ReadRegex: Raise correct error for empty char range (see #1)

Author: andreasabel

lib/Text/Regex/TDFA/ReadRegex.hs

@@ -15,8 +15,8 @@ import Text.ParserCombinators.Parsec((<|>), (<?>),
   sepBy1, option, notFollowedBy, many1, lookAhead, eof, between,
   string, noneOf, digit, char, anyChar)
 
-import Control.Applicative (asum)
 import Control.Monad (liftM, guard)
+import Data.Foldable (asum)
 import qualified Data.Set as Set(fromList)
 
 -- | An element inside @[...]@, denoting a character class.