[R] Why does a custom function called is.numeric.factor break lattice?

[Bert Gunter]

There is no multiple dispatch; just multiple misunderstanding.

The generic function is "is.numeric" . Your method for factors is
"is.numeric.factor".

You need to re-study.

Cheers,
Bert


Bert Gunter

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
   -- Clifford Stoll


On Mon, Nov 16, 2015 at 5:00 AM, sbihorel
<Sebastien.Bihorel at cognigencorp.com> wrote:
> Hi,
>
> Interesting, based upon my understanding of S3 methods, I would have assumed
> that my function would be applied to objects of class "numeric.factor". I
> was not aware of this multiple "dispatch".
>
> Thanks for pointing this out.
>
>
> On 11/15/2015 9:42 PM, Bert Gunter wrote:
>>
>> Think about it.
>>
>> I shall assume that you are familiar with S3 methods.  What do you
>> think would happen when xyplot code calls is.numeric() on a factor
>> object expecting it to call the is.numeric primitive but, instead,
>> finding a factor method defined, calls that? Note that your factor
>> method has no arguments, but the is.numeric() primitive does. Hence
>> when the code calls the primitive on the factor object, the error you
>> saw is thrown.
>>
>> I would say that this is a weakness of the informal S3 "class" system,
>> although you probably should not have been surprised that is.numeric
>> is called on factors as the "x" argument, so you were inviting trouble
>> by defining a factor method that overrides this behavior.
>> Nevertheless, I would argue that one cannot know in general when this
>> occurs for other S3 classes, and that therefore allowing methods for
>> is.numeric() is dangerous.
>>
>> Of course, full qualification in the original xyplot code
>> (base::is.numeric() rather than is.numeric() ) would avoid such
>> things, but that's a drag.
>>
>> Contrary opinions and corrections to any flawed understanding on my
>> part are welcome, of course.
>>
>> Cheers,
>> Bert
>>
>>
>> Bert Gunter
>>
>> "Data is not information. Information is not knowledge. And knowledge
>> is certainly not wisdom."
>>     -- Clifford Stoll
>>
>>
>> On Sun, Nov 15, 2015 at 6:22 PM, sbihorel
>> <Sebastien.Bihorel at cognigencorp.com> wrote:
>>>
>>> Hi,
>>>
>>> Pretty much everything is in the title of the post. An example is below.
>>>
>>> library(lattice)
>>> data <-
>>>
>>> data.frame(x=rep(1:10,8),y=rnorm(80),trt=factor(rep(1:4,each=20)),groups=rep(1:8,each=10))
>>> xyplot <- xyplot(y~x|trt,data,groups=groups)
>>>
>>> is.numeric.factor <- function(){
>>>    print('hello world')
>>> }
>>>
>>> xyplot <- xyplot(y~x|trt,data,groups=groups)
>>>
>>> Thanks for shedding some light on this.
>>>
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>
>
>