[R] S4: When is validObject issued? (or why S4 is killing me:( ..

[Vitalie S.]

>>
>>  setMethod("initialize","test2",
>>            function(.Object,...){
>>                .Object<-callNextMethod(.Object,...)
>
> validity is checked in the preceeding line
>
>>                .Object at b<-.Object at a   ## here "b" is initialized ##
>>
>>                .Object
>>            })
>>
>>> new("test2")
>> Error in if (object at a != object at b) print("values must be equal!!")
>> else TRUE : argument is of length zero
>>
>> This could mean only one thing, validity of "test2" is checked before
>> it gets to point when b is initialized (i.e. in callNextMethod). But
>> this is absurd (at leas from my point): callNextMethod constructs
>> object of type "test" and should have nothing to do with validity of
>> "test2". Shouldn't validity be checked only after the object is
>> created?
>
> The class of .Object is test2, and initialize should return a (valid)
> instance of test2, right? So the validity check on test2 has to have
> been performed before returning from callNextMethod.
>

Oh yes, that is indeed meaningful. So it's default coercion method from test1 to test2 which is called.  


>> Second my slot "a" is initialized in "test1"'s constructor and "b"
>> must be assigned only after the callNextMethod as in above
>> code. Seemingly unsolvable paradox? How to be?
>
> My own solution would avoid using initialize (because as you're
> discovering the contract is hard to satisfy in general) and instead
> use constructors

I am sure this "contract" and all this initialization/coercion/creation-sequence  stuff must be documented somewhere. Though all what I can find is a collection of your responses in R-help archive. Could you please suggest something to read (except R-core code)?


For now I am pretty much clarified. Thanks a lot!

>   test2 <- function(a=runif(1), b=a, ...) new("test2", a=a, b=b, ...)
>
> or similar, which I think helps to provide a bit of an interface
> between the class and it's implementation.
>
> One could think of additional solutions (creating a new 'test' inside
> 'test2', and not using callNextMethod(), for instance) but this is
> unlikely to be robust (e.g., when derived classes are relying on
> initialize,ANY-method to fill in slots with supplied arguments).
>
> Martin
>
>>
>> Many thanks.
>>
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>