R devel - Feb 2024 - Get list of active calling handlers?

The SO post https://stackoverflow.com/q/77943180 tried to call 
globalCallingHandlers() from a function, and it failed with the error 
message "should not be called with handlers on the stack".  A much 
simpler illustration of the same error comes from this line:

   try(globalCallingHandlers(warning = function(e) e))

The problem here is that try() sets an error handler, and 
globalCallingHandlers() sees it and aborts.

If I call globalCallingHandlers() with no arguments, I get a list of 
currently active global handlers.  Is there also a way to get a list of 
active handlers, including non-global ones (like the one try() added in 
the line above)?

Duncan Murdoch

On Tue, 6 Feb 2024, Duncan Murdoch wrote:
> The SO post https://stackoverflow.com/q/77943180 tried to call 
> globalCallingHandlers() from a function, and it failed with the error
message
> "should not be called with handlers on the stack".  A much
simpler
> illustration of the same error comes from this line:
>
>  try(globalCallingHandlers(warning = function(e) e))
>
> The problem here is that try() sets an error handler, and 
> globalCallingHandlers() sees it and aborts.
>
> If I call globalCallingHandlers() with no arguments, I get a list of 
> currently active global handlers.  Is there also a way to get a list of 
> active handlers, including non-global ones (like the one try() added in the
> line above)?
There is not. The internal stack is not safe to allow to escape to the
R level.  It would be possible to write a reflection function to
provide some information, but it would be a fair bit of work to design
and I don't think would be of enough value to justify that.

The original SO question would be better addressed to
Posit/RStudio. Someone with enough motivation might also be able to
figure out an answer by looking at the source code at
https://github.com/rstudio/rstudio.

Best,

luke

>
> Duncan Murdoch
>
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-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
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