Clarifying the receiver parameter
mernst at cs.washington.edu
Tue Oct 22 10:11:07 PDT 2013
Markus just gave us a link to his April email. (I didn't see that message
either, unfortunately.) I'm pasting Markus's April message here for the
> From: Markus Keller markus_keller at ch.ibm.com
> Date: Wed Apr 24 04:24:16 PDT 2013
> To: type-annotations-spec-observers
> The difference is that we're not talking about a 'this' expression here,
> but about the *declaration* of a method/constructor parameter.
> For references to 'this', qualification is necessary in general to
> support references to all enclosing instances. Since the grammar already
> allows qualifiers, it makes sense to keep full generality and also allow
> redundant qualifiers there.
> But there's no point in adding qualifiers for the declaration of the
> 'this' parameter of an instance method. The reason is consistency with
> all other declarations. You also cannot qualify a method name,
> constructor name, field name, or type name in a declaration.
> For constructors, the 308 spec decided to annotate the whole
> constructor, rather than allowing an explicit 'this' parameter like for
> instance methods. This works fine for top-level constructors, but it
> doesn't offer a way to annotate enclosing types of an inner class
> constructor. The syntactic kludge to allow annotations on the enclosing
> types is the superclass.'this' parameter on inner class constructors. A
> pure 'this' would declare the wrong element, so it needs to be
> qualified. And like the enclosing class' simple name is enough for a
> constructor name, the outer class' simple name is also enough to qualify
> the 'this' parameter of an inner class constructor.
> An alternative to the current spec would be to drop section 2.1 point 4
> : "A type annotation is permitted in front of a constructor declaration,
> [..]". Instead of that, we could allow a 'this' receiver parameter for
> all constructors, where the 'this' would point to the constructed
> object. This would obviate the need for any qualification on 'this', but
> it has the drawback that TYPE_USE annotations on the whole constructor
> declaration could clash with annotations on the 'this' parameter's type.
> I think the rationale for the current design is to avoid this clash.
More information about the type-annotations-spec-experts