[External] : Re: JEP draft: Implicit Classes and Enhanced Main Methods (Preview)

[Dan Heidinga]

On Fri, Feb 17, 2023 at 11:05 AM Ron Pressler <ron.pressler at oracle.com>
wrote:

> I’d like to make another point about the general approach of this JEP.
>
> We try to avoid a beginners’ dialect, but a beginners’ *subset* is also
> not what we’re proposing. While the feature is primarily motivated by
> education, it is also a natural, perhaps even obvious, feature for Java
> that’s perfectly in tune with the existing features of the language.
>
> Classes, packages, and modules are all programming-in-the-large
> constructs, and every Java method resides in a class that resides in a
> package that resides in a module. Yet, when you don’t need encapsulation,
> an unnamed module is implicitly provided; when you don’t need package
> namespacing, an unnamed package is implicitly provided. It makes sense to
> do the same for classes even to reduce the need for
> programming-in-the-large declarations in small programs. Of course, helping
> students is a bigger motivation that makes significantly raises this
> feature’s priority.
>
> The important question as is whether or not this feature fulfils the
> motivation of helping beginners (of course, it’s not the only feature we
> can or will do to that end). I think the answer is yes. So then the
> remaining question is, would subsetting the language to forbid static
> members significantly help students? I’m not sure.
>

Wouldn't forbidding static members impose a cliff on beginners?  As they
learn about static methods and fields and introduce the first static member
to their implicit class, they'd be forced to grow a class structure around
their program. Maybe that's a natural time to have to talk about defining a
class?

I think there's a benefit in letting students (and advanced users) use as
much of the language as possible in implicit classes.  Don't force them to
define the class until they do something that requires the class to have a
proper name.

--Dan

>
> — Ron
>
> On 17 Feb 2023, at 10:11, Ron Pressler <ron.pressler at oracle.com> wrote:
>
>
>
> On 16 Feb 2023, at 21:41, forax at univ-mlv.fr wrote:
>
>
> I still think that fields should not be allowed inside an implicit class,
> because when you remove the class declaration a field and a local variable
> are too similar and because an implicit class has no user defined
> constructor.
>
>
> I think your general point has some merit — I’ll get to that later — but
> first let me address the concrete points you raise.
>
>
> Here is a series of examples showing how confusing it can be.
>
>
> How would any of those be made easier to understand by the presence of a
> class declaration when you don’t know what a class is?
>
> By the way, we should certainly look into making some error messages —
> especially those encountered by beginners — easier to understand.
>
> Also conceptually, being able to define fields without constructors is
> problematic, because you are bypassing the the notion of encapsulation.
>
>
>
> Encapsulation from what? Encapsulation is a programming-in-the-large
> notion, but even at the technical level, an implicit class is well
> encapsulated by virtue of it being unnamed (and the default access remains
> package).
>
> Implicit class instance fields are more complex that usual class fields
> because of the lack of constructors.
>
>
> I’m not sure I understand the relationship you make to constructors (BTW,
> you can define initializers).
>
> Python and JS, both also first language have a notion of shared variables
> that can be introduced before object fields. Clojure and ML, both
> functional languages, also have a similar notion of shared variables. Even
> Haskell has constants. Surely you’d agree that at least final fields —
> constants — are necessary to do any kind of nice programming?
>
>
> Teaching using a simpler model is great but not if as a student you have
> to unlearn something previously introduced.
>
>
> I wholeheartedly agree, but what is the thing that needs to be unlearned?
>
> But now back to where I think your general point has merit. I think final
> fields are a must, but one could certainly argue that non-final fields are
> not. You can certainly do a lot of programming without them. But I think
> that allowing final fields and disallowing non-final fields *in Java* would
> be weird, because to designate something as final you need extra syntax, so
> we’d reject syntactically simpler, valid, code and accept more complex one.
> Moreover, there are things that are easier to do and tech with mutable
> fields.
>
> However, there’s the matter of static, which you used in your examples but
> didn’t explicitly discuss. Because a an implicit class is effectively a
> singleton (plus, the class cannot be referenced by other classes), there is
> no useful difference between an instance field and a static field, so I
> think we should entertain the notion of disallowing static members —
> fields, methods, or even member classes (although things that are
> implicitly static, such as records would obviously be allowed).
>
> One argument against that may be is that if an experienced Java programmer
> has an existing small program that they want to make prettier by turning
> into an implicit class — implicit classes are mainly motivated by learners,
> but they’re not *just* for them — then the process would be made harder by
> disallowing static members.
>
> In short, I think we must allow fields, but we can think about disallowing
> (explicitly) static members altogether.
>
> — Ron
>
>
>
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