On 07/08/10 19:42, John Dlugosz wrote:
So you give it two functions, one to do on the way down and another to do on a the way up? Yes. However, I'm not sure if the way-up function is the NowUp or Else template parameter. I'd have to look at the backus_turingaward_lecture.pdf reference to see which is which.
My own effort at this, straddling the compile-time and run-time boundaries, is posted on a new topic. I can't get the necessarily run-time test for done-ness to play nice with the compile-time metafunctions that are "run" (expanded) regardless of what might happen at run time.
I'm sure it's a documentation problem with Fusion. There is no guarantee that the 'end' iterator evaluates to a unique type that won't happen to match the iterator's type inside the normal work. For a cons (Lisp-like) list I'm rather certain that it will be, unless he's using some kind of type erasure in the implementation, but I don't want to make that assumption, and even if that were taken, it seems difficult to accomplish (compared with simply knowing a simple name for a type that could be used in a partial specialization or overloaded form). I'm assuming that there is intended to be some simple and natural way to iterate a collection other than using the canned fold or for_each functions! [snip]
John, I think I had a similar problem and didn't find a good answer. The problem was posted in a thread to this list whose last post was: http://thread.gmane.org/gmane.comp.lib.boost.user/58611/focus=58612 Anyway, the attached fusion::if_recur does what I was thinking as a solution. The test driver is also attached with the output. The test driver uses: http://svn.boost.org/svn/boost/sandbox/variadic_templates/boost/iostreams/ut... The output shows the reverse of the input going up the recursion stack and that's why I thought the if_recur might provide the "reverse processing" that you mentioned in post: http://article.gmane.org/gmane.comp.lib.boost.user/60210 However, the attached fusion::if_recur suffers the problem that the result_type is fixed by the ElseBtm::result_type. I'm not sure this can be used to solve the problem of reversing a heterogeneous fusion sequence since the result_type would have to be the reverse of the original type. Anyway, hope this helps. -regards, Larry /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_4v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.exe show_default=9 increment:state=0 increment:state=1 increment:state=2 increment:state=3 print_btm:state_btm=4 print_pair:state_now=3:state_up=4 print_pair:state_now=2:state_up=4 print_pair:state_now=1:state_up=4 print_pair:state_now=0:state_up=4 show_less_increment=4 Compilation finished at Sat Jul 10 08:20:10