r_session_debug {callr}R Documentation

Interactive debugging of persistent R sessions


The r_session$debug() method is an interactive debugger to inspect the stack of the background process after an error.


$debug() starts a REPL (Read-Eval-Print-Loop), that evaluates R expressions in the subprocess. It is similar to browser() and debugger() and also has some extra commands:

To exit the debugger, press the usual interrupt key, i.e. CTRL+c or ESC in some GUIs.

Here is an example session that uses $debug() (some output is omitted for brevity):

# ----------------------------------------------------------------------
> rs <- r_session$new()
> rs$run(function() knitr::knit("no-such-file"))
Error in rs_run(self, private, func, args) :
 callr subprocess failed: cannot open the connection

> rs$debug()
Debugging in process 87361, press CTRL+C (ESC) to quit. Commands:
  .where       -- print stack trace
  .inspect <n> -- inspect a frame, 0 resets to .GlobalEnv
  .help        -- print this message
  <cmd>        -- run <cmd> in frame or .GlobalEnv

3: file(con, "r")
2: readLines(input2, encoding = "UTF-8", warn = FALSE)
1: knitr::knit("no-such-file") at #1

RS 87361 > .inspect 1

RS 87361 (frame 1) > ls()
 [1] "encoding"  "envir"     "ext"       "in.file"   "input"     "input.dir"
 [7] "input2"    "ocode"     "oconc"     "oenvir"    "oopts"     "optc"
[13] "optk"      "otangle"   "out.purl"  "output"    "quiet"     "tangle"
[19] "text"

RS 87361 (frame 1) > input
[1] "no-such-file"

RS 87361 (frame 1) > file.exists(input)

RS 87361 (frame 1) > # <CTRL + C>
# ----------------------------------------------------------------------

[Package callr version 3.7.0 Index]