[Rd] "try"ing to understand condition handling

Prof Brian Ripley ripley at stats.ox.ac.uk
Tue Feb 20 08:35:51 CET 2007

Since you have not told us what 'the documents' are (and only vaguely 
named one), do you not think your own documentation is inadequate?

There are documents about the condition system on developer.r-project.org: 
please consult them.

I quess 'ctl-C' is your private abbreviation for 'control C' (and not a 
type of cancer): that generates an interrrupt in most (but not all) R 
ports.  Where it does, you can set up interrupt handlers (as the help page 

On Mon, 19 Feb 2007, Ross Boylan wrote:

> I'm confused by the page documenting tryCatch and friends.
> I think it describes 3 separate mechanisms: tryCatch (in which control
> returns to the invoking tryCatch), withCallHandlers (in which control
> goes up to the calling handler/s but then continues from the point at
> which signalCondition() was invoked), and withRestarts (I can't tell
> where control ends up).
> For tryCatch the docs say the arguments ... provide handlers, and that
> these are matched to the condition.  It appears that matching works by
> providing entries in ... as named arguments, and the handler matches
> if the name is one of the classes of the condition.  Is that right?  I
> don't see the matching rule explicitly stated.  And then  the handler
> itself is a single argument function, where the argument is the
> condition?
> My reading is that if some code executes signalCondition and it is
> running inside a tryCatch, control will not return to the line after
> the signalCondition.  Whereas, if the context is withCallHandlers,
> the call to signalCondition does return (with a NULL) and execution
> continues.  That seems odd; do I have it right?
> Also, the documents don't explicitly say that the abstract subclasses
> of 'error' and 'warning' are subclasses of 'condition', though that
> seems to be implied and true.
> It appears that for tryCatch only the first matching handler is
> executed, while for withCallHandlers all matching handlers are
> executed.
> And, finally, with restarts there is again the issue of how the name
> in the name=function form gets matched to the condition, and the more
> basic question of what happens.  My guess is that control stays with
> the handler, but then this mechanism seems very similar to tryCatch
> (with the addition of being able to pass extra arguments to the
> handler and  maybe a more flexible handler specification).
> Can anyone clarify any of this?
> P.S. Is there any mechanism that would allow one to trap an interrupt,
> like a ctl-C,  so that if the user hit ctl-C some state would be
> changed but execution would then continue where it was?  I have in
> mind the ctl-C handler setting a "time to finish up" flag which the
> maini code checks from time to time.
> Thanks.
> Ross Boylan
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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