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

forax at forax at
Sat Feb 18 07:11:29 UTC 2023

> From: "Dan Heidinga" <heidinga at>
> To: "Ron Pressler" <ron.pressler at>
> Cc: "Remi Forax" <forax at>, "amber-spec-experts"
> <amber-spec-experts at>
> Sent: Friday, February 17, 2023 5:56:49 PM
> Subject: Re: [External] : Re: JEP draft: Implicit Classes and Enhanced Main
> Methods (Preview)

> On Fri, Feb 17, 2023 at 11:05 AM Ron Pressler < [ mailto:ron.pressler at
> | ron.pressler at ] > 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

from my experience, the time to introduce the notion of class is when you start to have shared mutable state. What i do not like with the implicit class proposal is the fact that you can have fields without defining the class around. 
But i think there is a solution. 

What about the feature being renamed to "implicit record" instead of "implicit class" ? We have no discuss why the container of an implicit "class" has to be a class instead of an annotation, an interface, an enum or a record. 

Having the container to be an annotation is useless given that an annotation can not have a main. 
If the container is an interface, methods are abstract by default which is not what we want. 
If the container is an enum, then we are closer to the idea of Ron that it is a singleton, especially if the container defined one implicit enum member like "SINGLETON" . An enum can not be inherited and the default constructor is private which is are nice properties. 
if the container is a record with no component, it can not be inherited, the constructor is package visible and more importantly to me, a user can not defined instance fields ... 

I prefer implicit record to implicit class because with a record as container you can not introduce a shared mutable state by error, you have at least to write static in front of the field. 

I dread about students being able to write code like this 

String name; 

void setName(String name) { = name; } 
void hello() { System.out.println(name); } 
void main() { 

i.e to be able to declare mutable shared state without a class around (@Ron without class encapsulation). 
At least if the container is a record, "name" in the example above has to be static, from the student POV, an unusual variable. 

An implicit record has also the advantage that you do not have to introduce the notion of class to explain the notion of implicit container, given that records are far simpler at the beginning than class, having the implicit container being a record make sense because it's records all the way down. 

What do you think about having the implicit container being a record instead of a class ? 

>> — Ron


>>> On 17 Feb 2023, at 10:11, Ron Pressler < [ mailto:ron.pressler at |
>>> ron.pressler at ] > wrote:

>>>> On 16 Feb 2023, at 21:41, [ mailto:forax at |
>>>> forax at ] 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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