Sealed types -- updated proposal

Remi Forax forax at
Thu Jan 31 21:07:12 UTC 2019

You have forgotten that 
- if you have a sealed class (not sealed interface), using nesting has the side effect of creating inner classes. 
I don't know what is the policy of Google about inner classes that mix enclosing class access and inheritance but i suppose they are prohibited too. 

- for #4, I've proposed a simple scheme that allow tools to find the compilation unit of any auxiliary classes of a sealed type. 


> De: "Brian Goetz" <brian.goetz at>
> À: "amber-spec-experts" <amber-spec-experts at>
> Envoyé: Jeudi 31 Janvier 2019 20:12:50
> Objet: Re: Sealed types -- updated proposal

> Since this seems to be the open issue that has the most divergence….

> The basic problem is that the user is “forced” to choose between a convenient
> and readable way to declare the classes (a group of related, short declarations
> in one place), and exporting a flat namespace. And the downside of a non-flat
> namespace is that then the client has to say Node.AddNode instead of just
> AddNode, which is inconvenient. So we can have what’s nice for the declaration
> site, or what’s nice for the use site, but not both. Here are the options that
> have been proposed so far:

> Option 1: Do nothing; just tell clients to `import static Node.*`.

> Option 2: Automagically import-static the nested subtypes of Node when you
> import sealed type Node.

> Option 3: Do what we do for enums: when switching on an enum of type C, allow
> `case` labels to omit the `C.` prefix.

> Option 4: Relax the rules about public auxiliary types.

> #1 is a viable solution, and has the benefit that it requires no incremental
> complexity. IDEs will help here too. So doing nothing is an acceptable choice;
> this should be our null hypothesis.

> #2 is cute, but (a) import processing is already nastier than it looks, and (b)
> this feels odoriferously specific to the interaction between two features.

> #3 Seems pretty justifiable to me; enums and sealed types are sibling constructs
> (both are about controlling the number of things that can be a member of the
> value set), so having a similar rule for both is arguably reducing gratuitous
> asymmetries. It is also a smaller change than #2 or #4, and likely will cover a
> great deal of the pain-causing situations.

> People who maintain large codebases (JDK, Google) are pretty down on #4; we ban
> auxiliary classes in the JDK in part because it makes tooling support so much
> harder. Google does something similar. This one strikes me as something that
> seems enticing at first but ultimately would cause other problems. It also
> shares downside (b) from #2.

> Given that, I’m going to cross #2 and #4 off the list for consideration, and
> restrict the choices to #1 and #3 (open to new ideas that have not yet been
> discussed.) I have a preference for #3, but could be moved off it.

>> On Jan 9, 2019, at 1:44 PM, Brian Goetz < [ mailto:brian.goetz at |
>> brian.goetz at ] > wrote:

>> Auxilliary subtypes. With the advent of records, which allow us to define
>> classes in a single line, the “one class per file” rule starts to seem both a
>> little silly, and constrain the user’s ability to put related definitions
>> together (which may be more readable) while exporting a flat namespace in the
>> public API.

>> One way to do get there would be to relax the “no public auxilliary classes”
>> rule to permit for sealed classes, say: allowing public auxilliary subtypes of
>> the primary type, if the primary type is public and sealed.

>> Another would be to borrow a trick from enums; for a sealed type with nested
>> subtypes, when you import the sealed type, you implicitly import the nested
>> subtypes too. That way you could declare:
>> semi-final interface Node {
>>     class A implements Node { }
>>     class B implements Node { }
>> }

>> ​but clients could import Node and then refer to A and B directly:
>> switch (node) {
>>     case A(): ...
>>     case B(): ...
>> }

>> We do something similar for enum constants today.
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