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    ricejasonf commented #432
Louis Dionne
@ldionne
@ricejasonf Yes, I think it’s quite nice. I just hope the Clang/libc++ guys will want the patch.

Regarding your primality testing example, here's how I would do it:

#include <boost/hana.hpp>
namespace hana = boost::hana;


constexpr bool is_prime_impl(int n) {
    if (n <= 1)
        return false;
    else if (n <= 3)
        return true;
    else if (n % 2 == 0 or n % 3 == 0)
        return false;

    int i = 5;
    while (i*i <= n) {
        if (n % i == 0 or n % (i + 2) == 0)
            return false;
        i += 6;
    }
    return true;
}

template <typename N>
constexpr auto is_prime(N const&) {
    return hana::bool_c<is_prime_impl(hana::value<N>())>;
}


auto primes = hana::tuple_c<int,
    2,  3,  5,  7, 11, 13, 17, 19, 23, 29,
    31, 37, 41, 43, 47, 53, 59, 61, 67, 71,
    73, 79, 83, 89, 97,101,103,107,109,113,
    127,131,137,139,149,151,157,163,167,173,
    179,181,191,193,197,199,211,223,227,229,
    233,239,241,251,257,263,269,271,277,281,
    283,293,307,311,313,317,331,337,347,349,
    353,359,367,373,379,383,389,397,401,409,
    419,421,431,433,439,443,449,457,461,463,
    467,479,487,491,499
>;

int main() {
    auto integers = hana::make_range(hana::int_c<0>, hana::int_c<100>);
    hana::for_each(integers, [](auto i) {
        static_assert(decltype(hana::contains(primes, i) == is_prime(i)){}, "");
    });
}

For homogeneous problems, it's much easier and much faster to just use constexpr.

Note that hana::contains is quite slow (it could be optimized, at least for hana::types and hana::integral_constants, but it isn’t).
Jason Rice
@ricejasonf
ha ha.. i guess i was overcomplicating things
or rather it is a contrived use case
Maik Klein
@MaikKlein

How do you output different error messages?

Pseudo code:

constexpr auto size = hana::size(something);
// (size == 0) => print "Does not contain anything"
// (size > 1) =>print "Contains duplicates"
where "print" would be the compile time error.
Jason Rice
@ricejasonf
wouldn't static_assert work?
Maik Klein
@MaikKlein
I could do static_assert(hana::size(s) == hana::size_c<1>, "Something is wrong");
but then I wouldn't know if it i was 0 or bigger then one and checking that seems a bit cumbersome with static_assert
Jason Rice
@ricejasonf
static_assert(size == 0, "Does not contain anything");
static_assert(size > 1, "Contains duplicates");
This message was deleted
Maik Klein
@MaikKlein
hm okay I must be tired :d
Jason Rice
@ricejasonf
Maik Klein
@MaikKlein

Wait it should work with 1, but should not accept 0 or 2+. size == 0 doesn't work because 1 == 0 will trigger the assert, and static_assert triggers at false not true.

size != 0 doesn't work because it will also trigger at 1+. I could solve this but it would require some boolean shenanigans

Jason Rice
@ricejasonf
ah right
Maik Klein
@MaikKlein
    static_assert(size != 0 || size >= 1, "Empty");
    static_assert(size == 1 || size == 0, "More than 1");
Should be what I want, but that seems way too complicated for something so simple
Jason Rice
@ricejasonf
here is a much, much simpler solution and the good news is it only generates one warning :P
#include<type_traits>

template<int size, typename = void>
struct SizeChecker
{ };

template<int size>
struct SizeChecker<size,
  std::enable_if_t<(size == 0)>>
{
  static_assert(size == 0, "Does not contain anything");
};

template<int size>
struct SizeChecker<size,
  std::enable_if_t<(size > 1)>>
{
  static_assert(size > 1, "Contains duplicates");
};

int main()
{
  constexpr int size = 1;
  SizeChecker<size>{};
}
Maik Klein
@MaikKlein
haha
Jason Rice
@ricejasonf
sorry that's all i got
Maik Klein
@MaikKlein
It appears that I was very confused and you can actually write it like this static_assert( size != 0, "Must not be empty") and static_assert(size <= 1, "Too many")
I think the trigger at false must have confused me somehow
Jason Rice
@ricejasonf
ah right It would check the second one after asserting that it wasn't 0
Maik Klein
@MaikKlein
?
Jason Rice
@ricejasonf
meaning you can't get two failures
Maik Klein
@MaikKlein
I can only get one failure
Jason Rice
@ricejasonf
yeah.. nm :)
Maik Klein
@MaikKlein
because for 0, 0 <= 1 is true
:d
Jason Rice
@ricejasonf
@MaikKlein Regarding your question about 'previous template instantiations', precompiled header files might be the answer. I'm not sure to what extent it would make a difference, but it shaves off about 5 seconds (from 8 seconds total) from my convoluted primality test. A more extreme case could be tested by fiddling with the upper end of the range in expensive.hpp.
As you can see in the Makefile you can compile a .pch just like you can an object file.
Jason Rice
@ricejasonf
Apparently clang has modules implemented. I wonder if it would be feasible to export that map variable and tell clang to fully evaluate/optimize the AST to output a pch or header file that leaves all of the cruft behind.
Maik Klein
@MaikKlein
@ricejasonf interesting, haven't used pch's yet.
btw is there a way to benchmark the compile time? I tried -ftime-report in clang but it's not very helpful.
Maik Klein
@MaikKlein
I am thinking of something like
foo<a,b,c,d> at line 24 in foo.hpp took 1.54 seconds to instantiate.
Jason Rice
@ricejasonf
@MaikKlein idk. I was just using time make and it appears that Hana uses a ruby script that calls a make command
Maik Klein
@MaikKlein
I guess that works okay for isolated code, but I would really like to see where the bottleneck is in my current code base. I'll have a look at clang modules now and see if they give me any benefit.
Maik Klein
@MaikKlein
I guess I am also going to try templight, it even integrates kcachegrind
^ wtf is that image
ah it's his profile picture
Jason Rice
@ricejasonf
this oss?
your project that is
Maik Klein
@MaikKlein
My project will be open source
not sure if that is what you asked though :d. I am not the creator of templight
Jason Rice
@ricejasonf
is it on github?