Why does this not work?

DeltaInt.hs

{-# LANGUAGE TypeFamilies #-}

module DeltaInt where

class Changing a where
  -- Using type families here is a bit convenient, but means risking trouble later.
  type Delta a
  nil :: a -> Delta a
  apply :: a -> Delta a -> a
  diff :: a -> a -> Delta a

instance Changing Integer where
  type Delta Integer = Integer
  nil = const 0
  apply = (+)
  diff = (-)

v :: Delta Integer
v = diff 2 1