Re: [Boost-users] [proto] I'm lost

Subject: Re: [Boost-users] [proto] I'm lost
From: Eric Niebler (eric_at_[hidden])
Date: 2009-08-28 16:03:14

Mail Delivery System wrote:
> I need a transform property_b that evaluates another transform
> property_a and returns what property_a returns+1.
> Simple enough, here's what I do:
> 
> struct property_a : proto::or_<
>   proto::when< proto::terminal<_>, mpl::int_<42>()>
>   > {};
> 
> struct property_b : proto::or_ <
>   proto::when< _, mpl::next<property_a(_) >() >
>   > {};

proto::or_ in these two is unnecessary. For that matter, so is "(_)" in 
property_b.

> The complete file is attached at the end of this message as "Working
> example".
> 
> But when I move to my real application (of which I attach a very
> condensed version as "Non-working example") I cannot manage to make it
> compile. The code in #ifdef BUG...#endif gives an error, the #else
> branch doesn't (but doesn't compute what I need, which makes the fact it
> compiles secondary).
> I cannot see differences, other than my terminals now being in their own
> domain.

There is an important difference, and it looks like a simple oversight. 
Here is the definition of actual_width_transform in your non-working 
example:

struct actual_width_transform : proto::or_<
   //#define BUG
#ifdef BUG
   mpl::next<left_bound_transform (_)>()
#else
   left_bound_transform
#endif
   >
{};

Again, proto::or_ is unnecessary ... you only have one alternate. But I 
bet if you change the code to the following, it'll work:

struct actual_width_transform :
   proto::when<_, mpl::next<left_bound_transform>()>
{};

This is pretty much what you have in your "working example," which is 
why I think this is simply an oversight.

You also appear to have gotten your terminology mixed up, and I think 
that's contributing to your confusion. You're calling these things 
"_transform", but they're actually grammars with nested transforms. 
"mpl::next<left_bound_transform>()" is indeed a transform, but 
transforms cannot be used on their own. They must be part of a grammar. 
That's what proto::when is for; it associates transforms with rules in a 
grammar.

Hope that clears things up.

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com