnvFile
    case (getEntry env name) of
        (Just value) -> putStrLn value
        Nothing -> cliFail "ERROR: No value defined"
cliGet _ = cliFail "USAGE: fenv get <name>"

cliUnset :: [String] -> IO ()
cliUnset (name:_) = do
  env <- getEnvFile
  let env' = (removeEntry env name) in
    replaceEnvFile env'
cliUnset _ = cliFail "USAGE: fenv unset <name>"

cliSet :: [String] -> IO ()
cliSet (name:value:_) = do
  env <- getEnvFile
  let env' = (setEntry env name value) in
    replaceEnvFile env'
cliSet _ = cliFail "USAGE: fenv set <name> <value>"

cliList :: IO ()
cliList = do
  env <- getEnvFile
  printLevel env
  where
    printLevel (Entry key value rest) = putStrLn (key ++ ": " ++ value) >>= (printLevel rest)
    printLevel End = return ()

cliFail :: String -> IO ()
cliFail message = putStrLn message >>= return exitFailure

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
    if contents == "" then return End
    else return $ read contents

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