[Rd] S4 'object is not subsettable' in prototype

[Seth Falcon]

Martin Maechler <maechler at stat.math.ethz.ch> writes:
> I agree you've found an "infelicity" in the current setup,
> since
>
>   > setClass("A", representation(x="numeric"))
>   [1] "A"
>   > getClass("A")@prototype
>   <S4 Type Object>
>   attr(,"x")
>   numeric(0)
>   > length(getClass("A")@prototype)
>   [1] 1
>   > getClass("A")@prototype[1]
>   Error in getClass("A")@prototype[1] : object is not subsettable
>   > 
>
> It's not clear yet to me *what* should be changed.
> For my taste, if length(object) returns 1, 
> I'd typically want that object[1] should be valid;
> but I think we already have some (peculiar) exceptions to that
> rule. 

I disagree with the notion that length and "[" should be tightly
coupled.  An object having length doesn't have to mean that subsetting
makes any sense.  For example, an environment has length, but what
should env1[1] return?  And with S4 classes, there are many such
examples where "[i]" won't make sense.

+ seth