On 05/19/2010 09:48 PM, Michel Fortin wrote:
On 2010-05-19 19:01:51 -0400, Andrei Alexandrescu <seewebsiteforem...@erdani.org> said:I wrote a solution to the problem in native D. It goes like this: alias Container!(int, addable | purgeable) Messerschmidt; void messWith(Messerschmidt i) { ... use i's capabilities to add and purge ... }Are you sure this is necessary? I'm wondering how the above is different from: void messWith(C)(C i) if (IsAddable!C && IsPurgeable!C) { ... use i's capabilities to add and purge }
It's different in that messWith is a type-parameterized function (aka a template in C++) with the known tradeoffs: multiple instantiations, risk of code bloating, but good speed most of the time. Add to that that some people aren't comfortable with those.
where IsAddable just checks for an 'add' function and IsPurgeable checks for a 'purge' function. Obviously, one is a template and the other isn't. I'd expect the template function to be more performant since it doesn't require an indirection to call into the container. Is the need for runtime-swappable containers really common enough to justify adding it to the standard library? Won't adding this encourage people to use it without realizing the downside in performance and failed optimization opportunities because of the hidden dynamic dispatch? It's a quite nice idea, but I don't like the tradeoff.
These arguments are in line with mine. I tried above to convey that the situation is not all-win.
This criticism is valid for containers implementing interfaces too. In my first Java programs, I was always declaring variables as the List interface, then instantiating an ArrayList for them, thinking it'd make things more generic and easier to change later. Generic sometime is good, but if you do that with containers in D you're in for an important performance drop. Personally, I'd scrap anything that's not made of static calls (final functions in a class are fine) so people can't easily make these kind of mistakes (and then believe D is slow).
By the way, I dislike the name ArrayList. Is it just me, or "list" is most often associated with "linked list" in computer lingo? So when I see "ArrayList" it looks like an oxymoron. Steve, could I impose on you to rename ArrayList simply Array?
Also, addable and purgeable above being or'ed constants makes the system difficult to scale to new concepts. The template predicates on the other hand are infinitely extendable: if my containers have 'commit' and 'rollback' functions, I can define IsTransactional to check for the presence of the functions and make some algorithms that benefits from this. In fact, this can apply to anything, not just containers. Range are already using this pattern. Wouldn't it make things easier to learn if we could just reuse the same principle with containers?
Nice arguments! Andrei
