On Fri, Dec 13, 2013 at 10:25 PM, Matt R <habari.zerglings@gmail.com> wrote:
Interesting to discover that not all polymorphic functions are natural transformations. "fix" seems rather exotic -- are there any less unusual examples?

None of the church numerals of type (a -> a) -> (a -> a) are natural transformations either:

zero _ = id
one f = f
two f = f . f
three f = f . f . f
etc.

-- Kim-Ee