Re: [boost] PP lambda?

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).

Hi Aleksey,

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.

However...

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
pp-lib):

CHAOS_PP_EXPR(CHAOS_PP_REPEAT(
    5
  , ( 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.

Regards,
Paul Mensonides