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Subject: Re: [boost] [challenge!] find a common domain
From: Eric Niebler (eric_at_[hidden])
Date: 2010-05-19 17:42:55

On 5/19/2010 2:25 PM, Jeremiah Willcock wrote:
> On Wed, 19 May 2010, Eric Niebler wrote:
>> On 5/19/2010 2:08 PM, Marco wrote:
>>> I am a bit surprised by:
>>> // These should be ambiguous.
>>> BOOST_MPL_ASSERT((is_same<deduce_domain3<DD1, DD0, DD0>::type,
>>> not_a_domain>));
>>> BOOST_MPL_ASSERT((is_same<deduce_domain3<DD0, DD1, DD0>::type,
>>> not_a_domain>));
>>> BOOST_MPL_ASSERT((is_same<deduce_domain3<DD0, DD0, DD1>::type,
>>> not_a_domain>));
>>> I was expecting something like:
>>> BOOST_MPL_ASSERT((is_same<deduce_domain3<DD1, DD0, DD0>::type,
>>> default_domain));
>>> Am I missing something ?
>> This is a tricky case, and to understand why I want that result, you
>> need to know what proto uses domains for. In proto, you put an
>> expression in a domain to give it certain domain-specific behaviors.
>> (E.g. lambda expressions should have an operator() that evaluates the
>> lambda.) However, there are no domain-specific behaviors to the
>> default_domain. (E.g. "42" is in the default_domain. Obviously, it has
>> no domain-specific behavior itself, but "lambda::_1 + 42" should be in
>> the lambda domain.)
>> If you combine two expressions in different domains, and the only
>> super-domain they have in common is default_domain, then that would
>> effectively strip all domain-specific behavior from the new expression.
>> IMO, this would be surprising. Instead, I'd rather make this case
>> ambiguous -- proto doesn't know what domain-specific behaviors to give
>> the new expression so it gives up.
>> I acknowledge that it is a special case and that this result doesn't
>> follow naturally from the other rules.
> How do the rules for default_domain (and domains derived from it) fit
> into that picture? Expressions from different domains not derived from
> default_domain produce not_a_domain in the case of conflicts,


> while a
> pair in which one element is derived from default_domain will always
> choose that element as the domain of the new expression.

Here, you mean "the other element" -- that is, the domain that is not a
sub-domain of default_domain. The family of domains rooted at
default_domain is weaker than all others.

(I want this behavior for unified placeholders. Consider that _1 should
be a lambda, but it should combine freely with expressions in other

> In my
> implementation, if both domains derive from default_domain but are
> otherwise incompatible, the result will be default_domain,

That's not right for the reason I stated above. And that's the case that
hung me up for a while.

> but normal
> domains do not derive from default_domain and so end up with the
> not_a_domain behavior in the case of conflicts.

This part is right.

Eric Niebler
BoostPro Computing

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