R devel - Jun 2020 - Unexpected Error Handling by Generic in R 4.0.1

Hello R-devel community,

I posted a new R 4.0.1 behaviour to stack overflow
(https://stackoverflow.com/questions/62327810/inconsistent-error-handling-of-function-and-s4-generics-on-r-4-0-1),
where I think it is an undesired or unexpected change in 4.0.1. Attributes of
errors seem to be lost or obscured when encountered in an S4 generic context.

An example of this being undesirable comes in shiny applications where
my_reactive (an unevaluated reactive object) returns a shiny.silent.error
attribute which is lost upon error within an S4 generic function. The lack of
this attribute causes the entire application to exit with an error (with no
stack trace available). For example, within a shiny context:
> foo <- try(nrow(my_reactive()))
> attr(foo,"condition")
<shiny.silent.error: >

Where the S4 generic returns:
> bar <- try(BiocGenerics::nrow(my_reactive()))
> attr(bar,"condition")
<simpleError in (function (cond) .Internal(C_tryCatchHelper(addr, 1L,
cond)))(structure(list(message = ""), class =
c("shiny.silent.error", "validation", "error",
"condition"), stack.trace = [... lots more output here ...]: error in
evaluating the argument 'x' in selecting a method for function
'nrow': >
>From what I can tell from the release notes of 4.0.1, this does not appear
to be an expected breaking change so I am hesitant to update old code and shiny
applications to account for this behaviour. Any guidance would be appreciated.
Thank you,

Matthew Carlucci

CONFIDENTIALITY NOTICE: This e-mail message, including a...{{dropped:18}}

Thanks for the report.

This is due to a change restoring behavior that was disabled
temporarily to work around a bug
(https://bugs.r-project.org/bugzilla/show_bug.cgi?id=16111).
So it is again working as originally designed.

There are a number of places in the S4 dispatch code where errors are
caught and re-signaled with some additional information about the
dispatch context that might be helpful. Unfortunately all that is
retained from the original error is the message. It would be better to
signal a structured error object that includes the original error in a
slot.

Some options:

    Create and signal a structured error wrapping the original error in
    these cases.

    Revert the argument evaluation to not wrap errors.

    Drop wrapping of error from all other cases.

I don't have strong views on which way to go. But wrapping and
re-signaling from C would take a decent amount of effort so isn't likely
to happen without someone contributing a well-tested patch.

Best,

luke

On Thu, 25 Jun 2020, Matthew Carlucci wrote:
> Hello R-devel community,
>
> I posted a new R 4.0.1 behaviour to stack overflow
(https://stackoverflow.com/questions/62327810/inconsistent-error-handling-of-function-and-s4-generics-on-r-4-0-1),
where I think it is an undesired or unexpected change in 4.0.1. Attributes of
errors seem to be lost or obscured when encountered in an S4 generic context.
>
> An example of this being undesirable comes in shiny applications where
my_reactive (an unevaluated reactive object) returns a shiny.silent.error
attribute which is lost upon error within an S4 generic function. The lack of
this attribute causes the entire application to exit with an error (with no
stack trace available). For example, within a shiny context:
>
>> foo <- try(nrow(my_reactive()))
>
>> attr(foo,"condition")
>
> <shiny.silent.error: >
>
> Where the S4 generic returns:
>
>> bar <- try(BiocGenerics::nrow(my_reactive()))
>
>> attr(bar,"condition")
>
> <simpleError in (function (cond) .Internal(C_tryCatchHelper(addr, 1L,
cond)))(structure(list(message = ""), class =
c("shiny.silent.error", "validation", "error",
"condition"), stack.trace = [... lots more output here ...]: error in
evaluating the argument 'x' in selecting a method for function
'nrow': >
>
> From what I can tell from the release notes of 4.0.1, this does not appear
to be an expected breaking change so I am hesitant to update old code and shiny
applications to account for this behaviour. Any guidance would be appreciated.
>
> Thank you,
>
> Matthew Carlucci
>
> CONFIDENTIALITY NOTICE: This e-mail message, including a...{{dropped:18}}
>
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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
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu