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

[Brian Goetz]

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?

> 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!



On 2/20/2023 4:38 PM, forax at univ-mlv.fr wrote:
>
>
> ------------------------------------------------------------------------
>
>     *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: *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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