char_'s attribute type is polymorphic

The majority use case for parsing with Boost.Parser is Unicode-aware parsing. Those users should be able simply to use char_ and have it "just work". In the case of Unicode, that "just working" implies that every element of the input range should be a code point.

Some users will insist that their parsing needs are entirely ASCII. Yet other users cannot use Unicode, because they use some encoding that is not a subset of the Unicode encoding, like EBCDIC. For these users, they can just parse input sequences of char, and that will "just work" for them. For them, this means that every element of the input range that is parsed should be a char.

This is exactly what char_ does, and why it does it.

none is weird

Yes, and it's generally not a good programming practice to use a type which is so loose (anything can be assigned to it, it's implicitly convertible to anything, etc.). However, it is better than the alternative. Consider this semantic action:

[](auto & ctx) { _attr(ctx) = 42; }

If attached to an int-parser, this is fine. If attached to an epsilon parser (which has no attribute), this silently does nothing. However, in debug mode the assignment in this semantic action will hit a BOOST_ASSERT(false), and lead the user to a big inline comment about how they got there. This is a far more understandable failure mode for most programmers than the arbitrarily-deep template instantiation stack — and baffling type of ctx — that would result if the expression _attr(ctx) were ill-formed.

The use of none turns an entirely compile-time debugging operation into a run-time debugging one. Usually, this is the opposite of what we want as C++ users. In light of just how inscrutable error messages are that come from parser combinator libraries, using your favorite debugger to step through the stack to diagnose the problem is a much faster way to fix problems.

To demonstrate the difference, I added these three lines to the end of the object_init lambda in the Parsing JSON:

auto x = _locals(ctx);
if (x)
    std::cout << "Oops!  What x?";

The parser that object_init is attached to has no locals. Here is an example of how you can investigate this error at run time:

$ gdb --args example/json ../meta/libraries.json
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For help, type "help".
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Reading symbols from example/json...
(gdb) r
Starting program: /home/tzlaine/parser/build/example/json ../meta/libraries.json
json: /home/tzlaine/parser/include/boost/parser/parser.hpp:344: void boost::parser::none::fail() const: Assertion `false' failed.

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) up
#1  0x00007ffff7bdf859 in __GI_abort () at abort.c:79
79      abort.c: No such file or directory.
(gdb)
#2  0x00007ffff7bdf729 in __assert_fail_base (
    fmt=0x7ffff7d75588 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
    assertion=0x5555555f7a2c "false",
    file=0x5555555f7dd8 "/home/tzlaine/parser/include/boost/parser/parser.hpp", line=344,
    function=<optimized out>) at assert.c:92
92      assert.c: No such file or directory.
(gdb)
#3  0x00007ffff7bf0f36 in __GI___assert_fail (assertion=0x5555555f7a2c "false",
    file=0x5555555f7dd8 "/home/tzlaine/parser/include/boost/parser/parser.hpp", line=344,
    function=0x5555555f7db0 "void boost::parser::none::fail() const") at assert.c:101
101     in assert.c
(gdb)
#4  0x000055555555f99b in boost::parser::none::fail (this=0x7fffffffc2f0)
    at /home/tzlaine/parser/include/boost/parser/parser.hpp:344
344                 BOOST_ASSERT(false);
(gdb)
#5  0x000055555559d380 in boost::parser::none::operator bool<bool>() const (this=0x7fffffffc2f0)
    at /home/tzlaine/parser/include/boost/parser/parser.hpp:83
83                  fail();
(gdb)
#6  0x0000555555590a6b in _ZNK4json11object_initMUlRT_E_clIKN5boost4hana6detail8map_implINS6_10hash_tableIJNS6_6bucketINS4_6parser6detail9begin_tagEJLm0EEEENS9_INSB_7end_tagEJLm1EEEENS9_INSB_8pass_tagEJLm2EEEENS9_INSB_10locals_tagEJLm3EEEENS9_INSB_15rule_params_tagEJLm4EEEENS9_INSB_11globals_tagEJLm5EEEENS9_INSB_16trace_indent_tagEJLm6EEEENS9_INSB_17error_handler_tagEJLm7EEEENS9_INSB_13callbacks_tagEJLm8EEEENS9_INSB_22symbol_table_tries_tagEJLm9EEEENS9_INSB_7val_tagEJLm10EEEENS9_INSB_8attr_tagEJLm11EEEENS9_INSB_9where_tagEJLm12EEEEEEENS5_11basic_tupleIJNS5_4pairINS5_9type_implISC_E1_ENS4_4text20utf_8_to_32_iteratorIPKcS1B_NS18_25use_replacement_characterEEEEENS14_INS15_ISE_E1_ES1D_EENS14_INS15_ISG_E1_EPbEENS14_INS15_ISI_E1_ENSB_4nopeEEENS14_INS15_ISK_E1_ES1O_EENS14_INS15_ISM_E1_EPNS_12global_stateEEENS14_INS15_ISO_E1_EPiEENS14_INS15_ISQ_E1_EPKNSA_22callback_error_handlerEEENS14_INS15_ISS_E1_ES1O_EENS14_INS15_ISU_E1_EPSt3mapIPvNS4_3anyESt4lessIS2E_ESaISt4pairIKS2E_S2F_EEEEENS14_INS15_ISW_E1_EPNS_5valueEEENS14_INS15_ISY_E1_EPS1O_EENS14_INS15_IS10_E1_EPKNSA_4viewIS1D_S1D_EEEEEEEEEEEDaS1_ (__closure=0x7fffffffc9b1, ctx=...)
    at /home/tzlaine/parser/example/json.cpp:103
103             if (x)
(gdb) l
98              auto & globals = _globals(ctx);
99              if (globals.max_recursive_open_count < ++globals.recursive_open_count)
100                 throw excessive_nesting(_where(ctx).begin());
101             _val(ctx) = object();
102             auto x = _locals(ctx);
103             if (x)
104                 std::cout << "Oops!  What x?";
105         };
106
107         // We need object_insert because we can't just insert into the json::value
(gdb)

To find the problem, I just had to move up the stack, with GDB's "up" command, until I saw that I was in my own code. Then I listed the code surrounding the offending line, as you see above. If I were to keep going up the stack, I would move through the exact chain of template instantiations — at the exact lines of code where they appear — in a few seconds.

This is how the same problem looks with BOOST_PARSER_NO_RUNTIME_ASSERTIONS defined, the definition of which makes the code we added ill-formed instead of a run time error:

$ make json
Scanning dependencies of target json
[ 50%] Building CXX object example/CMakeFiles/json.dir/json.cpp.o

/home/tzlaine/parser/example/json.cpp: In instantiation of ‘json::<lambda(auto:58&)> [with auto:58 = const boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 11>, boost::hana::detail::bucket<boost::parser::detail::where_tag, 12> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::where_tag>::_, const boost::parser::view<boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >*> > >]’:
/home/tzlaine/parser/include/boost/parser/parser.hpp:3216:24:   required from ‘void boost::parser::action_parser<Parser, Action>::call(boost::hana::bool_<UseCallbacks>, Iter&, Sentinel, const Context&, const SkipParser&, boost::parser::detail::flags, bool&, Attribute&) const [with bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >; SkipParser = boost::parser::rule<json::ws>; Attribute = boost::parser::detail::nope; Parser = boost::parser::omit_parser<boost::parser::char_parser<char, void> >; Action = json::<lambda(auto:58&)>; boost::hana::bool_<UseCallbacks> = boost::hana::integral_constant<bool, false>]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:3175:17:   required from ‘boost::parser::detail::nope boost::parser::action_parser<Parser, Action>::call(boost::hana::bool_<UseCallbacks>, Iter&, Sentinel, const Context&, const SkipParser&, boost::parser::detail::flags, bool&) const [with bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >; SkipParser = boost::parser::rule<json::ws>; Parser = boost::parser::omit_parser<boost::parser::char_parser<char, void> >; Action = json::<lambda(auto:58&)>; boost::hana::bool_<UseCallbacks> = boost::hana::integral_constant<bool, false>]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:2745:35:   required from ‘auto boost::parser::seq_parser<ParserTuple, BacktrackingTuple>::dummy_use_parser_t<UseCallbacks, Iter, Sentinel, Context, SkipParser>::operator()(const Parser&) const [with Parser = boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >; bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >; SkipParser = boost::parser::rule<json::ws>; ParserTuple = boost::hana::tuple<boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >, boost::parser::opt_parser<boost::parser::delimited_seq_parser<boost::parser::action_parser<boost::parser::rule_parser<false, json::object_element, boost::hana::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, json::value>, boost::parser::detail::nope, boost::parser::detail::nope>, json::<lambda(auto:59&)> >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > > >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > >; BacktrackingTuple = boost::hana::tuple<boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, false> >]’
/home/tzlaine/boost_1_71_0/boost/hana/transform.hpp:62:42:   required from ‘constexpr auto boost::hana::transform_impl<S, boost::hana::when<boost::hana::Sequence<S>::value> >::transformer<F>::operator()(Xs&& ...) const [with Xs = {const boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >&, const boost::parser::opt_parser<boost::parser::delimited_seq_parser<boost::parser::action_parser<boost::parser::rule_parser<false, json::object_element, boost::hana::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, json::value>, boost::parser::detail::nope, boost::parser::detail::nope>, json::<lambda(auto:59&)> >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > > >&, const boost::parser::omit_parser<boost::parser::char_parser<char, void> >&}; F = const boost::parser::seq_parser<boost::hana::tuple<boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >, boost::parser::opt_parser<boost::parser::delimited_seq_parser<boost::parser::action_parser<boost::parser::rule_parser<false, json::object_element, boost::hana::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, json::value>, boost::parser::detail::nope, boost::parser::detail::nope>, json::<lambda(auto:59&)> >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > > >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > >, boost::hana::tuple<boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, false> > >::dummy_use_parser_t<false, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>, boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >, boost::parser::rule<json::ws> >*; S = boost::hana::tuple_tag]’
/home/tzlaine/boost_1_71_0/boost/hana/basic_tuple.hpp:115:39:   required from ‘static constexpr decltype(auto) boost::hana::unpack_impl<boost::hana::basic_tuple_tag>::apply(const boost::hana::detail::basic_tuple_impl<std::integer_sequence<long unsigned int, _Idx ...>, Xn ...>&, F&&) [with long unsigned int ...i = {0, 1, 2}; Xn = {boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >, boost::parser::opt_parser<boost::parser::delimited_seq_parser<boost::parser::action_parser<boost::parser::rule_parser<false, json::object_element, boost::hana::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, json::value>, boost::parser::detail::nope, boost::parser::detail::nope>, json::<lambda(auto:59&)> >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > > >, boost::parser::omit_parser<boost::parser::char_parser<char, void> >}; F = boost::hana::transform_impl<boost::hana::tuple_tag, boost::hana::when<true> >::transformer<const boost::parser::seq_parser<boost::hana::tuple<boost::parser::action_parser<boost::parser::omit_parser<boost::parser::char_parser<char, void> >, json::<lambda(auto:58&)> >, boost::parser::opt_parser<boost::parser::delimited_seq_parser<boost::parser::action_parser<boost::parser::rule_parser<false, json::object_element, boost::hana::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, json::value>, boost::parser::detail::nope, boost::parser::detail::nope>, json::<lambda(auto:59&)> >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > > >, boost::parser::omit_parser<boost::parser::char_parser<char, void> > >, boost::hana::tuple<boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, true>, boost::hana::integral_constant<bool, false> > >::dummy_use_parser_t<false, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>, boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >, boost::parser::rule<json::ws> >*>]’
/home/tzlaine/boost_1_71_0/boost/hana/unpack.hpp:47:29:   [ skipping 20 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/home/tzlaine/parser/example/json.cpp:262:5:   required from ‘void json::parse_rule(boost::parser::rule_parser<false, json::value_tag, json::value, boost::parser::detail::nope, boost::parser::detail::nope>::tag_type*, boost::hana::bool_<b>, Iter&, Sentinel, const Context&, const SkipParser&, boost::parser::detail::flags, bool&, Attribute&) [with bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, json::value*> > >; SkipParser = boost::parser::rule<json::ws>; Attribute = json::value; boost::parser::rule_parser<false, json::value_tag, json::value, boost::parser::detail::nope, boost::parser::detail::nope>::tag_type = json::value_tag; boost::hana::bool_<b> = boost::hana::integral_constant<bool, false>]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:3707:23:   required from ‘boost::parser::rule_parser<false, TagType, Attribute, LocalState, ParamsTuple>::attr_type boost::parser::rule_parser<false, TagType, Attribute, LocalState, ParamsTuple>::call(boost::hana::bool_<UseCallbacks>, Iter&, Sentinel, const Context&, const SkipParser&, boost::parser::detail::flags, bool&) const [with bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*> > >; SkipParser = boost::parser::rule<json::ws>; TagType = json::value_tag; Attribute = json::value; LocalState = boost::parser::detail::nope; ParamsTuple = boost::parser::detail::nope; boost::parser::rule_parser<false, TagType, Attribute, LocalState, ParamsTuple>::attr_type = json::value; boost::hana::bool_<UseCallbacks> = boost::hana::integral_constant<bool, false>]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:4155:32:   required from ‘auto boost::parser::parser_interface<Parser, GlobalState, ErrorHandler>::operator()(boost::hana::bool_<UseCallbacks>, Iter&, Sentinel, const Context&, const SkipParserType&, boost::parser::detail::flags, bool&) const [with bool UseCallbacks = false; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Context = boost::hana::detail::map_impl<boost::hana::detail::hash_table<boost::hana::detail::bucket<boost::parser::detail::begin_tag, 0>, boost::hana::detail::bucket<boost::parser::detail::end_tag, 1>, boost::hana::detail::bucket<boost::parser::detail::pass_tag, 2>, boost::hana::detail::bucket<boost::parser::detail::val_tag, 3>, boost::hana::detail::bucket<boost::parser::detail::attr_tag, 4>, boost::hana::detail::bucket<boost::parser::detail::locals_tag, 5>, boost::hana::detail::bucket<boost::parser::detail::rule_params_tag, 6>, boost::hana::detail::bucket<boost::parser::detail::globals_tag, 7>, boost::hana::detail::bucket<boost::parser::detail::trace_indent_tag, 8>, boost::hana::detail::bucket<boost::parser::detail::error_handler_tag, 9>, boost::hana::detail::bucket<boost::parser::detail::callbacks_tag, 10>, boost::hana::detail::bucket<boost::parser::detail::symbol_table_tries_tag, 11> >, boost::hana::basic_tuple<boost::hana::pair<boost::hana::type_impl<boost::parser::detail::begin_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::end_tag>::_, boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character> >, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::pass_tag>::_, bool*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::val_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::attr_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::locals_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::rule_params_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::globals_tag>::_, json::global_state*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::trace_indent_tag>::_, int*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::error_handler_tag>::_, const boost::parser::callback_error_handler*>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::callbacks_tag>::_, boost::parser::detail::nope>, boost::hana::pair<boost::hana::type_impl<boost::parser::detail::symbol_table_tries_tag>::_, std::map<void*, boost::any, std::less<void*>, std::allocator<std::pair<void* const, boost::any> > >*> > >; SkipParserType = boost::parser::rule<json::ws>; Parser = boost::parser::rule_parser<false, json::value_tag, json::value, boost::parser::detail::nope, boost::parser::detail::nope>; GlobalState = json::global_state&; ErrorHandler = boost::parser::callback_error_handler&; boost::hana::bool_<UseCallbacks> = boost::hana::integral_constant<bool, false>]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:1808:43:   required from ‘auto boost::parser::detail::skip_parse_impl(Iter&, Sentinel, const Parser&, const SkipParser&, const ErrorHandler&) [with bool Debug = true; Iter = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Sentinel = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Parser = boost::parser::parser_interface<boost::parser::rule_parser<false, json::value_tag, json::value, boost::parser::detail::nope, boost::parser::detail::nope>, json::global_state&, boost::parser::callback_error_handler&>; SkipParser = boost::parser::rule<json::ws>; ErrorHandler = boost::parser::callback_error_handler]’
/home/tzlaine/parser/include/boost/parser/parser.hpp:6369:53:   required from ‘auto boost::parser::parse(I&, S, const boost::parser::parser_interface<Parser, GlobalState, ErrorHandler>&, const boost::parser::rule<TagType, Attribute, LocalState, ParamsTuple>&, boost::parser::trace) [with I = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; S = boost::text::utf_8_to_32_iterator<const char*, const char*, boost::text::use_replacement_character>; Parser = boost::parser::rule_parser<false, json::value_tag, json::value, boost::parser::detail::nope, boost::parser::detail::nope>; GlobalState = json::global_state&; ErrorHandler = boost::parser::callback_error_handler&; TagType = json::ws; Attribute = boost::parser::detail::nope; LocalState = boost::parser::detail::nope; ParamsTuple = boost::parser::detail::nope]’
/home/tzlaine/parser/example/json.cpp:313:53:   required from here
/home/tzlaine/parser/example/json.cpp:103:13: error: could not convert ‘x’ from ‘boost::parser::none’ to ‘bool’
  103 |         if (x)
      |             ^
      |             |
      |             boost::parser::none
make[3]: *** [example/CMakeFiles/json.dir/build.make:63: example/CMakeFiles/json.dir/json.cpp.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:1668: example/CMakeFiles/json.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:1675: example/CMakeFiles/json.dir/rule] Error 2
make: *** [Makefile:827: json] Error 2

Some very familiar problems should be noted here:

Attribute types are flexible

This is how we get genericity in attribute generation. In the STL, we can use multiple types of container with the algorithms because iterators act as the glue that connects algorithms to containers. With attribute generation, there are instead arbitrary types being constructed and inserted into containers. Allowing the insertion to happen on arbitrary types that model the container concept is what allows generic use of different containers.

Sequences of character type are treated specially

Boost.Parser attempts to keep the rules for attribute generation simple. However, there are some rules for attribute generation that only apply to character types like char and char32_t. Sequences of these produce a std::string attribute, while sequences of every other type produce std::vectors. There are a couple of reasons for this.

First, strings and vectors are different. We know that strings are just arrays of numbers, but we have a whole different type for them, std::string. It has a different API, and other code that operates on text expects a string instead of some other container. Arrays of characters are already considered special by the standard library and common practice in C++.

Second, When you write a parser that parses multiple characters in a row, you are typically trying to produce a string attribute, rather than a few individual character values. When you use multiple non-character parsers in a row, you are typically trying to produce multiple values. For instance:

namespace bp = boost::parser;
auto parser_1 = bp::char_ >> bp::char_ >> -bp::char_;
auto parser_2 = +(bp::char_ - ' ') >> ' ' >> +(bp::char_ - ' ');

I don't know about you, but I've rarely written a parser like parser_1 and wanted to produce a boost::parser::tuple<char, char, std::optional<char>>. Similarly, I've rarely written a parser like parser_2 and wanted a std::vector<std::string>.

Boost.Parser therefore makes the common case the default behavior, and provides you with the merge[] and separate[] directives to let you opt-in to generating the less-common attributes.

Attribute compatibility rules are more strict than in Spirit

Consider this parser.

namespace bp = boost::parser;
auto parser = -(bp::char_ % ',');

But Boost.Parser and Spirit consider the attribute type of this parser to be optional<SEQ-OF<char>>. However, Spirit allows you to parse that into a std::vector<char> or a std::string, and Boost.Parser does not. Boost.Parser requires you to parse that into a std::optional<std::string>, or change the parser to -(bp::char_ % ',') | bp::attr(std::string{}). In other words, Spirit considers an optional of a-sequence-of-one-or-more to be equivalent to just a sequence, because the empty state of the sequence represents the empty optional state. Boost.Parser does not. Why the strictness?

I understand why Spirit works that way — there's no loss of information, so why not? However, I don't agree with that approach.

When I write operator-, I get a std::optional. That's a simple rule.

If I did write operator-, I was opting in to getting a std::optional. The code expresses that intent. If the library changes my written intent, there better be a damn good reason.

There is of course an exception to the "simple rule" above -- if I write -p1 >> *p2, and the ATTR(p1) is the same as ATTR(p2), attributes are the same, the optional value gets slurped up into the container. I consider this a "damn good reason", because this is a very common use case. For other, less-common cases, separate[] can be used to keep the attributes non-combining. So separate[-int_ >> *int_] has the attribute boost::parser::tuple<std::optional<int>, std::vector<int>>. This makes opting out of this exception very easy, and the intent remains visible in the code.

By contrast, "I wrote -+int_ but I really want a std::vector<int> instead of a std::optional<std::vector<int>>" is not a really common use case.

Also, Spirit-style looseness is more complicated than parser above indicates. Remember, int_ | eps and -int_ are supposed to be semantically equivalent. To do otherwise this would be a profound violation of the principle of least surprise. So, if they're equivalent, we would need to apply the same rule to int_ | eps. Also, we would probably need to apply it to if_(cond)[int_], which is also a std::optional<int>. This is a lot to remember, and this is complicated to implement and maintain.

I've been using Spirit 1 and later Spirit 2 since they were released. I did not know about the particular looseness discussed here; a user pointed it out on Github. In many years of using these libraries, I never fully learned all the attribute-compatibility rules, and was often surprised by them.

Having a small set of rules that the user can internalize is vital; if the attribute generated is different from my expressed intent, that's a problem. For this not to be a problem, I need to be able to understand the rules, so I can express my intent, and not be surprised.

Out-parameter attributes passed to parse() are cleared on parse failure

At the end of a call to any of the parse() overloads that takes an attribute out-param (including variants like callback_parse(), etc.), the parse either succeeds or fails. If the call fails, the attribute is explicitly "cleared" by assigning its default-constructed value.

This is done because it's the less bad of two options. Consider the other option first.

// Without explicit clearing.
namespace bp = boost::parser;
std::vector<int> result;
auto b = bp::parse("3 4 c", +bp::int_, bp::ws, result);
assert(!b);
assert(result == std::vector<int>({3, 4}));

This is odd — the parse failed, but the out-param has partial results in it anyway. This happens because the parser +bp::int_ only fails if it cannot match at bp::int_ at least once. Above, it matches it twice, meaning that it succeeds (if it had failed, it would have cleared its attribute). It does not know that there is nothing after it that could continue the parse, nor that it is being used in to do a full parse. So, the over-all parse fails, but the part of the parse that fills in the out-param attribute does not know do clear its attribute.

This is why the explicit clearing behavior happens at the end of parse(). This is not without its downsides, though. Consider this.

// With explicit clearing.
namespace bp = boost::parser;
std::string str = "-42";
int i = 3;
bool b = parse(str, bp::uint_, i);
assert(!b);
assert(i == 0);

Here, the explicit clearing replaces the previous value of 3, even though the parser never touched the value! Destroying users' variables' state without need may seem like a bad idea, but consider the alternative — In the previous example, we had spurious values left in the out-param attribute. Here, without clearing, we would have had a value left in the out-param attribute, not because it was a partial result of the parse, but because the parse never touched it. This is certain to be confusing, or at least surprising, behavior. I deemed it better to make the failed parse case consistent, to reduce confusion. The out-param attribute of type A is always equal to A() if the parser fails. It is equal to whatever the parser sets it to — or its previous value, if the parser does not mutate it — if the parse succeeds.