For what it's worth, I recently wrote a paper on what I call "polymorphic temporal media", of which "music" and "animation" are two examples. The basic data type is: data Media a = Prim a | Media a :+: Media a | Media a :=: Media a From this we can define a Music type: type Music = Media Note data Note = Rest Dur | Note Pitch Dur and an Animation type: type Animation = Media Anim type Anim = (Dur, Time -> Picture) and so on. It's then possible to define a polymorphic fold (i.e. a catamorphism) for the Media type: foldM :: (a->b) -> (b->b->b) -> (b->b->b) -> Media a -> b foldM f g h (Prim x) = f x foldM f g h (m1 :+: m2) = foldM f g h m1 `g` foldM f g h m2 foldM f g h (m1 :=: m2) = foldM f g h m1 `h` foldM f g h m2 and prove several standard laws about it, including: foldM (Prim . f) (:+:) (:=:) = fmap f foldM Prim (:+:) (:=:) = id and more importantly a Fusion Law, which states that if f' x = k (f x) g' (k x) (k y) = k (g x y) h' (k x) (k y) = k (h x y) then k . foldM f g h = foldM f' g' h' In the paper I use foldM to define a number of useful polymorphic functions on temporal media, such as a reverse function, a duration function, and most interestingly, a standard interpretation, or semantics, of polymorphic temporal media. I then prove some properties about these functions in which I avoid the use of induction by using the Fusion Law. Conceptually all of this is pretty "standard", actually, but used I think in an interesting context. If anyone would like a copy of the paper let me know. -Paul