Re: [Boost-users] Odd limitation in Fusion design ?

Subject: Re: [Boost-users] Odd limitation in Fusion design ?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2010-07-18 12:23:57

On 07/10/10 08:57, Larry Evans wrote:
> 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.
>  >>
[snip]
> 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/utility/indent_scoped_ostreambuf.hpp 
> 
[snip]
A revised version of this code is now in:

http://svn.boost.org/svn/boost/sandbox/variadic_templates/
   boost/fusion/algorithm/if_recur.hpp
   libs/fusion/sandbox/if_recur.cpp

The compilation and run output is in the attachment.

The indentation provided by indent_scoped_ostreambuf shows
the recursion depth pretty clearly.  The if_recur template
has another template parameter, the last one, which governs
whether the recursion is terminated on values or types.
The recursion termination on types was *much* harder to
implement.

I'm wondering if going up the recursion stack wouldn't
be faster if an explicit stack were used instead of the
way it's done in if_recur.  A nice side effect of the
explicit stack is the ability to short-circuit the
recursion as soon as a termination condition is met.
In contrast, with the example in if_recur.cpp, the
stack, since its implicit, must be unwound completely,
while a flag is used to tell the template functions,
specifically, the iter_find_target<*>::now_up<*>::call,
that the answer is already found.  In the example,
that flag is simply that IterUp is not the same as
IterEnd.

Anyway, HTH.

-regards,
Larry


###-*- mode: compilation; default-directory: "~/prog_dev/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/" -*-
Compilation started at Sun Jul 18 10:54:13

make run
install -d `dirname /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o`
/home/evansl/download/stlfilt/gfilt -compiler:/home/evansl/download/gcc/4.5-20100624/install/bin/g++ -c -Wall -ftemplate-depth-300 -O0 -g3 -fno-inline -std=gnu++0x -DCXX0X_VARIADIC_TEMPLATES -I/home/evansl/prog_dev/boost-svn/ro/sandbox/rw/variadic_templates -I/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/lje -I/home/evansl/prog_dev/boost-svn/ro/trunk  -DTEMPLATE_DEPTH=300  if_recur.cpp -MMD -o /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o 
COMPILE.cmd=/home/evansl/download/gcc/4.5-20100624/install/bin/g++   -c -Wall -ftemplate-depth-300 -O0 -g3 -fno-inline -std=gnu++0x -DCXX0X_VARIADIC_TEMPLATES -I/home/evansl/prog_dev/boost-svn/ro/sandbox/rw/variadic_templates -I/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/lje -I/home/evansl/prog_dev/boost-svn/ro/trunk -DTEMPLATE_DEPTH=300 if_recur.cpp -MMD -o /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o
BD Software STL Message Decryptor v3.10 for gcc 2/3/4
sed -e 's#^if_recur.o:#/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o:#' /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.d> /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o.dep
#rm /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.d
/home/evansl/download/gcc/4.5-20100624/install/bin/g++ -Wl,-rpath /home/evansl/download/gcc/4.5-20100624/install/lib64  /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.o -o /home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.exe
/home/evansl/prog_dev/boost-svn/ro/trunk/sandbox-local/build/gcc4_5v/boost-svn/ro/sandbox/rw/variadic_templates/libs/fusion/sandbox/if_recur.exe 
list_v=[int_val<-9>(-9), int_val<0>(0), int_val<-9>(-9), int_val<2>(2)]
target_v=int_val<-9>(-9)
  iter_next:
  iter_now=int_val<-9>(-9) int_val<0>(0) int_val<-9>(-9) int_val<2>(2) 
    iter_next:
    iter_now=int_val<0>(0) int_val<-9>(-9) int_val<2>(2) 
      iter_next:
      iter_now=int_val<-9>(-9) int_val<2>(2) 
        iter_next:
        iter_now=int_val<2>(2) 
          iter_btm:
          iter_now=
        now_up:
        iter_now=int_val<2>(2) 
        iter_up=
      now_up:
      iter_now=int_val<-9>(-9) int_val<2>(2) 
      iter_up=
    now_up:
    iter_now=int_val<0>(0) int_val<-9>(-9) int_val<2>(2) 
    iter_up=int_val<-9>(-9) int_val<2>(2) 
  now_up:
  iter_now=int_val<-9>(-9) int_val<0>(0) int_val<-9>(-9) int_val<2>(2) 
  iter_up=int_val<-9>(-9) int_val<2>(2) 
result_v=int_val<-9>(-9) int_val<2>(2) 

Compilation finished at Sun Jul 18 10:54:19