An update method for lists?

On 9/15/06, Martin Maechler <maechler at stat.math.ethz.ch> wrote:

"DeepS" == Deepayan Sarkar <deepayan.sarkar at gmail.com>
    on Fri, 15 Sep 2006 12:22:15 -0700 writes:

    DeepS> Hi, since lattice uses nested lists in various
    DeepS> situations, it has had an unexported function called
    DeepS> updateList for a while, which looks like

 >>     > lattice:::updateList

 >>     function (x, val)
 >>     {
 >>     if (is.null(x))
 >>         x <- list()
 >>     if (!is.list(x))
 >>         stop("x must be NULL or a list")
 >>     if (!is.list(val))
 >>         stop("val must be a list")
 >>     xnames <- names(x)
 >>     for (v in names(val)) {
 >>         existing <- v %in% xnames
 >>         if (existing && is.list(x[[v]]) && is.list(val[[v]]))
 >>             x[[v]] <- updateList(x[[v]], val[[v]])
 >>         else x[[v]] <- val[[v]]
 >>     }
 >>     x
 >>     }

[I'm not sure I'd allow NULL for 'x';  typing list() instead of
 NULL is not much more, but when the function name even includes  'list'
 I'd really require a list for 'x']

Makes sense.

You could hence collapse the first 6 lines to the single

   stopifnot(is.list(x), is.list(val))

I'll check if lattice needs some fixes with this.

    DeepS> Basically, it recursively replaces elements that have
    DeepS> been specified in val, leaving the other components
    DeepS> alone. I'm not aware of any other actual situation
    DeepS> where this is useful, but it certainly can be, so I
    DeepS> want to export this functionaliy. At least one other
    DeepS> person (Gabor) has also asked for that.

I've had a similar need only recently:
If a list is used to store "defaults" and you want a safe way to
change only a few of the values...
I presume you use this for manipulating the settings of lattice
parts ?

Yes, it's primarily used inside trellis.par.set, but many other places as well.

    DeepS> Now, as the name suggests, I think it might be
    DeepS> reasonable to export this as an update method for
    DeepS> "list" objects. Depending on what others (in
    DeepS> particular r-core) think, one of these things might
    DeepS> happen:

    DeepS> (1) I export it as updateList (or some other name) in lattice
    DeepS> (2) I export it as an S3 method update.list in lattice
    DeepS> (3) It gets added as an S3 method update.list in one of the base packages
or
    (4) it gets added as utility function updateList() to
        'utils' {= one of the base packages}

Yes, that a good option too (certainly better than (1))

which I'd favor momentarily.
- update() is typically for updating *models*
- it's not clear that this is *the* method for update()ing a list

I agree. Part of the reason I brought this up is because it is not
clear to me what justifies a new method for an existing generic. An
argument for is that one doesn't introduce yet another function, which
(I thought) might be enough if the other choice is to not have any
method at all.

I'm also a bit wondering if it wouldn't make sense to change the name to
something like assignInList().

I'm open to suggestions for the name. I didn't think too much about it
since it was unexported anyway.

    DeepS> The default option is (1), and I guess Sept 19 is the deadline for any
    DeepS> of these to be included in R 2.4.0.

Yes, that's true for (3) & (4) are higher if you provide a patch
to R-devel (not R-alpha) which includes a man page ...  [but
don't hurry, I'd like to see other comments]