Simple Class Extensions

  setIs(C2, C1)

1. Implicitly, by C1 being included in the representation of C2, in which case no explicit call to setIs is needed;

2. Explicitly, by a call to setIs, in which case C1 must either be a virtual class or included in C2 (in the sense of both having the same definition for all the slots of C1).

In the current version of R (1.5.0 and later), simple extensions are used in formal method dispatch without transforming the actual argument. That is, if a method exists corresponding to class C1 for some argument, and the actual argument has class C2, then the method can be selected. If it is, the method will be called with an untransformed argument (still class C2, not the defined class).

We are considering applying the same convention to slot assignments. Suppose a slot is assigned, say

  z@type <- x

The same rule would apply to named arguments supplied to the function new, assuming the class had no special initialize method defined.

There are a few details to note:

• Possibilities 1 and 2 are the only valid simple calls to setIs, because otherwise the two class representations conflict, so a coerce= argument must be supplied to setIs.

• Simple extensions are transitive: if C₃ is a simple extension of C₂, it is also a simple extension of C₁.

• Classes related by non-simple extensions are fine either for method dispatch or for slot assignment, but the supplied argument or slot value will be transformed to class C₁.

• The current API only requires that slot replacements be coercible to the target class, not that there be an extension. It's possible that this might be tightened if it seemed helpful in avoiding programming errors.