From maechler at stat.math.ethz.ch  Tue May 16 11:31:42 2017
From: maechler at stat.math.ethz.ch (Martin Maechler)
Date: Tue, 16 May 2017 11:31:42 +0200
Subject: [Rd] stopifnot() does not stop at first non-TRUE argument
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Message-ID: <22810.50942.577206.452639@stat.math.ethz.ch>

>>>>> Herv? Pag?s <hpages at fredhutch.org>
>>>>>     on Mon, 15 May 2017 16:54:46 -0700 writes:

    > Hi,
    > On 05/15/2017 10:41 AM, luke-tierney at uiowa.edu wrote:
    >> This is getting pretty convoluted.
    >> 
    >> The current behavior is consistent with the description at the top of
    >> the help page -- it does not promise to stop evaluation once the first
    >> non-TRUE is found.  That seems OK to me -- if you want sequencing you
    >> can use
    >> 
    >> stopifnot(A)
    >> stopifnot(B)
    >> 
    >> or
    >> 
    >> stopifnot(A && B)

    > My main use case for using stopifnot() is argument checking. In that
    > context, I like the conciseness of

    > stopifnot(
    > A,
    > B,
    > ...
    > )

    > I think it's a common use case (and a pretty natural thing to do) to
    > order/organize the expressions in a way such that it only makes sense
    > to continue evaluating if all was OK so far e.g.

    > stopifnot(
    > is.numeric(x),
    > length(x) == 1,
    > is.na(x)
    > )

I agree.  And that's how I have used stopifnot() in many cases
myself, sometimes even more "extremely" than the above example,
using assertions that only make sense if previous assertions
were fulfilled, such as

    stopifnot(is.numeric(n), length(n) == 1, n == round(n), n >= 0)

or in the Matrix package, first checking some class properties
and then things that only make sense for objects with those properties.


    > At least that's how things are organized in the stopifnot() calls that
    > accumulated in my code over the years. That's because I was convinced
    > that evaluation would stop at the first non-true expression (as
    > suggested by the man page). Until recently when I got a warning issued
    > by an expression located *after* the first non-true expression. This
    > was pretty unexpected/confusing!

    > If I can't rely on this "sequencing" feature, I guess I can always
    > do

    > stopifnot(A)
    > stopifnot(B)
    > ...

    > but I loose the conciseness of calling stopifnot() only once.
    > I could also use

    > stopifnot(A && B && ...)

    > but then I loose the conciseness of the error message i.e. it's going
    > to be something like

    > Error: A && B && ... is not TRUE

    > which can be pretty long/noisy compared to the message that reports
    > only the 1st error.


    > Conciseness/readability of the single call to stopifnot() and
    > conciseness of the error message are the features that made me
    > adopt stopifnot() in the 1st place. 

Yes, and that had been my design goal when I created it.

I do tend agree with  Herv? and Serguei here.

    > If stopifnot() cannot be revisited
    > to do "sequencing" then that means I will need to revisit all my calls
    > to stopifnot().

    >> 
    >> I could see an argument for a change that in the multiple argumetn
    >> case reports _all_ that fail; that would seem more useful to me than
    >> twisting the code into knots.

Interesting... but really differing from the current documentation,

    > Why not. Still better than the current situation. But only if that
    > semantic seems more useful to people. Would be sad if usefulness
    > of one semantic or the other was decided based on trickiness of
    > implementation.

Well, the trickiness  should definitely play a role.
Apart from functionality and semantics, long term maintenance
and code readibility, even elegance have shown to be very
important aspects of good code in ca 30 years of S and R programming.

OTOH, as mentioned above, the creation of good error messages
has been an important design goal of  stopifnot()  and hence I'm
willing to accept the extra complexity of "patching up" the call
used in the error / warning messages.

Also, as a change to what I posted yesterday, I now plan to follow
Peter Dalgaard's suggestion of using
             eval( ..<n> ) 
instead of   eval(cl[[i]], envir = <parent.frame(.)>)
as there may be cases where the former behaves better in lazy
evaluation situations.
(Other opinions on that ?)

Martin

    > Thanks,
    > H.

    >> 
    >> Best,
    >> 
    >> luke
    >> 
    >> On Mon, 15 May 2017, Martin Maechler wrote:
    >> 
    >>>>>>>> Serguei Sokol <sokol at insa-toulouse.fr>
    >>>>>>>> on Mon, 15 May 2017 16:32:20 +0200 writes:
    >>> 
    >>> > Le 15/05/2017 ? 15:37, Martin Maechler a ?crit :
    >>> >>>>>>> Serguei Sokol <sokol at insa-toulouse.fr>
    >>> >>>>>>> on Mon, 15 May 2017 13:14:34 +0200 writes:
    >>> >> > I see in the archives that the attachment cannot pass.
    >>> >> > So, here is the code:
    >>> >>
    >>> >> [....... MM: I needed to reformat etc to match closely to
    >>> >> the current source code which is in
    >>> >>
    >>> https://urldefense.proofpoint.com/v2/url?u=https-3A__svn.r-2Dproject.org_R_trunk_src_library_base_R_stop.R&d=DwIFAw&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=t9fJDOl9YG2zB-GF0wQXrXJTsW2jxTxMHE-qZfLGzHU&s=KGsvpXrXpHCFTdbLM9ci3sBNO9C3ocsgEqHMvZKvV9I&e=
    >>> >> or its corresponding github mirror
    >>> >>
    >>> https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_wch_r-2Dsource_blob_trunk_src_library_base_R_stop.R&d=DwIFAw&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=t9fJDOl9YG2zB-GF0wQXrXJTsW2jxTxMHE-qZfLGzHU&s=7Z5bPVWdGPpY2KLnXQP6c-_8s86CpKe0ZYkCfqjfxY0&e=
    >>> >> ]
    >>> >>
    >>> >> > Best,
    >>> >> > Serguei.
    >>> >>
    >>> >> Yes, something like that seems even simpler than Peter's
    >>> >> suggestion...
    >>> >>
    >>> >> It currently breaks 'make check' in the R sources,
    >>> >> specifically in tests/reg-tests-2.R (lines 6574 ff),
    >>> >> the new code now gives
    >>> >>
    >>> >> > ## error messages from (C-level) evalList
    >>> >> > tst <- function(y) { stopifnot(is.numeric(y)); y+ 1 }
    >>> >> > try(tst())
    >>> >> Error in eval(cl.i, pfr) : argument "y" is missing, with no default
    >>> >>
    >>> >> whereas previously it gave
    >>> >>
    >>> >> Error in stopifnot(is.numeric(y)) :
    >>> >> argument "y" is missing, with no default
    >>> >>
    >>> >>
    >>> >> But I think that change (of call stack in such an error case) is
    >>> >> unavoidable and not a big problem.
    >>> 
    >>> > It can be avoided but at price of customizing error() and
    >>> warning() calls with something like:
    >>> > wrn <- function(w) {w$call <- cl.i; warning(w)}
    >>> > err <- function(e) {e$call <- cl.i; stop(e)}
    >>> > ...
    >>> > tryCatch(r <- eval(cl.i, pfr), warning=wrn, error=err)
    >>> 
    >>> > Serguei.
    >>> 
    >>> Well, a good idea, but the 'warning' case is more complicated
    >>> (and the above incorrect): I do want the warning there, but
    >>> _not_ return the warning, but rather, the result of eval() :
    >>> So this needs even more sophistication, using  withCallingHandlers(.)
    >>> and maybe that really get's too sophisticated and no
    >>> more "readable" to 99.9% of the R users ... ?
    >>> 
    >>> I now do append my current version -- in case some may want to
    >>> comment or improve further.
    >>> 
    >>> Martin



[Rd] stopifnot() does not stop at first non-TRUE argument
Martin Maechler