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 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
