[Rd] Am I missing something about debugging?

Duncan Murdoch murdoch at stats.uwo.ca
Tue Jan 2 23:24:35 CET 2007

I don't think you're missing anything with the debug() function. It 
needs updating.

I don't think there's any structural reason why you shouldn't be able to 
do the things you're talking about in R, but they haven't been 

Mark Bravington put together a package (called debug) that does more 
than debug() does, but I haven't used it much, and I don't know if it 
does what you want.

I recently added things to the R parser to keep track of connections 
between R code and source files; that was partly meant as a first step 
towards improving the debugging facilities.  I'd be happy to help anyone 
who wants to do the hard work, but I don't think I'll be able to work on 
it before next summer.  (If you do decide to work on it, please let me 
know, just in case I do get a chance:  no point duplicating effort.)

Duncan Murdoch

On 1/2/2007 5:08 PM, Ross Boylan wrote:
> I would like to be able to trace execution into calls below the current
> function, or to follow execution as calls return.  This is roughly the
> distinction between "step" and "next" in many debuggers.
> I would also like to be able to switch to a location further up the call
> stack than the location at which I enter the debugger, to see the
> context of the current operations.
> Are there ways to do these things with the R debugger?  I've studied the
> man pages and FAQ's, and looked at the debug package, but I don't see a
> way except for manually calling debug on the function that is about to
> be called if I want to descend.  That's quite awkward, particularly
> since it must be manually undone (the debug package may be better on
> that score).  I'm also not entirely sure that such recursion
> (essentially, debugging within the debugger) is OK.
> I tried looking up the stack with things like sys.calls(), from within
> the browser, but they operated as if I were at the top level (e.g.,
> sys.function(-1) gets an error that it can't go there).  I was doing
> this in ess, and there's some chance the "can't write .Last.value" error
> (wording approximate) cause by having an old version is screwing things
> up).
> Since R is interpreted I would expect debugging to be a snap, but these
> limitations make me suspect there is something about the language design
> that makes implementing these facilities hard.  For example, the browser
> as documented in the Green book has up and down functions to change the
> frame (p. 265); these are conspicuously absent in R.

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