[Rd] comment causes browser() to exit (PR#9063)

Tim Hesterberg timh at insightful.com
Fri Jul 7 19:22:10 CEST 2006

>If I'm not mistaken, this works as documented.  ...

Thanks for the response.

The behavior with <return> is as documented -- hence my earlier
"enhancement" request.
The behavior with <a comment> is contrary to the documentation, hence
this as a "bug" report.

On the second point -- help(browser) says:
     Anything else entered at the browser prompt is interpreted as an R
     expression to be evaluated in the calling environment: ...
whereas the actual behavior is to interpret a comment as equivalent to "c",
and to exit the browser.

On the first point -- I would like to argue that the behavior of
return (and comments) should be changed, at least as a user option.
Here is a typical piece of code from a function; note the use of
comments and blank lines to improve readability:

  # If statistic isn't a function or name of function or expression,
  # store it as an expression to pass to fit.func.
  substitute.stat <- substitute(statistic)
  if(!is.element(mode(substitute.stat), c("name", "function")))
    statistic <- substitute.stat

  # Get name of data.
  data.name <- substitute(data)
    data.name <- "data"
  is.df.data <- is.data.frame(data)

  # How many observations, or subjects if sampling by subject?
  n <- nObservations <- numRows(data)  # n will change later if by subject

  # Save group or subject arguments?
  if(is.null(save.subject)) save.subject <- (n <= 10000)
  if(is.null(save.group))   save.group   <- (n <= 10000)

If I now stick a browser() in that function, and throw a line at a
time from the source file to R, it exits whenever I throw a blank line
or comment.  I try to remember to skip the blank lines and comments,
but I sometimes forget, and get very annoyed when I have to start over.

I could use "c" in some contexts, but not others:
* I often want to evaluate code that is not part of the
  defined function.
* I sometimes change objects and want to go evaluate some lines
  that were previously evaluated.

In the enhancement request I requested an option to turn off the current
behavior of return.  I personally would just change the default behavior,
and have both blank lines and comments do nothing.  This is simpler, and
people can always use c to quit the browser.

This behavior of browser() is the most annoying thing I've found
about using R.  As I anticipate using R a lot in the future, I would
appreciate very much if it is changed.  I spent a fair amount of time
trying to see if I could change it myself, but gave up.

Tim Hesterberg

Andy Liaw wrote:
>If I'm not mistaken, this works as documented.  As an example (typed
>directly into the Rgui console on WinXP):
>R> f <- function() {
>+     browser()
>+     cat("I'm here!\n")
>+     cat("I'm still here!\n")
>+ }
>R> f()
>Called from: f()
>Browse[1]> ## where to?
>I'm here!
>I'm still here!
>which I think is what you saw.  However:
>R> f()
>Called from: f()
>Browse[1]> n
>debug: cat("I'm here!\n")
>Browse[1]> ##
>I'm here!
>debug: cat("I'm still here!\n")
>I'm still here!
>From ?browser:
>(or just return) exit the browser and continue execution at the next
>synonym for c. 
>enter the step-through debugger. This changes the meaning of c: see the
>documentation for debug. 
>My interpretation of this is that, if the first thing typed (or pasted) in
>is something like a null statement (e.g., return, empty line, or comment),
>it's the same as 'c', but if the null statement follows 'n', then it behaves
>               _                         
>platform       i386-pc-mingw32           
>arch           i386                      
>os             mingw32                   
>system         i386, mingw32             
>major          2                         
>minor          3.1                       
>year           2006                      
>month          06                        
>day            01                        
>svn rev        38247                     
>language       R                         
>version.string Version 2.3.1 (2006-06-01)
>From: timh at insightful.com
>> I'm trying to step through some code using browser(), 
>> executing one line at a time.
>> Unfortunately, whenever I execute a comment line, the browser exits.
>> I previously reported a similar problem with blank lines.
>> These problems are a strong incentive to write poor code
>> -- uncommented code with no blank lines to improve 
>> readability -- so that I can use browser() without it exiting 
>> at inconvenient times.
>> Additional detail:  
>> (1) I'm running R inside emacs, with R in one buffer and a 
>> file containing code in another, using a macro to copy and 
>> paste a line at a time from the file to the R buffer.
>> (2) The browser() call is inside a function.  
>> Right now the lines I'm sending to the browser are not part 
>> of the function, though usually they are.
>> --please do not edit the information below--
>> Version:
>>  platform = i386-pc-mingw32
>>  arch = i386
>>  os = mingw32
>>  system = i386, mingw32
>>  status =
>>  major = 2
>>  minor = 3.1
>>  year = 2006
>>  month = 06
>>  day = 01
>>  svn rev = 38247
>>  language = R
>>  version.string = Version 2.3.1 (2006-06-01)
>> Windows XP Professional (build 2600) Service Pack 2.0
>> Locale:
>> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United 
>> States.1252;LC_MONETARY=English_United 
>> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
>> Search Path:
>>  .GlobalEnv, package:methods, package:stats, 
>> package:graphics, package:grDevices, package:utils, 
>> package:datasets, Autoloads, package:base
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
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