I'm also wondering what payload people want. There's already an error handling trace (similar but different to a normal stack trace) that captures the how the error propagates up to the point it's being handled so shows you exactly where the initial point was.
A payload can contain any details that the producer of that error believes are important. I don't know what you mean by the error handling trace, how does that help with the toy JSON parsing example? I want the payload to indicate the position in the input string where the parser encountered an error.
I want a function like `diffFiles(path_a, path_b)` to have an error set of `error { ReadError }` with more detailed information in the payload (e.g. file path, translated error code). The alternative is: `error { FileAReadErrorNotFound, FileBReadErrorNotFound, FileAReadErrorPermissionDenied, FileBReadErrorPermissionDenied, ...}`
