map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn] map f [x1, x2, ...] == [f x1, f x2, ...]
>>> map (+1) [1, 2, 3] [2,3,4]
map $ span $ toHtml "foo"
<map><span>foo</span></map>
toList (map f xs) = map f (toList xs)
toNonEmpty (map f xs) = map f (toNonEmpty xs)