Default convert() methods for base classes.

[t-kalinowski]

Users may expect something like this to work:

x <- 3
convert(x, class_character)

Concretely, I wonder if we should do:

method(convert, list(class_any, class_character)) <- function(from, to, ...) as.character(from)
method(convert, list(class_any, class_integer)) <- function(from, to, ...) as.integer(from)
method(convert, list(class_any, class_logical)) <- function(from, to, ...) as.logical(from)
...

One thing to consider is that individual package authors may want different behavior. E.g., a strict whole-number check when casting double to integer

method(convert, list(class_double, class_integer)) <- function(from, to, ...) {
  stopifnot("must be a whole number" = trunc(from) == from)
  as.integer(from)
}

We don't want different packages exporting different methods for this type of cast. We may want to dis-allow setting convert(, to) methods for base S3 classes for this reason.

[fithisux]

@t-kalinowski if convert method takes always 2 arguments, why do we need the list(_,_) part?

[t-kalinowski]

The list(_,_) is the second argument to method<- and is used by the generic to perform dispatch based on the argument types. https://rconsortium.github.io/S7/reference/method-set.html

[hadley]

I worry that this has a bunch of unintended consequences that are hard to reason about. In general, I think it's best to be as explicit as possible with coercion, even if it means some existing functions might not work.

In general, I think it's ok to rely on the convention that you should only define methods if you own either one of the classes involved or the generic. It would be nice if we could automate that as a check, but I suspect it's going to be hard.

[lawremi]

I've already defined all of the base/S3 conversions in my S7-based package. Definitely useful to have.

method(convert, list(class_any, class_logical)) <-
    function(from, to, ...) as.logical(from, ...)
method(convert, list(class_any, class_integer)) <-
    function(from, to, ...) as.integer(from, ...)
method(convert, list(class_any, class_double)) <-
    function(from, to, ...) as.double(from, ...)
method(convert, list(class_any, class_complex)) <-
    function(from, to, ...) as.complex(from, ...)
method(convert, list(class_any, class_character)) <-
    function(from, to, ...) as.character(from, ...)
method(convert, list(class_any, class_raw)) <-
    function(from, to, ...) as.raw(from, ...)
method(convert, list(class_any, class_list)) <-
    function(from, to, ...) as.list(from, ...)
method(convert, list(class_any, class_expression)) <-
    function(from, to, ...) as.expression(from, ...)
method(convert, list(class_any, class_function)) <-
    function(from, to, ...) as.function(from, ...)
method(convert, list(class_any, class_environment)) <-
    function(from, to, ...) as.environment(from, ...)
method(convert, list(class_any, class_name)) <-
    function(from, to, ...) as.name(from, ...)
method(convert, list(class_any, class_call)) <-
    function(from, to, ...) as.call(from, ...)

method(convert, list(class_any, class_data.frame)) <-
    function(from, to, ...) as.data.frame(from, ...)
method(convert, list(class_any, class_Date)) <-
    function(from, to, ...) as.Date(from, ...)
method(convert, list(class_any, class_factor)) <-
    function(from, to, ...) as.factor(from, ...)
method(convert, list(class_any, class_POSIXct)) <-
    function(from, to, ...) as.POSIXct(from, ...)
method(convert, list(class_any, class_POSIXlt)) <-
    function(from, to, ...) as.POSIXlt(from, ...)
method(convert, list(class_any, class_formula)) <-
    function(from, to, ...) as.formula(from, ...)

[fithisux]

The list(_,_) is the second argument to method<- and is used by the generic to perform dispatch based on the argument types. https://rconsortium.github.io/S7/reference/method-set.html

You are right.