From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2003-04-10 19:15:19
Aleksey Gurtovoy wrote:
> I wonder if our indispensable PP library could provide some kind of
> lambda facility that would allow one to do inline composition of
> macro functions, along the lines of the MPL (ideally) or Boost.Bind.
> For instance, instead of
> or something similar. I understand that it's quite a challenge to
> come up with such facility, but if we could, it would make an order
> of magnitude difference in the library's usability/power, IMO (if
> MPL's history is any indication).
The major problem with this is making it work on buggy preprocessors--which is
either not possible or so difficult that it wouldn't be worth it.
I'm working on a "strict" pp-lib that will be complete in 1-2 weeks. This
version of the pp-lib contains absolutely no hacks and therefore gives me a
great deal more freedom in what I can accomplish. This "strict" version is
already several magnitudes more powerful than the current CVS pp-lib--even
without lambda. As far as lambda is concerned, the facility _already_ exists in
the strict pp-lib.
There are three caveats: first, only two preprocessors, AFAIK, can handle the
strict pp-lib: wave and gcc. Not even EDG can handle it (and I'm not referring
to speed). Second, the strict pp-lib is not compatible with the CVS pp-lib in
many ways, and compatibility is not possible. Third, the parametric lambda
facility is implemented with variadics and placemarkers, which are part of C99,
but not C++. This was a deliberate design decision as the syntax is much
cleaner for the user then the non-variadic version was that I had implemented
previously. For your example above, it looks something like this (and I'm using
a different macro prefix so that the strict pp-lib can co-exist with the CVS
, ( CHAOS_PP_CAT_( (1), (0) ), =, (0) )
, int x
BTW, the CHAOS_PP_EXPR above has nothing to do with the lambda expression in
parameter 2. That is part of a recursive abstraction that is, by itself,
several orders of magnitude more powerful than the CVS pp-lib. E.g. the CVS
pp-lib supports three dimensions of repetition with BOOST_PP_REPEAT, and
requires over 256 macros to implement each one. The strict pp-lib needs only
about **three or four** macros to support up to 512 dimensions with a maximum of
512 total repetitions. The same is true for WHILE, FOR, ENUM, various types of
FOLD_RIGHT and FOLD_LEFT for different data structures, and even the
FOR_EACH_PRODUCT algorithms. Basically, you have full-fledged, generalized
recursion and full algorithmic support.
The major purpose of the "strict" pp-lib is to show what _could/can_ be done if
preprocessors would get their act together and to give me a clean implementation
to use as a reference implementation. The "strict" pp-lib literally blows the
CVS pp-lib out of the water.
So, the answer to the question, "can it be done?", is "yes." Can it be done on
buggy preprocessors, particularly VC++ and MWCW? Unlikely. Can it be done
without variadic macros? Yes, but it makes for ugly, hard-to-read lambda
expressions, which make it much easier just to define the target macro instead.
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk