R help - Nov 2005 - Method for $

Dear R experts,

I have defined a class "myclass" and would like the slots to be
extractable
not only by "@" but also by "$". I now try to write a method
for "$" that
simply executes the request object at slotname, whenever someone calls 
object$slotname for any object of class "myclass".
I don't manage to find out how I can provide this function with
"slotname",
so that one and the same function works for any arbitrary slotname a user 
might choose.

I have tried

setMethod("$", signature(x="myclass"), function(x,slotname){
 x at slotname 
 } 
)

This produced the error message: 
In method for function "$": expanding the signature to 
include omitted arguments in definition: name = "missing" 
Error in rematchDefinition(definition, fdef, mnames, fnames, signature) : 
        methods can add arguments to the generic only if '...' is an
argument
to the generic

My searches for a solution of this problem have not been successful. Can 
someone help? 

Thanks and regards,
Ulrike Gr??mping, Berlin

Ulrike Gr??mping wrote:
> Dear R experts,
> 
> I have defined a class "myclass" and would like the slots to be
extractable
> not only by "@" but also by "$". I now try to write a
method for "$" that
> simply executes the request object at slotname, whenever someone calls 
> object$slotname for any object of class "myclass".
> I don't manage to find out how I can provide this function with
"slotname",
> so that one and the same function works for any arbitrary slotname a user 
> might choose.
> 
> I have tried
> 
> setMethod("$", signature(x="myclass"),
function(x,slotname){
>  x at slotname 
>  } 
> )

Ulrike,

what about (untested!):
slot(x, slotname)

Best wishes from Dortmund and Lena,
Uwe Ligges

> This produced the error message: 
> In method for function "$": expanding the signature to 
> include omitted arguments in definition: name = "missing" 
> Error in rematchDefinition(definition, fdef, mnames, fnames, signature) : 
>         methods can add arguments to the generic only if '...' is
an argument
> to the generic
> 
> My searches for a solution of this problem have not been successful. Can 
> someone help? 
> 
> Thanks and regards,
> Ulrike Gr??mping, Berlin
> 
> ______________________________________________
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>>>>> "Ulrike" == Ulrike Gr??mping <groemping at
tfh-berlin.de>
>>>>>     on Fri, 18 Nov 2005 15:37:58 +0100 writes:
    Ulrike> Dear R experts,

    Ulrike> I have defined a class "myclass" and would like the
    Ulrike> slots to be extractable not only by "@" but also by
"$".

hmm, I know this is not what you've asked for, but
why would you want to do that?  To me, it seems a pretty harmful idea
that only leads to confusion and does not provide sensible
functionality.

When S4 classes where created, using '@' instead of '@' with a
*different* meaning was very much on purpose and I think it's a
bit too ambitious to try being smarter than John Chambers,
the creator of the S4 object system, on this.

Martin Maechler, ETH Zurich

    Ulrike> I now try to write a method for "$" that 
    Ulrike> simply executes the request object at slotname, whenever someone
calls
    Ulrike> object$slotname for any object of class "myclass".
    Ulrike> ..............................
    Ulrike> ..............................

Ulrike Groemping <groemping at tfh-berlin.de> wrote:
>I have defined a class "myclass" and would like the slots to be
extractable
>not only by "@" but also by "$". I now try to write a
method for "$" that
>simply executes the request object at slotname, whenever someone calls 
>object$slotname for any object of class "myclass".
>I don't manage to find out how I can provide this function with
"slotname",
>so that one and the same function works for any arbitrary slotname a user 
>might choose.
>...
I would caution against defining methods for $.

In addition to Martin Maechler and Duncan Temple Lange's warnings
about the danger of this, I would note that it could make R run much
slower.  I once tried defining a method for it in S-PLUS; that
converted $ into a generic function, which slowed down every call to $,
of which there are many.

Even if it wouldn't slow down R, as we work to make R and S-PLUS more
compatible you or someone else might try your code in S-PLUS, and
cause a big speed hit.
>Maybe I could (and should?) have defined the class with just one slot
>that contains the list, which would make it behave like I want it
>immediately.
Why not make it a list with an S3 class, rather than an S4 class?

Tim Hesterberg

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