Re: R6RS exception printing -- take #2

[Andy Wingo <wingo@pobox.com>]

Heya Andreas,

You proposed a choice between two options:

On Tue 01 Feb 2011 01:19, Andreas Rottmann <a.rottmann@gmx.at> writes:

>   exception-printer := port key args exception-printer -> nothing
>   exception-printer := port args thunk

The `key' argument is necessary, at least if you want one printer to
handle multiple keys (as the default printer might).  I prefer the
first, FWIW.

> 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).

There are other cases that it would be nice to use it: in the catch-all
in throw.c, and in general when printing exceptions from C.

> 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.

Let's keep the frame out of the exception-printer functions, but still
pass it to the procedure that does the exception printing dispatch
(perhaps called print-exception or something).  That way we can print
source information, then let exception printers do their thing.

> [ 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. ]

There are certainly some tangles here.  If you find The Right Thing, let
us know...

Andy
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