Hi! I've started refreshing the patch[0] to improve exception presentation for R6RS exceptions, initially discussed in [1]. I'd like to solicit clarification on a few points.
To recap, it was agreed to add something like the following internal API to boot-9.scm: exception-printer := port key args exception-printer -> nothing set-exception-printer! : key exception-printer -> nothing exception-printer : key -> exception-printer OR #f In the above, exception-printer is a procedure that's supposed to either display the exception it got passed (via `args') on `port', or "punt", by calling the exception printer passed to it. Note that, relative to the previous discussion, I've added the argument `key', to make it possible for the fallback (generic) exception printer, as passed to any registered exception printer as its last argument, to display the key of the raised exception. Another option (which I slightly prefer) would be this change, relative to the above API: exception-printer := port args thunk In this variant, the exception printer may call `thunk' to punt on handling the exception -- the `key' argument is not necessary anymore, as it can be closed over by the thunk. This exception registry would be used in the REPL error-handling code primarily, via a `print-exception' procedure querying the registry and invoking the matching printer (or the default one, if no printer is matching). Another open issue is the potential `frame' argument; it seems this is only used at one place in `(system repl error-handling)', inside `call-with-error-handling' (I did a quick grep for "display-error" and "Throw to" to identify the sites where I'd plug in the exception registry via the `print-exception' procedure). If we decide that `frame' should not be part of the exception-printer arguments, we'd lose source information in the exception printout in this case. As `call-with-error-handling' is only used in the REPL (where one could get source information via the ",bt" meta-command) and in `(web server)', I guess the loss would be bearable. [ Just a thought: it looks like it /might/ make sense to contemplate deprecating passing the frame information to `display-error' and untangle displaying the exception object and displaying a backtrace completely. ] Regards, Rotty -- Andreas Rottmann -- <http://rotty.yi.org/>
