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design.md

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design of error

  • interface is enforced by a concept that enforces operators to compare error classes and instances, print the error message
  • on implementing an error a static array of chars is given that says what has gone wrong
  • form that a static hash is generated that serves as error uid
  • errors can be chained so that we can track down what is the root cause of a surfacing error. this is implemented by a parent error instance that is owned by the error itself.
  • keep all memory stack allocated for speed.

tools

Anyhow !-operator

!-operator like error handling is a killer feature since it is so fast and easy. To implement sth like that in c++ IMO only Macros can do that (although I don't like MACROS) since we need to return the error when std::unexpected happens. So far I also only found means to do that with a gcc extension that is unavailable to clang

Possible Names:

  • ANYWHO()
  • PROPAGATE()
  • PROP()
  • P()
  • VALUE()
  • V()

adding context

We want to be able to add context to errors so we can give more informatino why the error happened. we will add the info:

  • Message
  • line
  • file we decide to not use dynamic strings but static char arrays with a max size of 128. Hence memory can still be allocated on the stack keeping it efficient. Anywho since context usually gets concatenated, longer messages are too verbose.

syntax possibilities

 // std::expected<std::string, IOError> readFile(const std::string path);
 std::string content = ANYWHO(readFile(path).transform_error([](const auto& error){return error.add_context(std::format("Failed to read instrs from {}", path)})));

This is too long.

 // std::expected<std::string, IOError> readFile(const std::string path);
 std::string content = ANYWHO(readFile(path).with_context(std::format("Failed to read instrs from {}", path)));

would be better very similar to rust anyhow. We could Define anywho::Result<Type, Error> that defines with_context and casts to std::expected<Type,Error>.

  1. Add function
 // std::expected<std::string, IOError> readFile(const std::string path);
 std::string content = ANYWHO(with_context(readFile(path), std::format("Failed to read instrs from {}", path)));
  1. Use context class instead of string to save also line and file

Q&A

  • Q: Why not use a AnyError abstract class as interface A: Possible, but I shun the overhead of v-tables and the nightmare that abundant inheritance can create.
  • Q: Does ithappen often enough that one problem causes several errors so that we can not afford the copy? A: ?