[Rd] unlist preserve common class?

[Gabriel Becker]

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_class objects is
even coherent, or would produce a new my_awesome_class object when
performed if it is.

That said, your example was of length one, 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> 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
>
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