[Rd] Unexpected failure when calling new() with unnamed arg and

[Hervé Pagès]

Thanks Martin for looking into this.  H.

On 05/13/2015 03:57 AM, Martin Maechler wrote:
>>>>>> Hervé Pagès <hpages at fredhutch.org>
>>>>>>      on Tue, 12 May 2015 15:18:42 -0700 writes:
>
>> Hi,
>
>> The man page for new() suggests that if 'a' is an object with slots
>> "slot1" and "slot2" and C is a class that extends the class of 'a',
>> then the 2 following calls should be equivalent:
>
>>     new("C", a, ...)
>>     new("C", slot1=a at slot1, slot2=a at slot2, ...)
>
>> This is generally the case but I just ran into a situation where it's
>> not. In the following example the former fails while the latter works:
>
>>     setClass("A", representation(slot1="numeric", slot2="logical"))
>>     setClass("B", contains="A", representation(design="formula"))
>>     setClass("C", contains="B")
>>     a <- new("A", slot1=77, slot2=TRUE)
>
>>     new("C", a, design=x ~ y)  # fails
>>     new("C", slot1=a at slot1, slot2=a at slot2, design=x ~ y)  # works
>
>> Note that new("B", a, design=x ~ y) works so the 3-level class
>> hierarchy is really needed in order to reproduce.
>
>> Probably related to this, I also noted that new("B") and/or new("C")
>> return invalid objects:
>
>>     c <- new("C")
>
>>     validObject(c)
>>     # Error in validObject(c) :
>>     #  invalid class “C” object: invalid object for slot "design"
>>     #  in class "C": got class "S4", should be or extend class "formula"
>
>>     is(c at design, "formula")
>>     # [1] FALSE
>
>>     class(c at design)
>>     # [1] "S4"
>
>> Note that 'c' can be fixed:
>
>>     c at design <- formula(NULL)
>
>>     validObject(c)
>>     # [1] TRUE
>
>> Maybe something that the default "initialize" method should take care
>> of?
>
>> Another singularity that is maybe at the root of all of this is that
>> the "formula" S4 class is virtual:
>
>>     showClass("formula")
>>     # Virtual Class "formula" [package "methods"]
>>     #
>>     # Slots:
>>     #
>>     # Name:   .S3Class
>>     # Class: character
>>     #
>>     # Extends: "oldClass"
>
>> so a bare call to new("formula") fails:
>
>>     new("formula")
>>     # Error in new("formula") :
>>     #   trying to generate an object from a virtual class ("formula")
>
>> Shouldn't new("formula") just return an "empty" S3 formula (like
>> formula(NULL) does), in the same way that new("integer") returns
>> an empty ordinary integer vector?
>
> Interesting .. and at least worth following.
>
> One problem and historical reason for the current setup seems
> that the "formula" S3 class is not from 'base' but 'stats' :
>
> R's source, src/library/methods/R/BasicClasses.R,
> lines 524 ff has the following comment block
>
> |  .OldClassesPrototypes is a list of S3 classes for which prototype
> |  objects are known & reasonable.  The classes will reappear in
> |  .OldClassesList, but will have been initialized first in
> |  .InitBasicClasses.  NB:  the methods package will NOT set up
> |  prototypes for S3 classes except those in package base and for "ts"
> |  (and would rather not do those either).  The package that owns the
> |  S3 class should have code to call setOldClass in its
> |  initialization.
>
> So, when John Chambers wrote this, he envisioned that the
> 'stats' package would do "the correct thing" for its own classes.
> OTOH, as history went, the stats package was never allowed to
> depend on methods.
> There are many other S3 classes from 'stats' which also end up
> similarly, being defined via  setOldClass() and that itself
> produces a VIRTUAL class.
> Also, another part of the (R source) comment above is no longer
> quite accurate, e.g., "data.frame" is in .OldClassesPrototypes
> but not in .OldClassesList ...
>
> As I do agree that "formula" is much more basic than these other classes,
> I'm currently looking at tweaks to the methods (and stats) code,
> to get this to work.... as indeed - you mentioned above -  we
> already allow "empty S3 formula" objects anyway.
>
> ... half an hour later : Indeed, I've been able to use the above information
> such that  new("formula") and new("formula", y ~ x)
> work.
>
> However, your code above now --- with my changes --- would  fail :
>
>   > setClass("A", representation(slot1="numeric", slot2="logical"))
>   > setClass("B", contains="A", representation(design="formula"))
>   > setClass("C", contains="B")
>   Error in reconcilePropertiesAndPrototype(name, slots, prototype, superClasses,  :
>     "B" is not eligible to be the data part of another class (must be a basic class or a virtual class with no slots)
>   >
>
> So, I am not yet committing my changes to R-devel.
> Hopefully more on this, later today.
>
> Martin Maechler,
> ETH Zurich
>
>
>> Thanks,
>> H.
>
>>   > sessionInfo()
>> R version 3.2.0 Patched (2015-04-17 r68202)
>> Platform: x86_64-unknown-linux-gnu (64-bit)
>> Running under: Ubuntu 14.04.2 LTS
>
>> --
>> Hervé Pagès
>> Fred Hutchinson Cancer Research Center
>
>   [..................]
>
>

-- 
Hervé Pagès

Program in Computational Biology
Division of Public Health Sciences
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