[Rd] S3 method dispatch for methods in local environments

[Viechtbauer, Wolfgang (SP)]

Thanks, Sebastian, for the pointer to the NEWS item. After some further search, I also found this in the R Blog:

https://developer.r-project.org/Blog/public/2019/08/19/s3-method-lookup/

Best,
Wolfgang

>-----Original Message-----
>From: R-devel [mailto:r-devel-bounces using r-project.org] On Behalf Of Sebastian
>Meyer
>Sent: Tuesday, 12 May, 2020 21:17
>To: r-devel using r-project.org
>Subject: Re: [Rd] S3 method dispatch for methods in local environments
>
>Dear Wolfgang,
>
>I think this new behaviour is related to the following R 4.0.0 NEWS item:
>
>> S3 method lookup now by default skips the elements of the search path
>between the global and base environments.
>
>Your environment "myenv" is attached at position 2 of the search() path
>and thus now skipped in S3 method lookup.
>
>I have just noticed that
>
>attr(methods(class="myclass"), "info")
>getS3method("print", "myclass")
>
>both still find your function in myenv although the generic's
>UseMethod() won't. I find this a bit confusing.
>
>A solution to make R >= 4.0.0 find your method is to register the S3
>method using the new function .S3method (intended for R scripts, not
>packages). After running
>
>.S3method("print", "myclass", myenv$print.myclass)
>
>your method will be found from the generic.
>
>Best regards,
>
>	Sebastian
>
>
>Am 12.05.20 um 20:05 schrieb Viechtbauer, Wolfgang (SP):
>> Dear All,
>>
>> In R 3.6.3 (and earlier), method dispatch used to work for methods stored
>in local environments that are attached to the search path. For example:
>>
>> myfun <- function(y) {
>>    out <- list(y=y)
>>    class(out) <- "myclass"
>>    return(out)
>> }
>>
>> print.myclass <- function(x, ...) print(formatC(x$y, format="f",
>digits=5))
>>
>> myfun(1:4)
>>
>> # prints: [1] "1.00000" "2.00000" "3.00000" "4.00000"
>>
>> rm(print.myclass)
>> myenv <- new.env()
>> myenv$print.myclass <- local(function(x, ...) print(formatC(x$y,
>format="f", digits=5)), myenv)
>> attach(myenv)
>> myfun(1:4)
>>
>> # still prints: [1] "1.00000" "2.00000" "3.00000" "4.00000"
>>
>> But since R 4.0.0, this no longer words and the above prints:
>>
>> $y
>> [1] 1 2 3 4
>>
>> attr(,"class")
>> [1] "myclass"
>>
>> Is this intended? And is there a way to still make this work?
>>
>> Best,
>> Wolfgang