R devel - Sep 2017 - S4 method implementation for S3 class

Hi all,

I'm trying to implement the matrix multiplication operator, which is
S4 generic, for an old-style S3 class. The following works as
expected:

x <- 1:10
class(x) <- "myClass"

setOldClass("myClass")
setGeneric("myMethod", function(x, y)
standardGeneric("myMethod"))
setMethod("myMethod", c("myClass", "myClass"),
function(x, y)
message("dispatched!"))

myMethod(x, x)
#> dispatched!

but I don't understand why the following won't:

setMethod("%*%", c("myClass", "myClass"),
function(x, y) message("dispatched!"))

x %*% x
#>      [,1]
#> [1,]  385

Is this approach wrong?

Regards,
I?aki

The %*% function is a primitive. As it says in the documentation under
?Methods_Details

     Methods may be defined for most primitives, and corresponding
     metadata objects will be created to store them. Calls to the
     primitive still go directly to the C code, which will sometimes
     check for applicable methods. The definition of ?sometimes? is
     that methods must have been detected for the function in some
     package loaded in the session and ?isS4(x)? is ?TRUE? for the
     first argument (or for the second argument, in the case of binary
     operators).

But:
> isS4(x)
[1] FALSE

I think this behavior is in the interest of performance. It avoids
adding S4 dispatch overhead to e.g. matrix objects.

In general, it's best to define an S4 class when using S4 dispatch,
but it sounds like you're stuck using some legacy S3 objects. In that
case, one would normally define an S3 method for `%*%()` that
delegates to a custom non-primitive generic, perhaps "matmult" in this
case. But since %*% is not an S3 generic, that's not an option.

It would help to hear more about the context of the problem.

Michael



On Fri, Sep 22, 2017 at 5:44 AM, I?aki ?car <i.ucar86 at gmail.com>
wrote:
> Hi all,
>
> I'm trying to implement the matrix multiplication operator, which is
> S4 generic, for an old-style S3 class. The following works as
> expected:
>
> x <- 1:10
> class(x) <- "myClass"
>
> setOldClass("myClass")
> setGeneric("myMethod", function(x, y)
standardGeneric("myMethod"))
> setMethod("myMethod", c("myClass",
"myClass"), function(x, y)
> message("dispatched!"))
>
> myMethod(x, x)
> #> dispatched!
>
> but I don't understand why the following won't:
>
> setMethod("%*%", c("myClass", "myClass"),
function(x, y) message("dispatched!"))
>
> x %*% x
> #>      [,1]
> #> [1,]  385
>
> Is this approach wrong?
>
> Regards,
> I?aki
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

2017-09-22 19:04 GMT+02:00 Michael Lawrence <lawrence.michael at
gene.com>:
> The %*% function is a primitive. As it says in the documentation under
> ?Methods_Details
>
>      Methods may be defined for most primitives, and corresponding
>      metadata objects will be created to store them. Calls to the
>      primitive still go directly to the C code, which will sometimes
>      check for applicable methods. The definition of ?sometimes? is
>      that methods must have been detected for the function in some
>      package loaded in the session and ?isS4(x)? is ?TRUE? for the
>      first argument (or for the second argument, in the case of binary
>      operators).
>
> But:
>> isS4(x)
> [1] FALSE
>
> I think this behavior is in the interest of performance. It avoids
> adding S4 dispatch overhead to e.g. matrix objects.
I see, thanks for the explanation.
> In general, it's best to define an S4 class when using S4 dispatch,
> but it sounds like you're stuck using some legacy S3 objects. In that
> case, one would normally define an S3 method for `%*%()` that
> delegates to a custom non-primitive generic, perhaps "matmult" in
this
> case. But since %*% is not an S3 generic, that's not an option.
>
> It would help to hear more about the context of the problem.
This is a problem that Edzer Pebesma and I are facing in our packages
units and errors respectively (you can find both on CRAN). They are
designed in a similar way: units or errors are attached to numeric
vectors as an attribute of an S3 class, and Ops, Math and Summary are
redefined to cope with such units/errors.

Then Edzer found that the %*% operator silently drops the attributes,
so we were trying, without success, to set a method for our respective
S3 classes to at least show a warning stating that the units/errors
are being dropped.

Ours are perfect use cases for S3 classes, and it would be a pitty if
we have to switch everything to S4 just to show a warning. Clearly
overkilling. Isn't there another way?

I?aki