[Rd] unlist preserve common class?

[Gabriel Becker]

Hi Spencer,

Another, potentially somewhat less disruptive/more general option would be
to add a stop.at.object or stop.at.nonlist (or alternatively list.only)
argument, which would basically translate to "collapse the list structure
to flat, but don't try to combine the leaf elements within the list. You
could then do whatever you wanted to said now-flat list as a second call.

i.e.,

flatlist <- unlist(structured_list, list.only = TRUE)
final_res <- cool_combiner_fun(flatlist)

I had to do something similar years ago when I was implementing xpath for
arbitrary R objects, because you can, e.g., always get x[1] out of x
infinitely many times, so I defined "Stopping functions". The fully general
case would be to do the same here, and accept, e.g., stopping.cond, but
that is probably too complex for unlist and might simply belong as a
completely separate function.

Best,
~G

On Thu, Dec 8, 2022 at 8:21 PM Spencer Graves <
spencer.graves using effectivedefense.org> wrote:

> Hi, Gabriel:
>
>
> On 12/8/22 8:20 PM, Gabriel Becker wrote:
> > Hi Spencer,
> >
> > My 2c.
> >
> > According to the docs, factors are special-cased. Other S3 'classes'
> > could be special-cased, such as Date in your example, I suppose, but it
> > is not clear how what you're describing could be implemented for the
> > general case.
> >
> > Suppose I define an S3 "class" called my_awesome_class, and have a list
> > of 3 of them in it, and no other guarantees are provided. What should,
> > or even could, R do in the case of unlist(list_of_awesomes)?
> >
> > There is no guarantee that I as an S3 developer have provided a c method
> > for my class such that we could say the unlist call above is equivalent
> > (roughly) to do.call(c, list_of_awesomes), nor that I provided any other
> > particular "mash this set of my_awesome_class objects into one". Nor is
> > it even guaranteed that the concept of combining my_awesome_classobjects
> > is even coherent, or would produce a new my_awesome_classobject when
> > performed if it is.
>
>
>           What about adding another argument to create, e.g.,
>
>
> unlist(x, recursive = TRUE, use.names = TRUE, attributeFunction=NULL)
>
>
>           Then assign the assign the results of the current "unlist(x,
> ...)"
> to, say, "ux", and follow that by
>
>
>
> if(!is.null(attributeFunction))attributes(ux) <- attributeFunction(x)
>
>
> return(ux)
>
>
>           An alternative could be to have a default attributeFunction,
> that
> computes the attributes of each component of x and keeps only the ones
> that are shared by all components of x.  This would be the same as the
> current behavior for factors IF each component had the same factor
> levels and would drop attributes that are different between components.
> For S4 classes, if the attributes were not ALL identical, then all the
> attributes would be dropped, as with the current behavior.  This should
> not be a problem for S3 generics, because they should always check to
> make sure all the required attributes are available.
>
> >
> > That said, your example was of length one,
>
>
>           My example was of length one to provide a minimal,
> self-contained
> example.  That was motivated by a more complicated example, which took
> me a couple of hours to understand why it wasn't working as I expected ;-)
>
>
>           Thanks for your reply.
>
>
>           Spencer Graves
>
>
> we could special case (the
> > default method of) unlist so that for x /not a list/, we're guaranteed
> that
> >
> > identical(unlist(list(x)), x) == TRUE
> >
> > This would simplify certain code, such as the one from your motivating
> > example, but at the cost of making the output of unlist across inputs
> > less consistent and less easy to reason about and predict. In other
> > words the answer to the question "what class is
> > unlist(list_of_awesomes)? " would become "it depends on how many of them
> > are in the list"... That wouldn't be a good thing on balance, imho.
> >
> > Best,
> > ~G
> >
> > On Thu, Dec 8, 2022 at 5:44 PM Spencer Graves
> > <spencer.graves using effectivedefense.org
> > <mailto:spencer.graves using effectivedefense.org>> wrote:
> >
> >     Consider:
> >
> >
> >       > str(unlist(list(Sys.Date())))
> >        num 19334
> >
> >
> >       > str(unlist(list(factor('a'))))
> >        Factor w/ 1 level "a": 1
> >
> >
> >                I naively expected "str(unlist(list(Sys.Date())))" to
> >     return an
> >     object of class 'Date'.  After some thought, I felt a need to ask
> this
> >     list if they think that the core R language might benefit from
> >     modifying
> >     the language so "str(unlist(list(Sys.Date())))" was of class 'Date',
> at
> >     least as an option.
> >
> >
> >                Comments?
> >                Thanks,
> >                Spencer Graves
> >
> >
> >       > sessionInfo()
> >     R version 4.2.2 (2022-10-31)
> >     Platform: x86_64-apple-darwin17.0 (64-bit)
> >     Running under: macOS Big Sur 11.7.1
> >
> >     Matrix products: default
> >     LAPACK:
> >
>  /Library/Frameworks/R.framework/Versions/4.2/Resources/lib/libRlapack.dylib
> >
> >     locale:
> >     [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
> >
> >     attached base packages:
> >     [1] stats     graphics  grDevices utils     datasets  methods   base
> >
> >     loaded via a namespace (and not attached):
> >     [1] compiler_4.2.2 tools_4.2.2
> >
> >     ______________________________________________
> >     R-devel using r-project.org <mailto:R-devel using r-project.org> mailing list
> >     https://stat.ethz.ch/mailman/listinfo/r-devel
> >     <https://stat.ethz.ch/mailman/listinfo/r-devel>
> >
>

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