YAML doesn't round-trip generics #715

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1. liuzicheng1987 commented on Aug 23, 2026

   

   liuzicheng1987

   on Aug 23, 2026

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   @zewt to be honest, I think this might even be a YAML issue. The YAML spec specifies that on/off are supposed to be interpreted as booleans.

   https://yaml.org/type/bool.html
2. liuzicheng1987 commented on Aug 23, 2026

   

   liuzicheng1987

   on Aug 23, 2026

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   @zewt the issue is in yaml-cpp:

   https://github.com/jbeder/yaml-cpp/blob/master/src/convert.cpp

   As you can see, on/off are interpreted as booleans. And they explicitly reference the same page that I shared as well.
3. zewt commented on Aug 23, 2026

   

   zewt

   on Aug 23, 2026

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   That's just where the conversion is happening, not why. It happens for numbers too.

       rfl::Generic a("1234");
       string yaml = rfl::yaml::write(a);
       rfl::Generic b = rfl::yaml::read<rfl::Generic>(yaml).value();
       string result = b.to_string().value_or("error");
   

   results in "error", since b contains a double instead of a string.

   And this fails:

       struct test
       {
           string a = "1234";
       };
       test value;
   
       string yaml = rfl::yaml::write(value);
       rfl::Generic generic = rfl::yaml::read<rfl::Generic>(yaml).value();
       auto value2 = rfl::from_generic<test>(generic);
   

   "a" has become a double inside "generic", so from_generic fails with "Failed to parse field 'a': rfl::Generic: Could not cast the underlying value to a string."

   It seems like yaml-cpp doesn't handle types at the yaml level at all. It just parses all values to strings, and then node.as() converts to whichever data type is asked for. Generic::ReflectionType asks for bools and doubles before strings, so it asks for a bool, yaml-cpp sees the string "true" and happily converts it. I don't think that's a yaml-cpp bug, it's just how yaml-cpp works.

   yaml-cpp also doesn't quote strings that look like numbers or bools when serializing, which would be needed for that to be possible. I don't think yaml-cpp is meant to be type-preserving in the way reflect-cpp wants, even though YAML itself is perfectly capable of it.

   I've switched back to JSON, since this doesn't seem like something that can be fixed easily (or maybe at all without changing the yaml backend). But this is a bit of a landmine for people using rfl::Generic with YAML.