Re: [boost] Re: Re: MPL containers and algorithms

From: David Abrahams (dave_at_[hidden])
Date: 2002-07-29 14:28:14

From: "Andrei Alexandrescu" <andrewalex_at_[hidden]>

> > Until very recently, MWCW had a hard-coded limit on template nesting
> depth.
> > If you want to support CWPro 7.2, you might need to use long type
vectors
> > in lieu of type lists just to avoid running into those limits. You can
do
> > as much loop unrolling as you want to; it won't help if you can't even
> > represent your inputs or results.
>
> Let's think we did not need to support bad compilers for a second. Would
> separation of data structures and algorithms still be a design decision
for
> MPL?

Weren't we just talking about how wicked good that compiler was <wink>?
Answer, regardless: yes.

> > > An s-expression is a tree alright. You wouldn't have to flatten it in
> > order
> > > to process it. LISP provides many examples of elegant tree
processing,
> > and
> > > all it uses as data structures is the dot list.
> >
> > Yes, but you need to flatten it in order to *treat it as a sequence* if
> all
> > of your sequence algorithms are hard-coded to work on slists.
>
> I'm completely at a loss. S-expressions *are* trees.

I know you're awfully smart, so I can only imagine you're playing dumb
here. Let me try again to illustrate:

Take any of your favorite typelist algorithm, e.g. the one that finds the
length of a sequence.
Now, without rewriting it, find the number of elements in a tree (not a
trivial, linear tree such as

    typelist<x, typelist<y, typelist<z, null> > >

but something that's roughly balanced).

Now, pick another algorithm, say one that finds the largest type in a
sequence. Use that to find the largest element in a balanced tree.

> > Hmm, either way you now you have support for different data
structures...
>
> It's a wonder LISP programmers never felt the need for that... and they
do
> trees all the time.

Which lisp implementation are you talking about? I admit I'm not a bigtime
lisp guy, but the ones I know of all have vectors... oh, and properties,
too. That's another thing that moves the data structures away from pure
S-expressions.

-Dave