Should properties have a default value?

[hadley]

This would be used by the default constructor when the property is not supplied.

If properties can have a default value, should it be supplied automatically? i.e. if the property is an R7 or S4 class, it could call the default constructor. This would have the advantage that you can always easily initialise a class>

[lawremi]

This makes sense to me. The methods package initializes slots by just calling new(class), i.e., that is how it generates the default prototype.

[hadley]

This means we'd need to add constructors for non-vector base types (e.g. function), and s3_class() would need to gain a constructor argument. Not sure what to do for unions. Maybe make it an error so the creator has to supply the default? That would probably push us further towards pre-defining some key s3 classes provided by base R. I think this is likely to look something like this:

s3_factor <- s3_class("factor", factor)
s3_data_frame <- s3_class("data.frame", data.frame)

# Needed because S3 functions error when called without arguments
s3_ordered <- s3_class(c("factor", "ordered"), function(x = character(), ...) ordered(x, ...))
s3_date <- s3_class("Date", function(x = integer()) .Date(x))

If your class defines a validator and it fails with the default property values, it'd be your responsibility to either adjust the property defaults or override the constructor.

Should we make the default a function? It seems like it would be convenient to do something like this:

new_property("created_at", s3_date, default = function() Sys.Date())

[lawremi]

For unions, the methods package uses the prototype of the first member of the union (where "NULL" is always considered first if present).

In my experience, it's a lot more common to want a constant as the default, so a function might be too complex. It's fairly straightforward to achieve the same effect in the class constructor. That has the minor added benefit of putting all code executed during object instantiation in one place.

[hadley]

Makes sense. What should default = NULL do? Does it make NULL the default, or does it use the default value from the type?

[lawremi]

It would be NULL. The automatic default would only apply if no default is provided.

[hadley]

Ah you're thinking the behaviour is conditional on whether or not the argument is missing()? I'm not a big fan of that sort of interface, but maybe it works ok here.

[lawremi]

Well the default value of default could be whatever code generates the automatic default, but that wouldn't be NULL.

[hadley]

Somewhat related to this discussion, but in the course of working on this code I realised that the R7 wrapper of the environment base class used new.env(parent = emptyenv()), but this wasn't wrapped in a function so every instance of the class got the same environment.