So I've been trying to warm up to generating several permutations of arbitrarily structured 'messages' based on several tests applied to a

`spec`

. I decided to play around with making different implementations of the following psuedo code (from wikipedia :P):```
function is_prime(n : integer)
if n ≤ 1
return false
else if n ≤ 3
return true
else if n mod 2 = 0 or n mod 3 = 0
return false
let i ← 5
while i×i ≤ n
if n mod i = 0 or n mod (i + 2) = 0
return false
i ← i + 6
return true
```

here is my first attempt with a bunch of nested

`if_`

s```
#include<boost/hana.hpp>
namespace hana = boost::hana;
constexpr auto if_ = hana::if_;
template<int c>
constexpr auto int_c = hana::int_c<c>;
template<bool c>
constexpr auto bool_c = hana::bool_c<c>;
auto primality_test = [](auto n)
{
return
if_(n <= int_c<1>, hana::nothing,
if_(n <= int_c<3>, hana::just(n),
if_(n % int_c<2> == int_c<0> || n % int_c<3> == int_c<0>, hana::nothing,
if_(hana::is_just(hana::while_(
[&](auto just_i) {
return hana::maybe(bool_c<false>,
[&](auto i) {
return bool_c<i * i <= n>;
}, just_i);
},
hana::just(int_c<5>),
[&](auto just_i) {
return hana::maybe(hana::nothing,
[&](auto i) {
return if_(n % i == int_c<0> || n % (i + int_c<2>) == int_c<0>, hana::nothing,
hana::just(i + int_c<6>));
}, just_i);
})), hana::just(n), hana::nothing)))); //practically lisp!
};
int main()
{
static_assert(decltype(primality_test(int_c<1009>)){} == hana::just(int_c<1009>), "");
static_assert(decltype(primality_test(int_c<1010>)){} == hana::nothing, "");
}
```

In my second go, I am working on chaining these conditions instead of nesting. I was going to try to avoid using

`while_`

, but it doesn't seem there is any other way without generating all of the possible `i`

values up front in a range or something.
Is

`while_`

the best choice here?
the second one is slighty slower

the builtin that is

@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_constant`

s, but it isn’t).
or rather it is a contrived use case

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.

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

```
static_assert(size == 0, "Does not contain anything");
static_assert(size > 1, "Contains duplicates");
```

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

```
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

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>{};
}
```

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

ah right It would check the second one after asserting that it wasn't 0

:d