Syntactic naturality seems like it's mostly a function of familiarity. That said, the example has, for a lot of mathematical reasons, a great deal of semantic n

Syntactic naturality seems like it's mostly a function of familiarity. That said, the example has, for a lot of mathematical reasons, a great deal of semantic naturality. It's far from immediately obvious, but `foldr (:) []` is the way to copy singly-linked lists. In particular, if you look at a linked list as a degenerate tree then what `foldr f z` does is replace all of the nodes with `f` and the single leaf with `z`. Since cons, i.e. (:), creates a node and nil, i.e. [], is that single leaf then `foldr (:) []` is a very natural way to do nothing at all. So it's kind of the most boring interesting example imaginable.