In AnalysisPageServer: A framework for sharing interactive data and plots from R through the web

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Condition Trapping with AnalysisPageServer

In R "conditions" are messages, warnings and errors. AnalysisPageServer contains an API, completely independent of its other components, to trap all of these and be able to examine the call stacks.

First let's make some functions so that we'll have non-trivial call stacks.

e2 <- function(...)  stop(...)
e1 <- function(...)  e2(...)

w2 <- function(...)  warning(...)
w1 <- function(...)  w2(...)

m2 <- function(...)  message(...)
m1 <- function(...)  m2(...)

To use this functionality wrap your code in a call to tryKeepConditions(), which returns an object of class class "AnalysisPageValueWithConditions". Despite the name this is completely independent of the rest of AnalysisPageServer. We'll build two such objects, one without an error and one with

library(AnalysisPageServer)
vwc <- tryKeepConditions({
  m1("first message")
  m1("second message")
  w1("a warning")
  42
})
vwcErr <- tryKeepConditions({
  w1("a warning before the error")
  e1("this is a bad error")
  42
})

All of the functions to access these types of objects begin with vwc..

Check if your expression resulted in an error, and, if not, see what the value was (NULL if it did result in an error):

vwc.is.error(vwc)
vwc.is.error(vwcErr)
vwc.value(vwc)
vwc.value(vwcErr)

To inspect the three types of conditions there are a family of functions called vwc.TYPE(s).condition(s)(), vwc.TYPE(s)() and vwc.TYPE(s).traceback(s)() where TYPE is message, warning or error. The (s) is present in the function names for messages and warnings since there can be more than one, but absent for errors, since there can be only one.

vwc.TYPE(s).condition(s)() returns the condition object, or, for messages and warnings, a list of all the condition objects:

vwc.messages.conditions(vwc)
vwc.messages.conditions(vwcErr)
vwc.warnings.conditions(vwc)
vwc.warnings.conditions(vwcErr)
vwc.error.condition(vwc)
vwc.error.condition(vwcErr)

vwc.TYPE(s) returns the condition messages as strings or character vectors:

vwc.messages(vwc)
vwc.messages(vwcErr)
vwc.warnings(vwc)
vwc.warnings(vwcErr)
vwc.error(vwcErr)

(vwc.error(vwc) would throw a (new) error if the expression did not result in an error, so we don't call it here.)

Finally, vwc.TYPE(s).traceback(s)() return tracebacks as character vectors. Note that the "real" tracebacks can be extracted from the condition objects as returned by vwc.TYPE(s).conditions(s)(), but these functions return something suitable for showing to an end user.

(Since this vignette was rendered with knitr the call stacks are fairly complicated, but if you run it in an R session you would just see the top few levels.)

vwc.messages.tracebacks(vwc)
vwc.messages.tracebacks(vwcErr)
vwc.warnings.tracebacks(vwc)
vwc.warnings.tracebacks(vwcErr)
vwc.error.traceback(vwc)
vwc.error.traceback(vwcErr)

SessionInfo

sessionInfo()

AnalysisPageServer documentation built on April 28, 2020, 6:32 p.m.