S3 method dispatch for methods in local environments

Viechtbauer, Wolfgang (SP) 
    on Tue, 12 May 2020 18:05:32 +0000 writes:

-----Original Message-----
From: Martin Maechler [mailto:maechler at stat.math.ethz.ch]
Sent: Tuesday, 12 May, 2020 21:05
To: Viechtbauer, Wolfgang (SP)
Cc: r-devel (r-devel at r-project.org)
Subject: Re: [Rd] S3 method dispatch for methods in local environments

Viechtbauer, Wolfgang (SP)
    on Tue, 12 May 2020 18:05:32 +0000 writes:

   > 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?

yes, most probably, unless

   > And is there a way to still make this work?

Using  the new

      .S3method(<generic>, <class>, <method_function>)

had been intended as substitute.  Can you try it with your
attached-environment (which makes sense!)  approach ?

Best,
Martin

S3 method lookup now by default skips the elements of the search path between the global and base environments. 

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

-----Original Message-----
From: R-devel [mailto:r-devel-bounces at r-project.org] On Behalf Of Sebastian
Meyer
Sent: Tuesday, 12 May, 2020 21:17
To: r-devel at 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