pedantic stuff:

typo here:

  seq (f, Maybe df) (g, dg) t

s/Maybe/Just/

and `par` should be

  par :: Anim a -> Anim b -> Anim (a, b)
  par (f, df) (g, dg) = (\t -> (f t, g t), max <$> df <*> dg)

---

i'm not sure about `par` - if my understanding of Anim is right, it'll take the shorter animation over its specified duration. what if it was something like:

  par a@(_, da) b@(_, db) = (ab, max <$> da <*> db)
    where ab t = (a `at` t, b `at` t)
  
  
  at :: Anim a -> Duration -> a
  at (f, Nothing) t = f t
  -- freeze when t exceeds dur, i.e. the animation is done
  at (f, Just dur) t = f (min dur t)

might be less elegant but i'd expect the combinator to respect an animation's duration.

or is that not what the duration represents? i.e. is this correct:

  stretch :: Float -> Anim a -> Anim a
  stretch factor (f, dur) = (\t -> f (t / factor), (factor *) <$> dur)