[R] [External] Re: Parser For Line Number Tracing

Sun Jan 19 22:28:22 CET 2025

John,

What would be nice is not necessarily trivial.

Consider an example. After running a line of code with an assignment statement, did you know the last value has been saved in .Last.value as in this snippet of code:

> x <- 12
> .Last.value
[1] 12
> y <- .Last.value**2
> y
[1] 144
> .Last.value
[1] 144

Now how many novices would know to use this, say in a complicated calculation where another statement uses the results from before?

So, what if R saved an unevaluated line of code as .Last_text_evaluated or something. For all I know, something like that may exist and perhaps might be shown by something like ls(all.names=TRUE) or something.

Novices likely will not know about, nor many experts. It might not even be set if an error happens. Even if available, no current mechanism exists to show it after each line.

oThere are ways to save your history and restore it and so on, and running R interactively may allow something as simple as using up-arrows to see the last statements. RSTUDIO allows you to see all recent commands in a window.

So, if a program STOPS, you should be able to at least see the preceding line of code.

But what do you suggest should be the last command in something like a multiple embedded loop? Is it the code for many lines, or just the one being run? How do you deal with commands spread over multiple lines?

-----Original Message-----
From: Sorkin, John <jsorkin using som.umaryland.edu> 
Sent: Sunday, January 19, 2025 3:47 PM
To: avi.e.gross using gmail.com; 'Ivo Welch' <ivo.welch using gmail.com>; luke-tierney using uiowa.edu
Cc: r-help using r-project.org
Subject: Re: [R] [External] Re: Parser For Line Number Tracing

Avi,
Yes, R (really S) was not designed for novice users but rather for experts. For better or worse it has evolved into a programming language used by tyros, and experts. Debugging tools should be easy to use, generally known, and helpful for tyro and expert.  It would certainly help if R reported the line number, or the code, that generated the error.
John

John David Sorkin M.D., Ph.D.
Professor of Medicine, University of Maryland School of Medicine;
Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center;
PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
Senior Statistician University of Maryland Center for Vascular Research;

Division of Gerontology and Paliative Care,
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
Cell phone 443-418-5382

________________________________________
From: R-help <r-help-bounces using r-project.org> on behalf of avi.e.gross using gmail.com <avi.e.gross using gmail.com>
Sent: Sunday, January 19, 2025 3:31 PM
To: 'Ivo Welch'; luke-tierney using uiowa.edu
Cc: r-help using r-project.org
Subject: Re: [R] [External] Re:  Parser For Line Number Tracing

Arguably, R was not designed or evolved for truly novice users, nor really was Python or just about all computer languages. As they evolved and became in many ways more powerful, they tended to get ever less user friendly in the way you are asking for and gotten so bloated that many features are not familiar even to expert users.

Compiled languages can have ways to keep track of what LINE of code is being worked on. You can sometimes compile them with a flag that preserves information needed or to have it run with less overhead.

Interpreted languages that can be typed in as you go along are a challenge in that a concept like line number may not be trivial to use. Do you remember what was the 325th line you typed in? R does not normally display line numbers in a prompt.

As noted, there is plenty of debug info, not to mention what you can examine while running inside a debugger, that can be helpful.

Another consideration to consider is that you can run snippets of R code in cells as in a Markdown document, Notebook, Quarto or Jupyter  notebook  along with pieces of text around it. RSTUDIO supports some of these and there are likely other variants out there.

Since each cell can contain a limited set of code, down to as little as one instruction, and you can run segments one after another, or all at once to a point, and there can be text with info between cells, you might find where things go wrong and localize them in many cases. Of course, if a cell contains a loop that breaks after N iterations, not so much.

It is a problem now that so many people do not study a programming language for itself, but only want to do the minimum needed to use it as a one-time tool for solving something else they are studying. These days, you can ask a pseudo-AI to write you a snippet of code that may work and learn next to nothing you can use to do more.

-----Original Message-----
From: R-help <r-help-bounces using r-project.org> On Behalf Of Ivo Welch
Sent: Sunday, January 19, 2025 12:01 PM
To: luke-tierney using uiowa.edu
Cc: r-help using r-project.org
Subject: Re: [R] [External] Re: Parser For Line Number Tracing

understood.

but, please, consider not people like me but unwary beginners and
students of R.  I have used R now for decades, and even I am baffled
by it.  Couldn't you make R code easier to debug not only for people
like me (who can indeed tweak their environments) but also for novice
users?

On Sun, Jan 19, 2025 at 8:46 AM <luke-tierney using uiowa.edu> wrote:
>
> On Sun, 19 Jan 2025, Ivo Welch wrote:
>
> > Hi Duncan — Wonderful.  Thank you.  Bug or no bug, I think it would be
> > a huge improvement for user-friendliness if R printed the last line by
> > default *every time* a script dies.  Most computer languages do so.
> >
> > Should I file it as a request for improvement to the R code
> > development team?  Maybe R can be improved at a very low cost to the
> > development team and a very high benefit to newbies.
>
> No. There are already many ways to influence the way the default error
> handler prints information about errors, mstly via options(). In
> particular you may want to look at entries in ?options for
>
> show.error.locations
> showErrorCalls
> showWarningCalls
>
> and adjust your options settings accordingly.
>
> Best,
>
> luke
>
> >
> > Regards,
> >
> > /ivo
> >
> > On Sun, Jan 19, 2025 at 2:39 AM Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
> >>
> >> On 2025-01-18 8:27 p.m., Ivo Welch wrote:
> >>> I am afraid my errors are worse!  (so are my postings.  I should have
> >>> given an example.)
> >>>
> >>> ```
> >>> x <- 1
> >>> y <- 2
> >>> nofunction("something stupid I am doing!")
> >>> z <- 4
> >>> ```
> >>>
> >>> and
> >>>
> >>> ```
> >>>> source("where-is-my-water.R")
> >>> Error in nofunction("something stupid I am doing!") :
> >>>    could not find function "nofunction"
> >>> ```
> >>>
> >>> and no traceback is available.
> >>
> >> Okay, I see.  In that case traceback() doesn't report the line, but it
> >> still is known internally.  You can see it using the following function:
> >>
> >> showKnownLocations <- function() {
> >>    calls <- sys.calls()
> >>    srcrefs <- sapply(calls, function(v) if (!is.null(srcref <- attr(v,
> >>
> >> "srcref"))) {
> >>      srcfile <- attr(srcref, "srcfile")
> >>      paste0(basename(srcfile$filename), "#", srcref[1L])
> >>    } else ".")
> >>    cat("Current call stack locations:\n")
> >>    cat(srcrefs, sep = " ")
> >>    cat("\n")
> >> }
> >>
> >> I haven't done much testing on this, but I think it can be called
> >> explicitly from any location if you want to know how you got there, or
> >> you can set it as the error handler using
> >>
> >>    options(error = showKnownLocations)
> >>
> >> For example, try this script:
> >>
> >>    options(error = showKnownLocations)
> >>    f <- function() showKnownLocations()
> >>    x <- 1
> >>    f()
> >>    y <- 2
> >>    nofunction("something stupid I am doing!")
> >>    z <- 4
> >>
> >> I see this output from source("test.R"):
> >>
> >>   > source("test.R")
> >>    Current call stack locations:
> >>    . . . . test.R#4 test.R#2
> >>    Error in nofunction("something stupid I am doing!") :
> >>      could not find function "nofunction"
> >>    Current call stack locations:
> >>    . . . . test.R#6
> >>
> >> The first report is from the explicit call in f() on line 2 that was
> >> invoked on line 4, and the second report happens during error handling.
> >>
> >> I supppose the fact that traceback() isn't showing you the line 6
> >> location could be considered a bug.
> >>
> >> Duncan Murdoch
> >>
> >>
> >
> > ______________________________________________
> > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide https://www.r-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
>
> --
> 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 using uiowa.edu
> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu/

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