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 = (-)

-- Fails:

v :: Delta Integer
v = diff 2 1

{- Error:

DeltaInt.hs:21:5:
    Couldn't match type `Delta a0' with `Integer'
    Expected type: Delta Integer
      Actual type: Delta a0
    In the return type of a call of `diff'
    In the expression: diff 2 1
    In an equation for `v': v = diff 2 1
Failed, modules loaded: none.
-}