Towards cleaner nesting

[Alan Malloy]

Very clear and useful. I would love for the nesting story to get simpler.
I've discovered a number of corner cases where I thought some construct
would be legal but in fact wasn't (e.g. an inner class with a static
constant), and what you describe sounds like it will make the language
easier to understand.

Two remarks. First, I'm not sure what you mean by "static lambda". Of
course it is already possible for a lambda to be declared as a static
member of some class (static Runnable r = () -> {};), so you must mean
something else, but I don't know what that is. Do you imagine a special
declaration mode for a lambda that asks the compiler to ensure that it
captures nothing?

Second, how do code-containing constructs which are not methods or classes
fit into this? I have field initializers in mind, but maybe static/instance
initializers are relevant too. Does it make sense to nest a method inside
of an instance initializer?

On Tue, Jan 7, 2020 at 12:31 PM Brian Goetz <brian.goetz at oracle.com> wrote:

> Everything about nesting in Java is a mess.  The terminology is a mess
> (top level classes, nested classes, inner classes, local classes, anonymous
> classes); the set of restrictions on what can nest in what is ad-hoc (can
> have local classes but not local interfaces; inner classes cannot have
> static members, including static nested classes), and the set of rules
> about what must be, can be, or cannot be static is also ad-hoc (nested
> classes can be static or not, nested interfaces are implicitly static, but
> local and anonymous classes may not be static, even though it might make
> sense.)  On top of that, we can nest classes in methods (sometimes) and
> methods in classes but not methods in methods (local methods).
>
> Not only does this make for a lot of accidental complexity in
> specification, implementation, and user's brains, but it means every
> feature interact with this complexity.  Nested records are implicitly
> static, but this meant that in 14 we can't have nested records in
> non-static classes, because, non-static classes can't have static members.
> (Yes, this could be fixed; hold your "why don't you just" suggestions.)
> And we borked up the implementation of local records the first time around,
> where they accidentally capture effectively final locals, which they
> shouldn't -- because we'd never really charted the "static local class"
> territory, and got it wrong the first time.  (Yes, this can be fixed too,
> and will be before 14 goes out.)
>
> So, I'd like to propose a simpler, general story of nesting (which is
> consistent with the ad-hoc rubbish we have) which we can get to in stages.
> The purpose of this mail is to discuss the model; in what increments we get
> there is a separate story.
>
> Goals:
>  - Anything (class, interface, record, enum, method) can be nested in
> anything;
>  - Some things are always static (enums, records, interfaces) when nested;
> the rest can be made static when desired;
>  - The rule about "no static members in nonstatic nested classes" has to
> go;
>  - Rules about whether members / locals from enclosing contexts can be
> specified in a single place, using local reasoning.
>
> The core of this is coming to an understanding of what "static" means.
> When construct X nests in Y (whether X and Y are classes, methods,
> interfaces, etc), for "X" to be "static" means that nesting is being used
> purely for purposes of namespacing, and not for purposes of having access
> to names (locals or nonstatic class members) from enclosing constructs.
>
> Unfortunately all the terms we might use for whether or not a symbol in an
> outer construct can be used in a nested construct -- such as "accessible"
> -- are overloaded with other meanings.  For purposes of this discussion,
> let's call this "capturable" (this is also overloaded, but less so.)  Each
> construct (class type or method) has two sets of names from outer
> constructs that are capturable -- a _statically capturable_ set SC(X), and
> a _non-statically capturable_ set NC(X).  We can define capturability using
> local reasoning:
>
> Base cases:
>  - Names of static members in X are in SC(X);
>  - Names of instance members of X (if X is a class) or effectively final
> locals of X (if X is a method) are in NC(X);
>
> Induction cases, where X is nested directly in Y:
>  - SC(Y) is in SC(X)
>  - If _X is not static_, then NC(Y) is in NC(X)
>
> We then say that X can capture names in SC(X) and NC(X); all we need to
> compute capturability is the capture sets of X's immediately enclosing
> construct, and whether X is static or not in that construct (modulo
> shadowing etc.)
>
> For the math-challenged, what this means is:
>  - A nested construct can access static members of all the enclosing
> constructs;
>  - A nested non-static construct can access instance members and
> effectively final locals of all enclosing constructs, up until we hit a
> static construct, and then capturing stops.  (So if Z is nested in Y is
> nested in static X, Z can access instance members / eff final locals of Y
> and X but not anything non-static from outside of X.)
>
> Note that this is consistent with what currently happens when X is a
> method as well as a class type; static methods in a class "capture" the
> static members of the enclosing class, and instance methods also capture
> the instance members of the enclosing class -- and also consistent with
> capturing in lambdas and anonymous classes, if we assume that these are
> always non-static constructs.
>
> We then say enums, records, and interfaces are _always_ static when
> nested, whether declared so or not, we eliminate the restriction about
> static members in non-static nested classes (now that we have a clear
> semantics for them), and allow local classes to be declared as static.
> (Eventually, we also relax the restrictions about methods in methods,
> static or not.)
>
> (Additionally, the model supports the notion of "static lambda" and
> "static anonymous class" with obvious semantics (can't capture anything);
> we can decide later whether adding this flexibility is worth the additional
> surface syntax.)
>
> This is a strict superset of the status quo, and yields a more flexible
> and regular language -- and hopefully a simpler spec (since so many of
> these cases are specified as ad-hoc corner cases.)
>
>
>
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