[Rd] Reference classes

[Jon Clayden]

On 22 October 2010 18:55, John Chambers <jmc at r-project.org> wrote:

>> As a suggestion, it would be nice if the accessors() method could be
>> used to create just "getters" or just "setters" for particular fields,
>> although I realise this can be worked around by removing the unwanted
>> methods afterwards.
>
> In other words, read-only fields.  There is a facility for that implemented
> already, but it didn't yet make it into the documentation, and it could use
> some more testing.  The generator object has a $lock() method that inserts a
> write-once type of method for one or more fields.  Example:
>
>> fg <- setRefClass("foo", list(bar = "numeric", flag = "character"),
> +             methods = list(
> +             addToBar = function(incr) {
> +                 b = bar + incr
> +                 bar <<- b
> +                 b
> +             }
> +             ))
>> fg$lock("bar")
>> ff = new("foo", bar = 1.5)
>> ff$bar <- 2
> Error in function (value)  : Field "bar" is read-only
>
> A revision will document this soon.
>
> (And no, the workaround is not to remove methods.  To customize access to a
> field, the technique is to write an accessor function for the field that, in
> this case, throws an error if it gets an argument.  See the documentation
> for the fields argument.  The convention here and the underlying mechanism
> are taken from active bindings for environments.)

OK, yes - I see. This is clearly much less superficial than removing
the setter method for a field which can be directly set anyway. I'll
have to try out field accessor functions and get a feel for the
semantics.

Many thanks for the rapid and very helpful response.

Regards,
Jon