I guess I'm totally confused about "annotate", then; I thought that (= y (annotate 'fn z)) would return #3(tagged mac #<procedure: f>) -- i.e., it would _replace_ the existing annotation, rather than _adding_ to it.
No; as nex3 observed, it's supposed to add it (cf. http://www.paulgraham.com/ilc03.html). Why? This is more general. Right now, we can define reptag to do what you want:
(def reptag (typ obj)
(annotate typ (rep obj)))
If we just had reptag, we couldn't define annotate.
Also, annotate obeys two useful identities:
(type (annotate x y)) --> x
(rep (annotate x y)) --> y
However, reptag does not:
def --> #3(tagged mac #<procedure>)
(rep (annotate 'fn def)) --> #3(tagged mac #<procedure>)
(rep (reptag 'fn def)) --> #<procedure>
Because of the type-replacing behaviour, that identity does not hold for tagged objects. I consider that a strike against it as well.
I think it's pretty clear that he intends option 1, as that's how it actually works, and as absz pointed out, is a strict superset of the functionality of option 2 (and has nicer properties, too).
