Joel de Guzman <joel@boost-consulting.com> writes:
David Abrahams wrote:
Whether results should be lazy (as in Fusion) or greedy, as in standard algorithms, or whether both variants should be provided somehow, are interesting questions. The answers change the usage pattern, i.e.: copy(transform(s1,op), output) or output = transform(s1,op) // if output supports it vs. transform(s1,op,output)
WRT Fusion, I am re-considering its laziness. MPL algorithms are sequence preserving. Fusion is not, due to laziness. i.e:
transform(tuple, op)
does not return a sequence like tuple. instead, it returns a view (transform_view).
However, I realize now, that in most use cases, I noticed that I had to eagerly generate the result into a tuple almost as immediately after each transform step.
Really; both the compile-time part (type) and the runtime part (value)?
After leaving this for a while in my thought queue, and after implementing phoenix2's lazy STL algorithms, I realized that the lazy Fusion algorithms could as well be implemented as phoenix2's lazy fusion algorithms.
Interesting. I'm not sure I understand what you mean yet, though.
If I implemented Fusion algorithms as non-lazy and sequence preserving like MPL, I could easily write lazy fusion algorithms through phoenix.
I'd be interested to see that.
I am still unsure about which is the best way to go. I just thought I'd share my recent thoughts on the issue.
Well, the use of segmented iterators and hierarchical algorithms (http://lafstern.org/matt/segmented.pdf) has some interesting implications. You can't just use iterator adaptors such as transform_iterator to implement laziness unless either: 1. You want to pay a big efficiency hit or 2. The adaptors expose the same "extended properties" (e.g. segmented_iterator_traits) as the iterators they're adapting. That requires knowledge of all extended properties. It seems to me that in order to do the traversal optimally, more than laziness is required: you'd have to build expression templates and then somehow analyze the traversal properties of the leaf sequences in order to evaluate the expressions. -- Dave Abrahams Boost Consulting www.boost-consulting.com