MyPath.hs

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeInType #-}
module MyPath where

import Data.Maybe
import Data.Typeable
import GHC.TypeLits (ErrorMessage(..), TypeError)
import System.Directory

parseAbs :: FilePath -> Maybe (Path Abs)
parseAbs = pure . Path -- simplified
parseRel :: FilePath -> Maybe (Path Rel)
parseRel = pure . Path
parseFn :: FilePath -> Maybe (Path Fn)
parseFn = pure . Path


data Path (a :: PathType) = Path FilePath
           deriving (Show, Eq, Ord, Typeable)

data PathType = Abs
              | Rel
              | Fn
              | Any


type family UpCast (from :: PathType) :: PathType where
  UpCast 'Abs = 'Abs
  UpCast 'Rel = 'Rel
  UpCast 'Fn  = 'Rel
  UpCast 'Any = TypeError ('Text "Any cannot be upcasted")


(</>) :: (UpCast r ~ 'Rel) => Path b -> Path r -> Path (UpCast b)
(</>) (Path a) (Path b) = Path (a ++ "/" ++ b) -- simplified