[Rd] unlist preserve common class?

[Duncan Murdoch]

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

> 
> 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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>>
> 
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