hana::less does not work for adapted structs

[PiotrNycz]

Consider this code godbolt
The codebase is c++17 - so I cannot just default compare operators...

For any adapted struct (in hana (actually boost::hana) way) I have this strange situation when hana::equal just works for implementing operator== while hana::less cannot be used for implementing operator<. For me documentation is unclear - should it work?

I dig a little into this problem - and it seems that when I call directly hana::less from operator< then less calls operator< so infinite recursion - so segfault.
While when it is called directly (not via operator<) then it does not compile with a set of static-assert, starting from complaining about adapted struct being not ordered - well...

When I compared sources for equal and less - indeed - support for adapted structs is missing in less.

The code:

#include <boost/hana/adapt_struct.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/less.hpp>

namespace demo
{
struct Some
{
    int a;
    int b;
};
}

BOOST_HANA_ADAPT_STRUCT(demo::Some,
    a, b);

namespace demo
{
bool operator==(const Some& lhs, const Some& rhs)
{
    return boost::hana::equal(lhs, rhs);
}


bool less(const Some& lhs, const Some& rhs)
{
    return boost::hana::less(lhs, rhs);
}

bool operator<(const Some& lhs, const Some& rhs)
{
    // uncommenting boost::hana::less makes this segfault - because operator< calls itself
    //return boost::hana::less(lhs, rhs);
    return less(lhs, rhs);
}
}

int main() {
    return demo::Some{1, 1} < demo::Some{1, 2};
}

The output:

In file included from <source>:3:
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp: In instantiation of 'constexpr auto boost::hana::less_t::operator()(X&&, Y&&) const [with X = const demo::Some&; Y = const demo::Some&]':
<source>:29:29:   required from here
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:51:43: error: static assertion failed: hana::less(x, y) requires 'x' to be Orderable
   51 |         static_assert(hana::Orderable<T>::value,
      |                                           ^~~~~
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:51:43: note: 'std::integral_constant<bool, false>::value' evaluates to false
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:54:43: error: static assertion failed: hana::less(x, y) requires 'y' to be Orderable
   54 |         static_assert(hana::Orderable<U>::value,
      |                                           ^~~~~
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:54:43: note: 'std::integral_constant<bool, false>::value' evaluates to false
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:57:53: error: static assertion failed: hana::less(x, y) requires 'x' and 'y' to be embeddable in a common Orderable
   57 |         static_assert(!is_default<less_impl<T, U>>::value,
      |                                                     ^~~~~
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:57:53: note: '!(bool)std::integral_constant<bool, true>::value' evaluates to false
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/less.hpp:62:27: error: use of deleted function 'static constexpr auto boost::hana::deleted_implementation::apply(T&& ...) [with T = {const demo::Some&, const demo::Some&}]'
   62 |         return Less::apply(static_cast<X&&>(x), static_cast<Y&&>(y));
      |                ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/core/dispatch.hpp:14,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/value.hpp:18,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/concept/constant.hpp:19,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/core/to.hpp:15,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/bool.hpp:17,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/string.hpp:15,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/detail/struct_macros.hpp:28,
                 from /opt/compiler-explorer/libs/boost_1_84_0/boost/hana/adapt_struct.hpp:15,
                 from <source>:1:
/opt/compiler-explorer/libs/boost_1_84_0/boost/hana/detail/dispatch_if.hpp:21:31: note: declared here
   21 |         static constexpr auto apply(T&& ...) = delete;
      |                               ^~~~~
Compiler returned: 1

P.S.
I tried also to use hana::lexicographical_compare- but I did not manage to use it for adapted structs - any help would be welcome. I ended up with usinghana::all_of`...

[ricejasonf]

The compiler error is that your type Some does not satisfy the concept hana::Orderable which requires an implementation of hana::less for your type. So, I am not seeing how your commented call to boost::hana::less would segfault if it would not compile.

In any case, you would need to implement hana::less. Unfortunately a struct cannot implement hana::Iterable to get the lexicographical_compare function to make this easy, but here is a example that skirts around that by converting Some to a tuple in the implementation.
https://godbolt.org/z/f5YK4j3ov

EDIT: For the documentation, see Reference documentation -> Concepts -> Struct to see the "refined concepts". Notice that Orderable is not there.

EDIT: BTW The lexicographical_compare requires Iterable which requires a definition for drop_front. Hence, implementing it for Structs in general is not straightforward.

[PiotrNycz]

1. SEGFAULT explanation:

The segfault is when calling hana::less from operator< -- then, as I wrote, hana::less calls operator< which calls hana::less - so infinite recursion:

#include <boost/hana/adapt_struct.hpp>
#include <boost/hana/equal.hpp>
#include <boost/hana/less.hpp>

namespace demo
{
struct Some
{
    int a;
    int b;
};
}

BOOST_HANA_ADAPT_STRUCT(demo::Some,
    a, b);

namespace demo
{
bool operator==(const Some& lhs, const Some& rhs)
{
    return boost::hana::equal(lhs, rhs);
}

bool operator<(const Some& lhs, const Some& rhs)
{
    // uncommenting boost::hana::less makes this segfault - because operator< calls itself
    return boost::hana::less(lhs, rhs);
}
}

int main() {
    return demo::Some{1, 1} < demo::Some{1, 2};
}

2. Well...

So, in general, I need to implement hana::less_impl to use hana::less for every struct (hundreds of them I have) in my project, to use hana::less to implement operator< for my structs. Well, instead I can just implement my operator< with std::tie, can't I - and just forget about hana library?

My goal is to use BOOST_HANA_ADAPT_STRUCT(demo::Some, a, b, ... tens of fields here...); when implementing operator<. I did not want to repeat these fields agains in operator< -- because then - these ADAPT_STRUCT is useless.
The other thing is that I do not want to copy all fields into the hana tuple -- I know, I can use std::reference_wrapper - or just use std::tie.

Just for information - I did this to have implementation of operator< and still use ADAPT_STRUCT:

template <typename T> // assuming T has hana::accessors
int compare(const Some& lhs, const Some& rhs) // it behaves as C++20 operator<=>
{
//    return boost::hana::less(lhs, rhs);
    int result{};
    static_cast<void>(boost::hana::all_of(boost::hana::accessors<T>(),
        [&](const auto& field)
        {
            if (result != 0) return false;
            auto get = boost::hana::second(field);
            auto&& lhs_field = get(lhs);
            auto&& rhs_field = get(rhs);
            if (lhs_field < rhs_field) { result = -1; return false; }
            if (rhs_field < lhs_field) { result = 1; return false; }
            return true;
        }));
    return result;
}

bool operator<(const Some& lhs, const Some& rhs)
{
    return compare(lhs, rhs) < 0;
}

For me, it is hard to believe that adding support for adapted structs in hana::less is so hard, it was done for hana::equal and it just works. But ok, for C++20 it is not needed...

[ricejasonf]

So, in general, I need to implement hana::less_impl to use hana::less for every struct (hundreds of them I have) in my project, to use hana::less to implement operator< for my structs. Well, instead I can just implement my operator< with std::tie, can't I - and just forget about hana library?

The only benefits to implementing hana::less that I see immediately are that you get hana::greater, hana::min, and hana::max for free and your data type is then hana::Orderable. Beyond that I am not sure there is any other benefit.

In general, adapted structs are not considered ordered because it is the user who should opt in and define how that ordering should work. I made a more general solution that uses hana::accessors to fix the issue with copying and allows us to use the hana::lexicographical_compare function with a predicate. To opt in, I made a "concept" to indicate the struct is to be ordered lexicographically. (I did not test this thoroughly.)

namespace boost::hana {
  template <typename T>
  struct less_impl<T, T,
            hana::when<demo::concept::LexicographicallyOrderedStruct<T>::value>> {
    static constexpr auto apply(demo::Some const& x,
                                demo::Some const& y) {
      constexpr auto accessors = hana::transform(hana::accessors<T>(),
                                                 hana::second);
      return hana::lexicographical_compare(accessors, accessors,
        [&](auto accessor, auto) {
          return hana::less(accessor(x), accessor(y));
        });
    }
  };
}

https://godbolt.org/z/sbrPzMf3a