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

14 Jun 2015

      Paul Fultz II  writes:
...
[...]
Its not a real hash function. It just would return an object(ie type) where
identity(ie `std::is_same`) can be used. That is it will apply the function
as a transform to the key, and use the result to lookup the key using name
lookup. It could default to being identity:
auto m = make_hash_map(
        identity,
        make_pair(type<T>, x),
        make_pair(type<U>, y)
    );
    assert(m[type<T>] == x);
However, if the user wants store intregral constants, and wants it to work
with any integral constant, then a custom "hash" function could be used,
which
would convert every integral constant of the same value to the same type:
auto hash_intregral_constant = [](auto x) -> 
        integral_constant 
    { return {}; };
auto m = make_hash_map(
        hash_intregral_constant,
        make_pair(int_<0>, x),
        make_pair(int_<1>, y)
    );
    assert(m[0_c] == x);
    assert(m[std::integral_constant()] == x);
Of course, now the user can only use integral constants as keys. However, if
the user wanted to use non integral constants and have them compare using
`std::is_same` then the user could just write something like this:
auto m = make_hash_map(
        overload_linear(hash_intregral_constant, identity),
        make_pair(0_c, x),
        make_pair(1_c, y),
        make_pair(type<T>, x),
        make_pair(type<U>, y)
    );
    assert(m[type<T>] == x);
    assert(m[0_c] == x);
    assert(m[std::integral_constant()] == x);
That's an interesting idea. Like I said elsewhere, I'll be investigating
associative sequences for GSoC around the end of the review. I created 
this issue [1] to remind me to have a second look at hash-based associative
sequences when the time comes.

For completeness, I had looked at this possibility [2] last summer when I 
first implemented Map, but I think I gave up on it because it was slightly 
too complicated and I had other more urgent things to do. The main determinant
for using this will be compile-time performance, so I'll have to benchmark.

Regards,
Louis

[1]: https://github.com/ldionne/hana/issues/118
[2]: https://goo.gl/30sk5N

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

Louis Dionne