[Rd] as.list method for by Objects
maechler at stat.math.ethz.ch
Fri Feb 2 09:07:44 CET 2018
>>>>> Henrik Bengtsson <henrik.bengtsson at gmail.com>
>>>>> on Thu, 1 Feb 2018 10:26:23 -0800 writes:
> On Thu, Feb 1, 2018 at 12:14 AM, Martin Maechler
> <maechler at stat.math.ethz.ch> wrote:
>>>>>>> Michael Lawrence <lawrence.michael at gene.com>
>>>>>>> on Tue, 30 Jan 2018 15:57:42 -0800 writes:
>> > I just meant that the minimal contract for as.list() appears to be that it
>> > returns a VECSXP. To the user, we might say that is.list() will always
>> > return TRUE.
>> Indeed. I also agree with Herv'e that the user level
>> documentation should rather mention is.list(.) |--> TRUE than
>> VECSXP, and interestingly for the experts among us,
>> the is.list() primitive gives not only TRUE for VECSXP but
>> also of LISTSXP (the good ole' pairlists).
>> > I'm not sure we can expect consistency across methods
>> > beyond that, nor is it feasible at this point to match the
>> > semantics of the methods package. It deals in "class
>> > space" while as.list() deals in "typeof() space".
>> > Michael
>> Yes, and that *is* the extra complexity we have in R (inherited
>> from S, I'd say) which ideally wasn't there and of course is
>> not there in much younger languages/systems such as julia.
>> And --- by the way let me preach, for the "class space" ---
>> do __never__ use
>> if(class(obj) == "<classname>")
>> in your code (I see this so often, shockingly to me ...) but rather use
>> if(inherits(obj, "<classname>"))
> Second this one. But, soon (*) the former will at least give the
> correct answer when length(class(obj)) == 1
> and produce an error
Not quite; I think you you did not get the real danger in using
'class(.) == *':
What you say above would only be true if there were only S3 classes!
Try the following small R snippet
myDate <- setClass("myDate", contains = "Date")
## Object of class "myDate"
##  "2018-02-02"
(d <- myDate(Sys.Date()))
class(d) == "Date" # is FALSE (hence of length 1)
inherits(d, "Date") # is TRUE
> So, several of these cases will be caught at run-time in a
> near future.
Maybe. But all the others are still wrong, as I show above.
> (*) When _R_CHECK_LENGTH_1_CONDITION_=true becomes the default
> behavior - hopefully by R 3.5.0.
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