Subsetting S7 objects

[lgatto]

I hope I haven't missed anything obvious in the documentation, but how should I implement [ for an S7 object?

I see that there's a check_subsettable() that says that S7 objects are not subsettable and working with trying to work with

method(`[`, ....)

complains about not being able to convert signature to a valid class.

[lgatto]

So here's an example of what I want to do. This works in a script:

library(S7)

Klass <- new_class("Klass",
  properties = list(
    data1 = class_list,
    data2 = class_data.frame))

`[.Klass` <- function(x, i)
    Klass(data1 = x@data1[i],
           data2 = x@data2[i, ])

> k <- Klass(data1 = as.list(1:3),
+ data2 = data.frame(x = 1:3, y = letters[1:3]))
> k
<Klass>
 @ data1:List of 3
 .. $ : int 1
 .. $ : int 2
 .. $ : int 3
 @ data2:'data.frame':	3 obs. of  2 variables:
 .. $ x: int  1 2 3
 .. $ y: chr  "a" "b" "c"
> k[1:2]
<Klass>
 @ data1:List of 2
 .. $ : int 1
 .. $ : int 2
 @ data2:'data.frame':	2 obs. of  2 variables:
 .. $ x: int  1 2
 .. $ y: chr  "a" "b"

But if I put the same in a package, it fails with Error in check_subsettable(x) : S7 objects are not subsettable..

Back to my script, the S7 syntax

method(`[`, list(Klass, class_numeric)) <-
    function(x, i)
        Klass(data1 = x@data1[i],
              data2 = x@data2[i, ])

don't work, throwing

Error: Can't convert `signature` to a valid class. Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <list>.

[mjskay]

I think [ only dispatches on the first argument, so you need method(`[`, Klass) <- ...

The error Class specification must be an S7 class object, the result of `new_S3_class()`, an S4 class object, or a base class, not a <list> hints at this obliquely, since single-dispatch methods just take the class, not a list of classes.

[lgatto]

Thank you.