How can I place nothings in a list and filter them out?

[UndarkAido]

I make heavy use of macros in https://github.com/DiscordPP/plugin-endpoints and in merging it with https://github.com/DiscordPP/plugin-objects I decided I needed to macro-ify object definitions. But to make a long story short, what am I doing wrong here and/or is there a better way to do this?

#include <metalang99.h>
#if !ML99_VERSION_COMPATIBLE(1, 12, 0)
#error Please, update your Metalang99 to v1.12.0 or higher!
#endif

#define Fields v(field(std::string, name), field(std::string, fact), exclude_field(std::string, hideme), field(std::string, comment), field(std::string, desc))

template<typename t>
class field {
};

void func(
#define field(TYPE, NAME) field<TYPE> NAME = {}
#define exclude_field(TYPE, NAME) ML99_nothing()
#define PRED(s, data, elem) BOOST_PP_NOT(BOOST_PP_EQUAL(elem, nil))
        ML99_LIST_EVAL_COMMA_SEP(ML99_listFilter(v(ML99_isNothing), ML99_list(Fields)))
#undef exclude_field
#undef field
        ){
}

CLion claims this evaluates to

field<std::string> name = {}, field<std::string>
fact = {},
(0callUneval,ML99_nothing,),
field<std::string> comment = {}, field<std::string>
desc = {}

[Hirrolot]

This will do the trick:

void func(
#define field(TYPE, NAME) ML99_JUST(field<TYPE> NAME = {})
#define exclude_field(TYPE, NAME) ML99_NOTHING()

ML99_LIST_EVAL_COMMA_SEP(ML99_listMap(v(ML99_maybeUnwrap), ML99_listFilter(v(ML99_isJust), ML99_list(Fields))))

#undef exclude_field
#undef field
        ){
}

Expansion:

void func(
field<std::string> name = {} , field<std::string> fact = {} , field<std::string> comment = {} , field<std::string> desc = {}
        ){
}

[UndarkAido]

Thank you, that worked! I hadn't tried ML99_maybeUnwrap because the docs say it "emits a fatal error on ML99_nothing()". Maybe that needs to be changed or elaborated on?

[Hirrolot]

The call to ML99_listFilter yields only values constructed with ML99_just, so it's perfectly safe to call ML99_maybeUnwrap after that.