identifying the correct callback

[kariem2k]

Hello,
I am trying to find a way to identify the proper callbacks for my rules. Is there a good way to know the call backs for the grammars? Thank you very much

// Expression value: [expr] (expr can be any string within [])
struct ExpressionValue
{
    static constexpr auto rule = dsl::square_bracketed(dsl::token(dsl::while_(-LEXY_LIT("]"))));
    static constexpr auto value = lexy::as_string<std::string>;
};

Getting the following error

/opt/compiler-explorer/libs/lexy/trunk/include/lexy/grammar.hpp:371:27: error: static assertion failed due to requirement '_detail::error<test::ExpressionValue>': missing value callback overload for production
  371 |             static_assert(_detail::error<Production, Args...>,
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/_detail/lazy_init.hpp:130:24: note: in instantiation of function template specialization 'lexy::production_value_callback<test::ExpressionValue>::operator()<>' requested here
  130 |         return emplace(LEXY_FWD(fn)(LEXY_FWD(args)...));
      |                        ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/_detail/config.hpp:30:23: note: expanded from macro 'LEXY_FWD'
   30 | #define LEXY_FWD(...) static_cast<decltype(__VA_ARGS__)>(__VA_ARGS__)
      |                       ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/action/base.hpp:170:23: note: in instantiation of function template specialization 'lexy::_detail::lazy_init<std::basic_string<char>>::emplace_result<lexy::production_value_callback<test::ExpressionValue>>' requested here
  170 |         context.value.emplace_result(context.value_callback(), LEXY_FWD(args)...);
      |                       ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/dsl/token.hpp:112:70: note: in instantiation of function template specialization 'lexy::_detail::final_parser::parse<lexy::_pc<lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, test::ExpressionValue, void>, lexy::_pr<lexy::ascii_encoding>>' requested here
  112 |                 return lexy::whitespace_parser<Context, NextParser>::parse(context, reader,
      |                                                                      ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/dsl/token.hpp:112:70: note: in instantiation of function template specialization 'lexyd::token_base<lexyd::_lit<char, ']'>>::p<lexy::_detail::final_parser>::parse<lexy::_pc<lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, test::ExpressionValue, void>, lexy::_pr<lexy::ascii_encoding>>' requested here
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/dsl/token.hpp:68:66: note: in instantiation of function template specialization 'lexyd::token_base<lexyd::_token<lexyd::_whl<lexyd::_ccomp<lexyd::_ccp<U']'>>>>>::p<lexyd::token_base<lexyd::_lit<char, ']'>>::p<lexy::_detail::final_parser>>::parse<lexy::_pc<lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, test::ExpressionValue, void>, lexy::_pr<lexy::ascii_encoding>>' requested here
   68 |             return lexy::whitespace_parser<Context, NextParser>::parse(context, reader,
      |                                                                  ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/dsl/base.hpp:166:30: note: (skipping 22 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
  166 |         return impl.template finish<Continuation<NextParser>>(context, reader, LEXY_FWD(args)...);
      |                              ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/action/base.hpp:211:32: note: in instantiation of function template specialization 'lexyd::_lst<lexyd::_prd<test::Statement>, void>::p<lexyd::_eof::p<lexy::_detail::final_parser>>::parse<lexy::_pc<lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, test::DSLGrammar>, lexy::_pr<lexy::ascii_encoding>>' requested here
  211 |     auto rule_result = parser::parse(context, reader);
      |                                ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/action/base.hpp:237:24: note: in instantiation of function template specialization 'lexy::_do_action<lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, test::DSLGrammar, lexy::_pr<lexy::ascii_encoding>>' requested here
  237 |     auto rule_result = _do_action(context, reader);
      |                        ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/action/parse.hpp:164:22: note: in instantiation of function template specialization 'lexy::do_action<test::DSLGrammar, lexy::parse_action<void, lexy::string_input<lexy::ascii_encoding>, lexy_ext::_report_error<lexy::stderr_output_iterator>>::result_type, lexy::_ph<lexy::_pr<lexy::ascii_encoding>>, void, lexy::_pr<lexy::ascii_encoding>>' requested here
  164 |         return lexy::do_action<Production, result_type>(handler(input_holder, sink), _state,
      |                      ^
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/action/parse.hpp:173:12: note: in instantiation of function template specialization 'lexy::parse_action<void, lexy::string_input<lexy::ascii_encoding>, lexy_ext::_report_error<lexy::stderr_output_iterator>>::operator()<test::DSLGrammar>' requested here
  173 |     return parse_action<void, Input, ErrorCallback>(callback)(Production{}, input);
      |            ^
<source>:524:20: note: in instantiation of function template specialization 'lexy::parse<test::DSLGrammar, lexy::string_input<lexy::ascii_encoding>, lexy_ext::_report_error<lexy::stderr_output_iterator>>' requested here
  524 |     auto x = lexy::parse<test::DSLGrammar>(lexy::string_input<ascii_encoding>(input), lexy_ext::report_error);
      |                    ^
1 error generated.

[foonathan]

Is there a good way to know the call backs for the grammars?

/opt/compiler-explorer/libs/lexy/trunk/include/lexy/grammar.hpp:371:27: error: static assertion failed due to requirement '_detail::error<test::ExpressionValue>': missing value callback overload for production
  371 |             static_assert(_detail::error<Production, Args...>,
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This error message that test::ExpressionValue is missing a callback that accepts no values. The instantiation of _detail::error<Production, Args...> means that Production requires a callback that takes those Args.... I'll see about making that clearer.

The reason it needs to accept no values, is because none of the rules produce any values. You want to wrap the dsl::token in dsl::capture for that: https://lexy.foonathan.net/reference/dsl/capture/#capture.

[kariem2k]

@foonathan Just wanted to say Lexy is a piece of art. Thank you very much. Once it clicks, it simplifies things a lot. Thank you for doing this well thought of piece of art!