[Rd] unlist preserve common class?

[Spencer Graves]

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))

$d0
[1] "2022-12-09" "2022-12-10"

$d1
[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))

$d0
[1] "2022-12-09"

$d1
[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.


	  Thanks,
	  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.
>>
>> 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
>>>
>>> ______________________________________________
>>> R-devel using r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
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