The problem (which is a deliberate design decision) is that Zig's error unions, which have special built-in support at the syntax level, are just tagged unions

The problem (which is a deliberate design decision) is that Zig's error unions, which have special built-in support at the syntax level, are just tagged unions with an integer payload. In other words, when you return an error (either implicitly via `try` or directly with `return error.MyError`), that error payload is just an integer indicating which error was returned. So to use OP's parser example, there's not a built-in way to attach some sort of diagnostic info to the error itself, the way you would with, for example, python exceptions. Using the built-in syntax, you can't do the equivalent of `raise ParserException(token, line_number)` and then unpack that error later in your code to display a nice error message. The workaround would be to store that info in some sort of global struct (which has all the usual drawbacks of global variables); add a context/diagnostics parameter to your function, or manually manage your own error union in userspace, which means you lose the nice `try` syntax.