[Rd] Conventions: Use of globals and main functions

[William Dunlap]

Duncan Murdoch wrote:
>  Scripts are for throwaways, not for anything worth keeping.

I totally agree and have a tangentially relevant question about the <<-
operator.  Currently 'name <<- value' means to look up the environment
stack until you find 'name'  and (a) if you find 'name' in some frame bind
it to a new value in that frame and (b) if you do not find it make a new
entry for it in .GlobalEnv.

Should R deprecate the second part of that and give an error if 'name' is
not already present in the environment stack?  This would catch misspelling
errors in functions that collect results from recursive calls.  E.g.,

collectStrings <- function(list) {
    strings <- character() # to be populated by .collect
    .collect <- function(x) {
        if (is.list(x)) {
            lapply(x, .collect)
        } else if (is.character(x)) {
            strings <<- c(strings, x)
        }
        misspelledStrings <<- c(strings, names(x)) # oops, would like to be
told about this error
        NULL
    }
    .collect(list)
    strings
}

This gives the incorrect:
> collectStrings(list(i="One", ii=list(a=1, b="Two")))
[1] "One" "Two"
> misspelledStrings
[1] "One" "Two" "i"   "ii"

instead of what we would get if 'misspelledStrings' were 'strings'.
> collectStrings(list(i="One", ii=list(a=1, b="Two")))
[1] "One" "Two" "a"   "b"   "i"   "ii"

If someone really wanted to assign into .GlobalEnv the assign() function is
available.

In S '<<-' only had meaning (b) and R added meaning (a).  Perhaps it is
time to drop meaning (b).  We could start by triggering a warning about it
if some environment variable were set, as is being done for non-scalar &&
and ||.

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Sun, Aug 25, 2019 at 5:09 PM Duncan Murdoch <murdoch.duncan using gmail.com>
wrote:

> On 25/08/2019 7:09 p.m., Cyclic Group Z_1 wrote:
> >
> >
> > This is a fair point; structuring functions into packages is probably
> ultimately the gold standard for code organization in R. However, lexical
> scoping in R is really not much different than in other languages, such as
> Python, in which use of main functions and defining other named functions
> outside of main are encouraged. For example, in Scheme, from which R
> derives its scoping rules, the community generally organizes code with
> almost exclusively functions and few non-function global variables at top
> level. The common use of globals in R seems to be mostly a consequence of
> historical interactive use and, relatedly, an inherited practice from S.
> >
> > It is true, though, that since anonymous functions (such as in lapply)
> play a large part in idiomatic R code, as you put it, "[l]exical scoping
> means that all of the problems of global variables are available to writers
> who use main()." Nevertheless, using a main function with other functions
> defined outside it seems like a good quick alternative that offers similar
> advantages to making a package when functions are tightly coupled to the
> script and the project may not be large or generalizable enough to warrant
> making a package.
> >
>
> I think the idea that making a package is too hard is just wrong.
> Packages in R have lots of requirements, but nowadays there are tools
> that make them easy.  Eleven years ago at UseR in Dortmund I wrote a
> package during a 45 minute presentation, and things are much easier now.
>
> If you make a complex project without putting most of the code into a
> package, you don't have something that you will be able to modify in a
> year or two, because you won't have proper documentation.
>
> Scripts are for throwaways, not for anything worth keeping.
>
> Duncan Murdoch
>
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