[Rd] Extending [ method to S4 class and drop argument (PR#9211)

[Martin Morgan]

Or more in the spirit of method dispatch, and to get some insight into
why this design decision is not a bug:

> setClass("test",representation(x="character"))
[1] "test"
> 
> setMethod("[",
+           signature(x="test", drop="missing"),
+           function(x, i, j, ..., drop) {
+               cat("here\n")
+               callNextMethod(x, i, j, ..., drop=TRUE)
+       })
[1] "["
> 
> setMethod("[","test",function(x, i, j, ..., drop) {
+     cat("there\n")
+     print(drop)
+ })
[1] "["
> 
> a = new("test",x="fred")
> a[1]
here
there
[1] TRUE

'drop' is 'missing', and 'missing' satisfies the implicit 'ANY'
condition in the signature of the original method. With this insight,
the design decision to 'ignore' the default argument seems appropriate
to me -- developers implementing the generic might well want to
dispatch on it, since after all it is in the generic signature. This
could lead to an explosion of methods if many arguments have 'default'
values, but such an explosion probably points to a nice distinction
between arguments-for-dispatch and arguments-for-evaluation.

To tell the truth, I hadn't really understood why the error originally
reported occurred, until writing the method with drop="missing" just
now.

Martin

"Gabor Grothendieck" <ggrothendieck at gmail.com> writes:

> Try this:
>
>> setClass("test",representation(x="character"))
> [1] "test"
>> setMethod("[","test",function(x,i,j,...,drop) {
> +    print(i)
> +    if (missing(drop)) drop <- TRUE
> +    print(drop)
> + })
> [1] "["
>> a = new("test",x="fred")
>> a[1]
> [1] 1
> [1] TRUE
>
>
> On 9/8/06, jverzani at gmail.com <jverzani at gmail.com> wrote:
>> Full_Name: John Verzani
>> Version: 2.4.0 alpha (2006-09-05 r39134)
>> OS: linux, gentoo 2.6.17
>> Submission from: (NULL) (163.238.43.26)
>>
>>
>> When extending the "[" method to a new S4 class, the default value for the drop
>> argument is not being found. Here is a small example:
>>
>> setClass("test",representation(x="character"))
>> setMethod("[","test",function(x,i,j,...,drop=TRUE) {print(i);print(drop)})
>> a = new("test",x="fred")
>> a[1]
>>
>> resulting in:
>>
>> [1] 1
>> Error in print(drop) : argument "drop" is missing, with no default
>>
>> I'm expecting TRUE for the value of drop. That's correct isn't?
>>
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>>
>
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-- 
Martin T. Morgan
Bioconductor / Computational Biology
Fred Hutchinson Cancer Research Center
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PO Box 19024
Seattle, WA 98109

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