Re: [boost] [Hana] Announcing Hana's formal review next week (June 10th)

Subject: Re: [boost] [Hana] Announcing Hana's formal review next week (June 10th)
From: Louis Dionne (ldionne.2_at_[hidden])
Date: 2015-06-17 08:36:47

Larry Evans <cppljevans <at> suddenlink.net> writes:

> 
> On 06/13/2015 09:52 AM, Louis Dionne wrote:
> > Larry Evans <cppljevans <at> suddenlink.net> writes:
> > 
> >>
> >> On 06/09/2015 11:00 AM, Louis Dionne wrote:
> >> [snip]
> >>>
> >>> Indeed, you are using a Map when what you really want is a Tuple, since
> >>> your keys are integers. Here's the version I would have written:
> >>>
> >>> [...]
> >>>
> >>> However, I can easily imagine another example where you actually need 
> >>> the functionality of a Map. See below for performance comments.
> >>>
> >> [snip]
> >> Why couldn't map be implemented as a tuple, as shown here:
> >>
> >> https://gist.github.com/cppljevans/8e545e8d83946cd74311
> >>
> >> Wouldn't that eliminate the difference in performance between
> >> map and tuple?
> > 
> > The problem with this approach is that equality of keys can't be more 
> > general than type identity. In other words, in your example, writing
> > 
> >     get<_ulong<1>>(mud)
> > 
> > would fail because there is no _ulong<1> key, but only a _uint<1>, even 
> > though they compare equal. I want to determine the equality of keys with 
> > the `equal` function, which is more general. Of course, in the specific 
> > case of a Map mapping types (and only types) to values, then your approach
> > could be used, and it is in fact exactly what I had in mind.
> > 
> 
> OK.  I hadn't thought of that.  OTOH, why not just patch the gist map
> with a method for doing the get_key using the more general `equal`
> function.  Then you could have both the fast find with get, and the
> more general get_key using the `equal` function?

That's an idea. My initial idea was to simply detect when a map is created
with all the keys being Types. In that case, lookup by == is just lookup
by is_same, and you can use an optimized representation for the Map, and
an optimized version of the lookup algorithm. Such a Map might be created
with map_t<...> or something, I haven't figured that one out yet. It's 
similar to the way you can use tuple_t<...> to create a tuple optimized
for types.

> BTW, I'm trying to do this at the moment; however, I'm having
> a tough time figuring out how to do it.  I've looked at:
> 
> https://github.com/ldionne/hana/blob/master/include/boost/hana/map.hpp#L44
> 
> and guessed that maybe I can start by using superclasses for
> my gist map the same as the at map.hpp#L44, and then maybe
> reading http://ldionne.com/hana/index.html#tutorial-extending.
> Is that what you'd recommend?

Yes, that is what I would recommend. There's a new, extended version of this
section in the develop branch, but you'd have to read the source if you wanted 
to see it. See [1] if you're interested. Otherwise, the section on the master
branch might be enough to get you started.

Regards,
Louis

[1]: https://goo.gl/Kl7RDN