My main objection is the current state of arc.arc : Every iteration construct works with lists or strings, but it won't work well with something that has 'car and 'cdr (they check by using 'acons and 'alist, which use (is (type foo) 'cons) - meaning I had to hack into 'acons and 'alist. For that matter I had to hack into 'isa too)
Generally is-a means "object is this and only this", meaning single inheritance, which was really my meaning. But a scanner isn't a cons (it's a subset of cons; it will fail on scar and scdr). It does has-a 'car and has-a 'cdr.
I think the reason you keep saying has-a is that you're using conses as an example. A cons does has-a car and has-a cdr, but this is too restrictive. For example, say you wanted to make a type that acts like a list, that is, it supports map, each, reduce, all, rev, some, len, nthcdr, and so on.
arc> (= a (my-listtype 'foo nil t 'bar))
(foo nil t bar)
arc> (car a) ; has-a car
foo
arc> (cdr a) ; has-a cdr
(nil t bar)
arc> (map no a) ; has-a map (?!)
(nil t nil nil)
arc> (rev a) ; has-a rev (?!)
(bar t nil foo)
arc> (some no a) ; has-a some (?!)
t
arc> (all no a) ; has-a all (?!)
nil
See what I mean? (Note: I fully agree that it would be really cool if the above actually worked, I'm just arguing that the name has-a makes no sense here.)
has-a scanner interface just means that: it has-a car and has-a cdr. 'map et al. now require an object which has-a scanner interface, and will simply use basic 'car and 'cdr operations to work. This supports genericity: just write 'map et al once, then any new type you create just needs to give 'car and 'cdr, and say it has-a scanner interface, and the existing map will work.
Now suppose we have another type, which has-a 'collide function, which handles the events where it is collided with in the game space. A ship has-a collide function, and the basic collission detection code is something like this:
(thread
(while t
((afn (game-elements)
(iflet (first . rest) game-elements
(each e rest
(when (overlapping first e)
(collide first) (collide e))))
game-elements)))
The above now works on ships. Now suppose we add a new type of game element: a missile. We simply define its 'collide function, and declare it as having that function; now it magically works without changing the collision-detecting code.
