I'll look at this in more detail (though not for a few days), but the 
essence I think is that nothing in the current version allows one to say 
the S3 equivalent of

setClass("VCorpus", contains = "list", ....)

Seems a (minor, I hope) glitch that what Martin K.  tried didn't work: 
   setOldClass(c("VCorpus", "Corpus", "list"))

As far as I can see, setOldClass() should understand the basic data 
types as valid superclasses.

Martin Morgan's code is indeed a workaround.  And as Martin Maechler 
commented, the intention is to make S3 and S4 classes play together as 
much as possible.


Morgan wrote:
> Hi Martin Kober --
>
> Martin Kober wrote:
>   
>> Hello,
>>
>> I am the maintainer of the stringkernels package and have come across
>> a problem with using S3 objects in my S4 classes.
>>
>> Specifically, I have an S4 class with a slot that takes a text corpus
>> as a list of character vectors. tm (version 0.5) saves corpora as
>> lists with a class attribute of c("VCorpus", "Corpus", "list"). I
>> don't actually need the class-specific attributes, I only care about
>> the list itself.
>>
>> Here's a simplified example of my problem:
>>
>>     
>>> setClass("testclass", representation(slot="list"))
>>>       
>> [1] "testclass"
>>     
>>> a = list(a="1", b="2")
>>> class(a) = c("VCorpus", "Corpus", "list") # same as corpora in tm v0.5
>>>       
>
> This has the feel of a workaround, but
>
> setOldClass(class(a)[1:2]); setIs("Corpus", "list")
>
> helps here, first registering c('vCorpus', 'Corpus') as an S3 hierarchy
> and then defining the relationship between the root of the hierarchy and
> (the S4 class) 'list'.
>
> Martin (Morgan)
>
>   
>>> x = new("testclass", slot=a)
>>>       
>> Error in validObject(.Object) :
>>   invalid class "testclass" object: 1: invalid object for slot "slot"
>> in class "testclass": got class "VCorpus", should be or extend class
>> "list"
>> invalid class "testclass" object: 2: invalid object for slot "slot" in
>> class "testclass": got class "Corpus", should be or extend class
>> "list"
>> invalid class "testclass" object: 3: invalid object for slot "slot" in
>> class "testclass": got class "list", should be or extend class "list"
>>
>> The last line is a bit confusing here (``got class "list", should be
>> or extend class "list"''). There's an even more confusing error
>> message when I try to assign the slot later on:
>>
>>     
>>> y = new("testclass")
>>> y@slot = a
>>>       
>> Error in checkSlotAssignment(object, name, value) :
>>   c("assignment of an object of class \"VCorpus\" is not valid for
>> slot \"slot\" in an object of class \"testclass\"; is(value, \"list\")
>> is not TRUE", "assignment of an object of class \"Corpus\" is not
>> valid for slot \"slot\" in an object of class \"testclass\"; is(value,
>> \"list\") is not TRUE", "assignment of an object of class \"list\" is
>> not valid for slot \"slot\" in an object of class \"testclass\";
>> is(value, \"list\") is not TRUE")
>>
>> The last part of the message claims that ``is(value, "list") is not
>> TRUE'', but is(a, "list") is certainly TRUE. (??)
>>
>> On a side note, it does work when "list" is the first entry in class().
>>
>>
>> I tried to use setOldClass, but seemingly using list is not possible
>> because it does not extend oldClass, or I didn't find out how to do
>> it:
>>     
>>> setOldClass(c("VCorpus", "Corpus", "list"))
>>>       
>> Error in setOldClass(c("VCorpus", "Corpus", "list")) :
>>   inconsistent old-style class information for "list"; the class is
>> defined but does not extend "oldClass"
>>
>>
>> Intuitively I would have thought that, because the underlying data is
>> of type list, it would "fit" into an object slot requiring a list,
>> irrespective of S3 class attributes. The only thing I can think of is
>> a manual solution removing the class attribute.
>>
>> Is there a way to define lists with S3 class attributes such that they
>> are accepted as lists in S4 object slots? Or any other ways to solve
>> this?
>>
>>
>> Thanks in advance
>> Best Regards
>> Martin
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>     
>
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>
>   

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