Hello,
Double thanks, for the correction and for the lesson.
In particular, I had never noticed the f2() example's behavior.
It's always good to learn something new.
Rui Barradas
Às 08:57 de 29/07/21, Martin Maechler escreveu:
Rui Barradas
on Thu, 29 Jul 2021 07:52:02 +0100 writes:
> Hello,
> R 4.1.0 on Ubuntu 20.04, session info below.
> I am not an English native speaker but in help("browser"),
> section Arguments, argument expr, there is a wrong verb
> tense:
> "invoked" should be "invoke", right?
> expr An expression, which if it evaluates to TRUE the
> debugger will invoked, otherwise control is returned
> directly.
> sessionInfo() R version 4.1.0 .....
Thank you, Rui. Indeed, there's a typo there.
I claim that it is a missing 'be': "be invoked" almost surealy
was intended.
As we are on the R-devel mailing list, however, let's dig and learn a bit more:
Note that the *default* is `expr = TRUE`
which is already a bit "special" for an "expression"..
Let's try to understand what was meant --- NB a strategy I
strongly recommend even if you're somewhat experienced :
ff <- function(x) { y <- x^2; browser("in ff():", expr = (y == 4)); y }
ff(1)
[1] 1
ff(2)
Called from: ff(2)
Browse[1]> debug at #1: y
Browse[2]> ls.str()
x : num 2
y : num 4
Browse[2]> c
[1] 4
ff(3)
[1] 9
So indeed, it does behave as I expected.
A further experiment, play with
f2 <- function(x, e=1) { y <- x^2; browser("in ff():", expr = e); y }
shows that 'evaluates to TRUE' is also
not as precise as it could be, and even "wrong":
'expr = pi' also behaves as TRUE, and even
'expr = NA' behaves the same.
I don't know when/how `expr` was introduced (probably taken
from 'S / S+' ..), but to me it seems actually somewhat a
misnomer because in that generalized sense, *every*
argument passed to an R function is an "expression".
Instead, what counts is that a low-level as.logical(expr) is not TRUE.
So, yes, the documentation about `expr` definitely needs to be
changed.
Unless I get convincing suggestions for improvements, I'll commit
\item{expr}{a \dQuote{condition}. By default, and whenever not false
after being coerced to \code{\link{logical}}, the debugger will be
invoked, otherwise control is returned directly.}
(and also amend the formulation a bit later on the help page
where expr is mentioned again).
Martin
> Thanks to all R Core Team members for their great work for
> all of us.
> Hope this helps,
> Rui Barradas
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