On 12/9/22 4:33 AM, Duncan Murdoch wrote:
On 08/12/2022 9:20 p.m., 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_class objects is
even coherent, or would produce a new my_awesome_class object when
performed if it is.

For the non-recursive case of unlist, do.call(c, list_of_awesomes) is a pretty reasonable expectation.  Wouldn't the simplest change be to make no change to unlist, but suggest this alternative in the documentation?

Duncan Murdoch

Hi, Duncan and Gabrien:

          That's ultimately what I did.  My real problem was more like:

(todaytomorrow <- list(d0=Sys.Date()+0:1, d1=Sys.Date()+2:3))

[1] "2022-12-09" "2022-12-10"

[1] "2022-12-11" "2022-12-12"

          I wanted the minimum of the maxima.  So I naively did:

(tt2 <- sapply(todaytomorrow, min))

   d0    d1
19335 19337

          So I next tried:

> (tt3 <- as.Date(tt2))

Error in as.Date.numeric(tt2) : 'origin' must be supplied

I believe that the default "origin" for "as.Date.numeric" should be "1970-01-01". I implemented that several years ago in Ecfun:

> (tt4 <- Ecfun::as.Date1970(tt2))

          d0           d1
"2022-12-09" "2022-12-11"

However, before getting here, I first misdiagnosed the problem with "tt2", believing that "min" not "sapply" was stripping the attributes.

          After fixing that problem, I came to Duncan's solution:

> (tt4 <- lapply(todaytomorrow, min))

[1] "2022-12-09"

[1] "2022-12-11"

(maximin <- do.call('max', tt4))
[1] "2022-12-11"

Conclusion: It would help to document Duncan's solution using "do.call" and avoiding "unlist" and "sapply". I brought it to the attention of this group, because I wondered if you might want to change the language -- or at least the documentation, as Duncan suggested.

          Spencer Graves

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.


On Thu, Dec 8, 2022 at 5:44 PM Spencer Graves <
spencer.gra...@effectivedefense.org> wrote:


  > 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.

           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

[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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