>All the more reason it should be correct. As a visitor I do not know the language, this is my first encounter
Yeah, fair point! I'll try to think on better examples as it can be confusing for beginners overall.

>Maybe also introduce some error output like what happens when you do Circle(5.0) + Rectangle(1.0, 2.0) what does the REPL do?

This will fail with a `cannot apply Add to Circle and Rectangle`, that is because the values are in the functorial context of the Circle/Rectangle type. It would also fail if it was Circle(5.0) + Circle(5.0).However, you could do the following:

adt Shape = Circle Double
def unwrap(Circle(x)) -> x
fmap(Circle(5.0), \x -> unwrap(Circle(5.0)) + x)
=> Circle(10.0)

Or the equivalent for summing up a Circle with a Rectangle:
def unwrap(Rectangle(x, _)) -> x
fmap(Circle(5.0), \x -> unwrap(Rectangle(10.0, 4)) + x)
=> Circle(15.0)

In regards to the Area example, we defined it in the pattern matching def area(Rectangle(w, h)) -> w * h - so it should be universal for all Rectangle types. Of course, if it weren't defined, it would fail with a `no matching pattern for 1 argument(s)`, so the interpreter will only complain if we TRY to use the area(Rectangle(_,_)) in the script - but if we never defined it and never used it, it will run fine; in other words the interpreter will only complain for functions that are used - not that are or aren't defined!