-- for IdempotentIO
import Control.Monad (when)

-- for Link
import Control.Monad.Loops (andM)
import System.Posix.Files (fileExist, getSymbolicLinkStatus, isSymbolicLink, readSymbolicLink, createSymbolicLink)
import System.Directory (getHomeDirectory, getCurrentDirectory)


class IdempotentIO thing where
  check       :: thing -> IO Bool
  set, ensure :: thing -> IO ()
  ensure thing = do
    isSet <- check thing
    when (not isSet) $ set thing


data Link = Link
  { fromPath :: String
  , toPath :: String
  , qualifyPath :: String -> String
  }


instance IdempotentIO Link where

  check link = andM
    [ fileExist $ qualified fromPath
    , fmap isSymbolicLink . getSymbolicLinkStatus $ qualified fromPath
    , fmap (== qualified toPath) . readSymbolicLink $ qualified fromPath
    ]
    where qualified path = qualifyPath link $ path link

  set link = createSymbolicLink (qualified toPath) (qualified fromPath)
    where qualified path = qualifyPath link $ path link


main = ensure $ Link "foo" "bar" id