How to signal a parse error from a value callback?

[Korsar13]

std::set<std::string> block_names;

static constexpr auto ident = dsl::identifier( dsl::ascii::alpha, dsl::ascii::alpha_digit_underscore );

struct block_name
{
	static constexpr auto rule  = ident;
	static constexpr auto value = lexy::callback<std::string>(
		[]( const auto& lx ){
			auto s = std::string{ lx.begin(), lx.end() };
			if ( !block_names.insert( s ).second )
			{
				// error "redefinition" ?
			}
			return std::move(s);
		}
	);
};

[foonathan]

You can't signal a parse error from a callback because those aren't parse errors - they're semantic errors. You probably want to format into your own diagnostics anyway, so can populate that in there. An example of that can be found here: https://github.com/foonathan/lauf/blob/752b53df4b3f6c9affeb300f35bf9a28271a6849/src/lauf/frontend/text.cpp#L253 Or you just generate an AST and do semantic errors later.