R help - May 2007 - S4 object slot of type 'call'

I'm using an S4 object with a slot of type 'call': I would like to
be able to initialize
it with something like NULL or NA (indicating that there is no information in
the slot)
but the value should comply with the fact that it must be of type call.

Is there any simple way to obtain this?

Thanks for any hint,

	Roberto

-- 
	<r/>
| Roberto Brunelli - [scientist at  Fondazione Bruno Kessler-irst]
|           'Home can be anywhere, for it is a part of one's self'

------------------
ITC -> dall'1 marzo 2007 Fondazione Bruno Kessler
ITC -> since 1 March 2007 Fondazione Bruno Kessler

Roberto Brunelli wrote:
> I'm using an S4 object with a slot of type 'call': I would like
to be able to initialize
> it with something like NULL or NA (indicating that there is no information
in the slot)
> but the value should comply with the fact that it must be of type call.
>
> Is there any simple way to obtain this?

This looks fine:


 > setClass("testCall", representation = representation(call =
"call"))
[1] "testCall"
 > test <- new("testCall")
 >
 > test
An object of class "testCall"
Slot "call":
`<undef>`()



Although, I found a bug while playing around:

cl <- call("round", 10.5)
cl[] <- NULL
cl
## CRASH using R-2.5.0 on Windows XP


Uwe Ligges


> Thanks for any hint,
> 
> 	Roberto
>

Roberto Brunelli <brunelli at itc.it> writes:
> I'm using an S4 object with a slot of type 'call': I would like
to
> be able to initialize it with something like NULL or NA (indicating
> that there is no information in the slot) but the value should
> comply with the fact that it must be of type call.
>
> Is there any simple way to obtain this?
One possibility is to create a NullCall class:

   setClass("NullCall", contains="call")

This can be put into a slot of type 'call' and identified by its
class.  One advantage of this is that you can write a few methods for
NullCall and in many cases avoid explicit tests like:

   if (class(obj) == "NullCall") ... else ...

For example, you could define a show metho that printed 'NULL' or
something.

It is also possible to define a class union and use that as the slot
type.  I don't like this solution, but YMMV.

+ seth

-- 
Seth Falcon | Computational Biology | Fred Hutchinson Cancer Research Center
http://bioconductor.org