It's unclear from your mail what you actually tried to do, but here are 
a few comments that may be relevant.

The syntactic form for() is indeed implemented as a primitive 
function.   Some primitives can and are used as generic functions, but 
`for` is not currently one of them.

 > setGeneric("for")
Error in genericForPrimitive(f) :
  methods may not be defined for primitive function "for" in this 
version of R

Allowing methods for it would be possible in a future version.  This 
would be a little odd, since the syntax does not look like a function 
call. Still, it's an interesting idea, and I don't know of anything 
offhand to prevent its implementation.  The natural interpretation would 
be for the signature of the generic to be the second argument 
(primitives don't intrinsically have argument names, so we would make 
one up, `seq` is used in the documentation, although something like 
`object` would be more suggestive).

Your comments about coercing are unclear and you showed no examples of 
what supposedly went wrong.  In fact, that approach works fine:

 > setClass("foo",representation(bar="list"))
[1] "foo"
 > setAs("foo","list",function(from)from@bar)
 > xx = new("foo", bar = list(1,2,3))
 > as(xx, "list")
[[1]]
[1] 1

[[2]]
[1] 2

[[3]]
[1] 3

 > for(i in as(xx, "list")) dput(i)
1
2
3

Of course, this is not exactly defining methods for the iterator, but 
would be a sensible workaround in practice.

Along the same lines, if one asks how the underlying ideas fit naturally 
into R, as opposed to making R behave more like other languages, here's 
my take on that.  The `for` operator is defined to work on vectors of 
various kinds.  If a new class is supposed to be "like" a vector, then 
the two natural choices are to define a method to coerce it to a vector 
(as in the example above) or to make it a subclass of "vector" (or of a 
specific vector class):

 > setClass("baz", contains = "vector")
[1] "baz"
 > yy = new("baz", list(1,2,3))
 > for(i in yy) dput(i)
1
2
3

Which choice works best depends on what the "real" meaning of the class 
is (there's discussion of these and other alternatives in section 9.3 of 
"Software for Data Analysis").


John Chambers

Stavros Macrakis wrote:
> Inspired by Rudolf Biczok's query of Fri, Jan 23, 2009 at 1:25 AM, I
> tried to implement iteration in a generic way using S4. (Though I am
> admittedly still struggling with learning S4.)
>
>   
>> setClass("foo",representation(bar="list"))
>>     
> [1] "foo"
>   
>> x<-new("foo",bar=list(1,2,3))
>>     
>
> Given this, I would not expect for(i in x)... to work, since R has no
> way of knowing that x@bar should be used as is.  What would it do if
> the representation included two lists?  What if list(1,2,3) is used by
> the class foo to represent something else?
>
> But I did hope that I could put in place some definitions so that the
> *class* could define an iterator.
>
> First I tried overloading `for` to allow the definition of iterator
> classes, but as a primitive function, `for` cannot be overloaded.
>
> Then I tried to see how the Containers package handles iterators:
>
>   
>> library(Containers);.jinit();.jpackage("Containers")
>> ah = MaxHeap(); ah$insert(3)
>> for (i in ah) print(i)
>>     
> [1] NA
>   
>> as.list(ah)
>>     
> [[1]]
> [1] NA
>
> Bit it appears that the Containers package's Iterators don't interface
> with R's `for` or type conversion system.
>
> So I gave up on iterators, but thought I'd try automatic conversion to lists.
>
> So I defined an automatic conversion from foo to list, since `for`'s
> seq argument is specified as "An expression evaluating to a vector
> (including a list...)":
>
>     setAs("foo","list",function(from)from@bar)
>
> This and various variants (using "numeric" or "vector" instead of
> "list") all give errors.  Is there perhaps some 'sequence' superclass
> that I am ignorant of?
>
> I *was* able to overload lapply:
>
>   
>> setMethod("lapply","foo",function(X,FUN,...) lapply(X@bar,FUN,...))
>> lapply(x,dput); NULL
>>     
> 1
> 2
> 3
> NULL
>
> but of course that doesn't affect `for` and other places that expect sequences.
>
> Is there in fact some generic way to handle define iterators or
> abstract sequences in R?
>
>           -s
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>   

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