Re: [Boost-users] [mpl] Fusion/MPL examples

From: Zeljko Vrba (zvrba_at_[hidden])
Date: 2007-06-06 12:34:34

On Wed, Jun 06, 2007 at 10:52:28AM -0400, David Abrahams wrote:
> 
> By "down-to-earth" I suppose you mean "like the problem I'm trying to
> solve?"
> 
No.  I mean examples other than implementing DSLs.  I can compare it to
the first time I learned about pointers from Wirth's Pascal book: he
introduces pointers, shows how to allocate a variable on the heap and
how to access it, but.. I kept wondering about what pointers are *useful*
for all the way until the example with a linked list.  [I was _very_
young then, so I have also wondered about what the linked list was good
for, but.. :)]

> >
> > I'm in the process of writing a recursive data structure with very
> > similar operations at every level, but still with small behavioral
> > variations.  It seems like a perfect problem for metaprogramming,
> 
> Why?
> 
Hm, good question.  I might be prejudicing the solution.  Because I have
a recursive algorithm and data structure that I'd like to "unroll" at
compile-time.

> 
> Maybe because that's not the most appropriate approach for your
> problem.  Did you go through the book's criteria for when
> metaprogramming might apply, in section 1.1.6?
> 
Yes.  So, to summarize the questions 1-5:

1. Abstractions of the problem domain: there is no 'domain'.  It's a single
   specialized data structure and associated code (hierarchical scheduler).
   The scheduling algorithm is the same at each level in the hierarchy
   (+ a special case at the root).

2. No large amounts of boilerplate code.  Just (almost) duplicated code for
   three (almost) identical classes.

3. I don't need to choose component implementations.

4. Yes, I do want strong type-checking and behavioral customization
   (different data structures in scheduler nodes).

5. Yes, I want to do it all in C++.

>
> I have been working on something that sounds similar to what you're
> doing, but I'm using basic generic programming to do it.  Have you
> seen http://lafstern.org/matt/segmented.pdf ?
>
No, I'll take a look.  Thanks.  It may also be the case that I'm missing
the boundary between generic programming and metaprogramming.

>
> would be of much help to you.  If you're not specifically trying to
> raise the abstraction level of your client's code to the point where
> they can be seen to be using a DSEL, you probably don't want
>
No, I'm not trying to do that.  I'm trying to refactor the code so that
it becomes "manifestly correct" when read together with a high-level
description of the algorithm.

Best regards,
  Zeljko.