[Rd] stopifnot

[Martin Maechler]

>>>>> Suharto Anggono Suharto Anggono via R-devel 
>>>>>     on Sun, 31 Mar 2019 15:26:13 +0000 writes:

 [.............]
 [........ "eval() inside for()" not giving call in error message .....]
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    > "Details" section of 'stopifnot' documentation in current R 3.6.0 alpha
    > (https://svn.r-project.org/R/branches/R-3-6-branch/src/library/base/man/stopifnot.Rd)
    > has this.

    >   Since \R version 3.6.0, \code{stopifnot()} no longer handles potential
    >   errors or warnings (by \code{\link{tryCatch}()} etc) for each single
    >   expression but rather aims at using the correct
    >   \code{\link{sys.call}(<n>)} to get the most meaningful error message in
    >   case of an error.  This provides considerably less overhead.

    > I think part of the first sentence starting from "but rather" should be removed because it is not true.

You are right that it is not accurate... I'll modify it,
including keeping the  "considerably less overhead"
which had been one important reason for changing from 3.5.x to
the current version.

    > The next paragraph:

    >   Since \R version 3.5.0, expressions \emph{are} evaluated sequentially,
    >   and hence evaluation stops as soon as there is a \dQuote{non-TRUE}, as
    >   indicated by the above conceptual equivalence statement.
    >   Further, when such an expression signals an error or
    >   \code{\link{warning}}, its \code{\link{conditionCall}()} no longer
    >   contains the full \code{stopifnot} call, but just the erroneous
    >   expression.

    > As I said earlier (https://stat.ethz.ch/pipermail/r-devel/2019-February/077386.html), the last sentence above is not entirely true. 

You are right to some degree:  That really was true for R 3.5.x,
but is no longer entirely accurate.

It is still true currently interestingly thanks to the "eval() in for()"
behavior that the error/warning message is most of the time only
about the relevant part and not mentioning the full stopifnot(..) call.


    > It may say something like:
    > Further, when such an expression signals an error, stopifnot() in R 3.5.x makes its conditionCall() the erroneous expression, but no longer since R 3.6.0.


    > Is it OK that, for
    > do.call(stopifnot, list(exprs = expression())) ,
    > the whole expression object is taken as one?

You are right; that's not so nice.
On one hand, this is fine, as there is nothing not TRUE :

   > stopifnot()          
   > stopifnot(exprs = {})

but here,

   > do.call(stopifnot, list(exprs = expression())) 
   Error in do.call(stopifnot, list(exprs = expression())) : 
     expression() are not all TRUE

I'm about to commit a version [mostly from your suggestions],
where the above do.call() works as well.

    > End portion from running
    > example(stopifnot)
    > in R 3.5.0:

    stpfnt> stopifnot(all.equal(pi, 3.141593),  2 < 2, all(1:10 < 12), "a" < "b")
    > Error in eval(ei, envir) : pi and 3.141593 are not equal:
    >   Mean relative difference: 1.102658e-07

    > To me, "in eval(*)" is rather surprising and annoying and doesn't add clarity. Yes, stop() gives the same. But, in this case, just "Error", like in R before version 3.5.0, feels better to me. If
    > stop(simpleError(msg, call = if(p <- sys.parent()) sys.call(p)))
    > were used in 'stopifnot', just "Error" would be given in this case.

And you are right again... in my current version I do use your

   stop(simpleError(msg, call = if(p <- sys.parent()) sys.call(p)))

and that does solve the above.

Martin