Heterogenous lists [ was Re: [boost] BGL: property "num" members ]

From: Emily Winch (emily_at_[hidden])
Date: 2002-01-15 17:02:32

-----Original Message-----
From: David Abrahams <david.abrahams_at_[hidden]>

>----- Original Message -----
>From: "Jeremy Siek" <jsiek_at_[hidden]>

>> Also, it would be nice if we could get the heterogenous list into boost
>> soon. Though I would prefer the name mpl::associative_list instead.
>
>Agreed. Emily, are you planning to submit?

Yep. Only, I went to tidy it up, and then I realised that every design
decision I made was with the intent to mock up a quick proof-of-concept for
the paper, and not to actually produce decent code :-)

So then I discovered a nasty issue with my plot to have clones of the
standard algorithms that work on this kind of list. In brief: How do the
names get out of the list and into the functors called by the algorithms?
and crucially, How can I do that while keeping my algorithms general to
_all_ compile time object lists (e.g. boost::tuple, or some future
heterogenous_vector), and without making the algorithm implementations
smell?

That begs the question, do the functors need to know the names? Can anyone
think of a use for an associative_list that would require that? The paper
mentions some, but none of them to my mind are sufficient to justify the
capability without more concrete evidence that they'd be useful to someone
in real life.

I'm just hoping that there aren't _other_ attributes (apart from names) that
items in hypothetical heterogenous container implementations might need to
introduce to functors. Otherwise each element of a heterogenous container
would need a heterogenous container of attributes :-)

Emily.