Implicit Record Was: JEP draft: Implicit Classes and Enhanced Main Methods (Preview)

[forax at univ-mlv.fr]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Ron Pressler" <ron.pressler at oracle.com>, "Dan Heidinga"
> <heidinga at redhat.com>, "amber-spec-experts" <amber-spec-experts at openjdk.org>
> Sent: Tuesday, February 21, 2023 12:33:31 AM
> Subject: Re: Implicit Record Was: JEP draft: Implicit Classes and Enhanced Main
> Methods (Preview)

> You seem to have a very strange notion of what "works" means. These all work
> just fine. No one is suggesting that local and "shared" variables are unified.
> If you find it confusing to teach fields early, then wait -- that's one of the
> choices.

>> Scope rules are different, being static is different, initialization rules are
>> different, inference rules are different, even colors in IDEs are different.

> Correct, and well understood. Your point?
Implicit class remove the "class" shell, so you now have two syntactically identical things that are semantically different. 
IMO, introducing dangling fields, a new feature in Java, does not worth the confusion it creates. 

>> You did, by re-using the term "variables" for both local variable and fields.

> Sorry, no. They are both variables, but they are different sorts. Just like
> "instance vs static" variables, or "final vs mutable" variables. They are all
> variables (they have a name, and a type, and hold a value), and yet they each
> have different characteristics (and the characteristics can be combined; you
> can have shared final static variables, and local mutable variables, and....)

> I think you're taking a "I would prefer it work this way" and bootstrapping it
> into "the alternative is broken" (that's what "doesn't work" means.) You should
> know by now that this is the best way to not have your arguments taken
> seriously!
I think you are talking about variable in general, i'm focused on local variable and fields in the context of implicit class, hence the difficulty to understand each other. 
Implicit class makes the syntax of fields identical to the syntax of local variables and at the same time, the way to differentiate then is to explain what an implicit class is. 

Again, i'm not against the notion of implicit container, i just think that implicit record makes more sense than implicit class, and i would like we discuss about that, 
here is example where the container leaks, 

void main() { 
System.out.println(this); 
} 

what it should print ? 

regards, 
Rémi 

> On 2/20/2023 4:38 PM, [ mailto:forax at univ-mlv.fr | forax at univ-mlv.fr ] wrote:

>>> From: "Brian Goetz" [ mailto:brian.goetz at oracle.com | <brian.goetz at oracle.com> ]
>>> To: "Remi Forax" [ mailto:forax at univ-mlv.fr | <forax at univ-mlv.fr> ]
>>> Cc: "Ron Pressler" [ mailto:ron.pressler at oracle.com | <ron.pressler at oracle.com>
>>> ] , "Dan Heidinga" [ mailto:heidinga at redhat.com | <heidinga at redhat.com> ] ,
>>> "amber-spec-experts" [ mailto:amber-spec-experts at openjdk.org |
>>> <amber-spec-experts at openjdk.org> ]
>>> Sent: Monday, February 20, 2023 9:33:34 PM
>>> Subject: Re: Implicit Record Was: JEP draft: Implicit Classes and Enhanced Main
>>> Methods (Preview)

>>>> As i said earlier, it does not work because fields and local variables have
>>>> different semantics,
>>>> fields are initialized with a default value while local variables need to be
>>>> initialized before use.
>>>> So the curtain is just a veil that will be pierced by any students moving
>>>> declarations around.

>>> Of course it "works", it just might not work how you would prefer it to.
>> var greetings = 0;

>> does not work,

>> void main() {
>> int greetings;
>> System.out.println(greetings);
>> }

>> does not work,

>> void main() {
>> static int greetings;
>> System.out.println(greetings);
>> }

>> does not work,

>> void main() {
>> int greetings = 0;
>> for(;;) {
>> int greetings = 0;
>> }
>> }

>> does not work too.

>> Scope rules are different, being static is different, initialization rules are
>> different, inference rules are different, even colors in IDEs are different.

>>> Prior to learning about fields, the user can perceive local variables (declared
>>> in a method) and "shared" variables (accessible to all members of the class.)
>>> They can learn about their characteristics. Then, when they learn about classes
>>> and fields and accessibility, they can learn that the variables they were
>>> calling "shared" are really fields. The distinction between locals and fields
>>> is there from the beginning, though for most use cases, they will not notice
>>> the difference. When they're ready to learn the fine differences, there's not
>>> anything to unlearn.

>>>> From my personal experience, unifying local variable and field leads to more
>>>> pain than gain, mostly because local

>>> Who said anything about unification of fields and locals? Where did you get such
>>> an idea that this is what is being proposed?
>> You did, by re-using the term "variables" for both local variable and fields.

>>> First students will learn about statements. Then they will probably learn about
>>> local variables. They can be taught that they disappear when the method exits,
>>> and each invocation of the method gets a fresh copy. Then they can learn about
>>> multiple methods, and then that there are variables that can be shared across
>>> methods and retain their values across method invocations, and while their
>>> declaration syntax is the same (they're both variables, after all), the _place_
>>> in which they are declared is different (which is what makes them shared), and
>>> shared variables have slightly different characteristics (though not so
>>> different they have to learn this immediately). They can learn the
>>> characteristics of shared variables when it makes sense to teach this. And when
>>> the curtain is pulled back, they learn all fields have the characteristics of
>>> these shared variables.
>> Record components behave as you said, they have slightly different
>> characteristics than local variables and it's hard to not notice the difference
>> syntactically.

>> record Hello(String name) {
>> void hello() {
>> System.out.println("hello " + name);
>> }

>> void bonjour() {
>> System.out.println("bonjour " + name);
>> }
>> }

>> Class fields are far more different than just the lifetime but have a very
>> similar syntax.

>> Rémi
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