This is a lot like when you use try, it is followed by a list of catch statements. I wouldn't call that unmaintainable.

> The second thing that strikes me is that each error handler takes a `leaf::match` that is a variadic template, but what are its valid parameters? Also, the lambda itself takes a different number parameters, and the relation between what is inside `leaf::match<...>` and the rest of lambda parameters is totally non-obvious. So... ok, bad for learnability, it seems I have to check the docs every single time I want to handle an error.

Yes it helps to read the docs. You don't have to use match, it's just a way to select a subset of enumerated values automatically. If you want to write a handler for some enum type my_error_code, you could just say:

[]( my_error_code ec )

{

  switch(ec)

  {

    case err1:

    case err2:

      .... // Handle err1 or err2 errors

    case err5:

      .... // Handle err5 errors

  }

}

But you could instead use match to split it into two handlers:

[]( leaf::match<my_error_code, err1, err2> )

{

  // Handle err1 or err2 error

},

[]( leaf::match<my_error_code, err5> )

{

  // Handle err5 errors
}

> Having to "tell" LEAF that an enum is error code by directly poking into its "implementation" namespace feels "dirty". Yes, I know, that's how C++ works, even `std::` has "customization points", but it still feels "dirty".

This is done to protect the user from accidentally passing to LEAF some random type as an error type (since LEAF can take pretty much any movable object). I'm considering removing is_e_type.

> Then, macros, `LEAF_AUTO` and `LEAF_CHECK`. This doesn't belong to modern C++. (Indeed, how will it play with upcoming modules?)

You don't have to use the helper macros. They're typical for any error handling library that can work without exceptions. You don't need them if you use exceptions.

> So exception handlers: `leaf::catch_`. The same questions/problems apply as for matching. We have `input_file_open_error`, the lambda expects a file name, but the file name is nowhere supplied when throwing an error. Oh, wait, that's the purpose of `preload` I guess. I overlooked that one with the sample code using `leaf::match`.

Helps to read the docs.

> From what I've read, I don't like it and I can't see myself using it: Overall, it feels as if it's heavily biased towards error-handling based on return values, exceptions being a 2nd-class citizen.

The library is neutral towards exceptions vs. error codes, if anything it lets you deal with return values as easily as with exceptions. In my own code I prefer exceptions, so most definitely not 2nd class.

> Whether an error handler executes depends on whether the error-configuration part of the code (preload/defer) has executed. Does not bode well for robust error handling.

You can write the code so the same error handler is matched regardless of whether e.g. e_file_name is available:

[]( catch_<file_read_error>, e_file_name const * fn )

{

  if( fn )

    // use e_file_name

  else

    // e_file_name not available
}

You could write a handler that takes all pointer arguments and then it would match all errors and you can do your own logic.