For a dynamic language like arc multi dispatch will have an efficiency cost.
I don't think the efficiency cost for dynamic dispatch on the first argument would be significantly different, and I'm not worried about either one. I think they're at worst as inefficient as a sequence of if statements, which is to say they would take constant time (given a constant number of methods to check and constant time for each check).
Constant time can still be painful in the right quantity, and making the decision at compile time would indeed help, but Arc doesn't have the necessary static type information to go on (as you know). So multiple dispatch and single dispatch alike have to suffer a bit.
It is one of the reasons people hate clos.
Well, I've heard nothing but good things about CLOS, but then I haven't heard that much. ^_^ Is there some good anti-CLOS reading material you can recommend? XD
In what way is multi dispatch better then the alternatives?
It's partly a matter of name compression. ^_-
; no dispatch
(collide-spaceship-with-spaceship s s)
(collide-spaceship-with-asteroid s a)
(collide-asteroid-with-spaceship a s)
(collide-asteroid-with-asteroid a a)
; single dispatch (on the first argument)
(collide-with-spaceship s s)
(collide-with-asteroid s a)
(collide-with-spaceship a s)
(collide-with-asteroid a a)
; double dispatch
(collide a s)
(collide s a)
(collide a s)
(collide a a)
Yeah, I stole Wikipedia's example here. Another example is a comparison function that needs to compare values of many different types. Yet another is a "write" function which has different behavior for each type of writer object and each type of object to be written.
When it comes right down to it, I just believe multiple dispatch ought to be a simpler concept than single dispatch, since it imposes fewer restrictions on the programmer. Unfortunately, it seems to replace one arbitrary restriction with a bunch of (what I see as) arbitrary design decisions. ^_^;
For every one of your examples,
we can use double dispatch design pattern;
For example:
(collide o1 o2) call
(collide-with-spaceship o2 o1) if o1 is a spaceship
or (collide-with-asteroid o2 o1) if o1 is an asteroid.
I think that you can even abstract this design pattern
with a macro (but I did not try it).
So you still did not convince me that multi dispatch is
a good feature.
