test

test.hs

{-# LANGUAGE OverloadedStrings #-}
import Lengthable
import Prelude hiding (length)
import qualified Data.Text
import Data.Text(Text(..))

text :: Text
text = "abcdefg"

main = do print $ length [1,2,3]
          print $ length text

---- lengthable.hs
module Lengthable where
import qualified Data.Text
import qualified Data.List

class Lengthable a where
  length :: a -> Int

instance Lengthable Data.Text.Text where
  length = Data.Text.length
  
instance Lengthable [a] where
  length = Data.List.length