[PRL] Re: isomorphic fixed points

[Peter R. Douglass]

Dale has pointed out a mistake in my previous email.

 > The set of all reals is not a
 > fixed point of the Maybe functor, and ...

  Actually, the set of reals is a fixed point of the Maybe functor (in 
Set).  Witness:

f ::  Maybe Real -> Real

f Nothing = 0
f Just n (where n is numerically equal to a Natural) = n + 1
f Just x = x

g :: Real -> Maybe Real

g 0 = Nothing
g n (where n is numberically equal to a positive Natural = Just (n - 1)
g x = Just x


f . g = id(Real)
g . f = id(Maybe Real)


--PeterD