Demo code for https://noamlewis.wordpress.com/2015/02/19/ad-hoc-polymorphism-type-families-ambiguous-types-and-javascript/

tf.hs

{-# OPTIONS -Wall #-}
{-# LANGUAGE TypeFamilies, FlexibleInstances #-}

import Data.Map

class Indexable a where
  type Index a
  type Result a
  index :: a -> Index a -> Result a

instance Indexable [a] where
  type Index [a] = Int
  type Result [a] = a
  index = (!!)

newtype Str = Str { getStr :: String }

instance Indexable Str where
  type Index Str = Int
  type Result Str = Char
  index = (!!) . getStr

instance Indexable (Map String a) where
  type Index (Map String a) = String
  type Result (Map String a) = a
  index = (!)

f :: (Indexable a, Index a ~ Int) => a -> Result a
f x = index x (0::Int)