Re: [boost] [mpl] is_lambda_expression bug?

Subject: Re: [boost] [mpl] is_lambda_expression bug?
From: Edward Diener (eldiener_at_[hidden])
Date: 2013-02-10 10:48:42

On 2/10/2013 1:12 AM, James Hirschorn wrote:
> Jeffrey Lee Hellrung, Jr.-2 wrote
>>   If it's not documented, I don't know what propertiesis_lambda_expression
>> has and what guarantees it makes. Its name suggestssome, I admit, but who
>> knows what the author's intents were? Without someadditional
>> documentation, the code defines its behavior, hence I'd say
>> it'stautologically correct :)I can sympathize if you think the above
>> implies this metafunction should bean auxiliary or detail namespace; I
>> would agree.
>
> I would think (but I don't really know) that having undocumented functions
> in the main namespaces would itself be a violation of boost policies, even
> if it worked as expected. On this basis I think I will make a report and see
> what they say.
> Even if it was hidden, it seems like horrible style to have
> is_lambda_expression fail on lambda expressions (there is a documented
> is_sequence metafunction, and there is an is_placeholder in the mpl
> namespace that seems to work as expected).
>
>> Is this just an FYI, or are you genuinely in need to detecting
>> Boost.MPLlambda expressions?
>
> Yes, there is a need. I want a function to be instantiated according to its
> template parameters (e.g. one definition if T1 is a lambda_expression and
> another if T1 is a sequence (or is there overlap?)).

In the latest version of tti on the trunk, I have an 
is_lambda_expression metafunction as a detail of the implementation. You 
can find it at tti/detail/dlambda.hpp. I use it at one point in the 
implementation when introspecting a nested type and allowing the user to 
optionally apply a lambda expression when found, a suggestion made by 
Jeffrey Hellrung when tti was reviewed.