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The error message is not informative enough. It will print the errors that are marked by label combinator, and add into a message list. But for a complex parser this error messages are usually lack of context and are hardly useful. For example. the error message for the demo/json.h currently looks like this

cmake --build . --target json-demo  11.90s user 0.48s system 99% cpu 12.447 total    
/home/jimmy/newDisk/Repo/cppparsec/demo//demo1.json                                                                                                  
terminate called after throwing an instance of 'cppparsec::parse_failed'                                                                                                    
  what():  parse failed. line: 2, column: 3. 2 errors occurred.                       
details (lastest on top):                                                                                                                                                   
 - expect character: }                   
 - "                                                                                                 

This only shows that something happened when parsig the character ", and it's saying it's expecting }. We don't know what's the context, and what parser it is currenly in.
Describe the solution you'd like
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Make error message more descrptive. Some information should be included:

• The proximity of the error -- where it happened, what does text around it looks like.
• The cause of the error. Not only ", but some messages like error when parsing the character " in parser: ch. [empty error]. The last bracket marks which continuation it invoked.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
Maybe add some compilication options to compile with all errors turned on, so we can read a trace of all errors. It's really hard to debug without knowing where things go wrong.

set up fuzzers

Currently the parser is unbarely slow. The calculator demo takes about 1 second to parse a simple arithmetic expression. It's expected becaues a lot of implementations are based on pass by value. It was intened to speed up the development process.

It''s nice to clean up all references and benchmark to see how much it will speed up before doing optmizations on the program's structure.

address sanitizer reports stack buffer overflow when token in core.h is calling the match callback passed from satisfy.

Parsers can have dependencies with each other, and we can't guarantee a parser always lives longer than those who depend on it. The current solution is simply copying the parser all the time. The plan is to use shared_ptr.

shared_ptr always has less overhead than std::functional when there is closure.

These two combinators are located at cppparsec/combinator.h:line 222. The implementation is roughly the same as sep_by. The expected behavior is documented in the comment.