Validator not triggered when `new_class` derives from base class

[tschwarzl]

Related to #413

The validator is also not triggered when new_class derives from a base type as showcased in this example e.g.:

list_derived_class <- new_class(
  "list_derived_class",
  parent = class_list,
  validator = function(self) {
    if(length(self) > 0)
      "raising non-empty test error"
  }
)

x <- list_derived_class()
x[["A"]] = 1
x[["B"]] = 2

This is rather counter intuitive behavior when deriving from a class.

One suggestion would be that S7_base_class provided by e.g. class_list would overwrite the base setters installing a hook to trigger the validator.

[t-kalinowski]

Thanks for opening this.

The issue is that [[<- doesn’t interact with any parts of S7 in this case, so it doesn’t trigger validation.

For example, in the snippet above, class(x) is c("list_derived_class", "list", "S7_object"), and the [[<- method for list is the one being invoked.

One workaround is to define a [[<- method on the S7 class that explicitly calls validate().

In the example method below, I’m using NextMethod() to delegate to the [[<- method for list. Note that using NextMethod() with S7 is technically “off-label” usage; the ultimate intention is for S7_data() and S7::super() to address this need. However, S7_data() needs a little more work, and NextMethod() can be a reliable solution in the interim.

library(S7)

list_derived_class <- new_class(
  "list_derived_class",
  parent = class_list,
  validator = function(self) {
    if(length(self) > 0)
      "raising non-empty test error"
  }
)

method(`[[<-`, list_derived_class) <- function(x, ...) validate(NextMethod())

x <- list_derived_class()
x[["A"]] = 1
#> Error: <list_derived_class> object is invalid:
#> - raising non-empty test error

^{Created on 2024-11-15 with reprex v2.1.1}

[tschwarzl]

Thank you, @t-kalinowski for the nice explanation and the workaround.

It would be very nice if when the S7_base_class would provide a wrapper around "list" with validate() call in place - e.g. a S7_list which overwrites [[<-

c("list_derived_class", "S7_list", "list", "S7_object")