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*From: *"Ron Pressler" <[email protected]>
*To: *"Remi Forax" <[email protected]>
*Cc: *"Dan Heidinga" <[email protected]>, "Brian Goetz"
<[email protected]>, "amber-spec-experts"
<[email protected]>
*Sent: *Monday, February 20, 2023 10:47:57 AM
*Subject: *Re: Implicit Record Was: JEP draft: Implicit Classes
and Enhanced Main Methods (Preview)
I was trying to understand how you tie shared variables to
encapsulation and what you meant by something that students later
need to unlearn, and I think I understand now. You’re okay with
shared mutable state as long as it’s clear that it’s not shared
with the whole world but only with some explicit unit, and since
an implicit class appears to be the whole world, then its fields
seem to be globally shared, and that’s what you want to avoid your
students learning. Is that correct?
yes. But it's one of the arguments.
The real problem is dangling fields. Conceptually fields are far more
"attached" to a class than methods (at least until you explain
classes) but syntactically with an implicit class, you do not see
that. Allowing dangling fields force the teacher to explain what an
implicit class is which defeat the teaching purpose of it.
Now, we’re not in the business of telling teachers how to teach,
and I assume different teachers teach in different ways. Implicit
classes are not exclusively a teaching construct, but a natural
Java construct — a natural extension of implicit modules and
packages.
Agree, but there are some fundamentals, introducing fields without the
notion of class is hard to envision.
Expert programmers may also appreciate implicit classes just as
they appreciate JShell and launching source-code programs, both of
which are also explicitly motivated, at least in part, by
education (see JEPs 222 and 330).
yes,
Here we are discussing about ergonomics, i.e. is the "natural"
extension is implicit class or implicit record ?
With JEP 330, it's fear easier to create one file scripts written in
Java instead of Python mostly because Python on Windows does not work
as well as Python on Linux. I've seen several of those scripts on
github, and there are not all using "class" as the top-level
containers, some are using "interface", i've not seen a lot of scripts
using records but record is a more novel construct. Implicit class is
not necessary the right default.
That natural Java construct *allows* teachers to teach in the
order and style they choose without *forcing* classes on them.
Some teachers may teach just basic control flow and (local)
variables, perhaps records, and treat List, Map, and Set as
built-in constructs without teaching any OOP in the first course
and without teaching students how they can implement their own
Lists etc. Some may choose to show shared variables (even if only
to demonstrate their danger) while some may choose not to,
although I assume everyone will teach constants. If you want to
teach mutable fields only after introducing classes, that’s great
and you can certainly do that (perhaps while enjoying the enhanced
main and postponing static). But I don’t think the language should
enforce a particular teaching style, where shared mutable state
*must* be taught only in the context of classes. Even if you
believe that doing otherwise is bad pedagogy, there is nothing
that fundamentally ties shared variables to the ability to create
class instances.
Agree, i'm not suggesting that there is a right way to teach, i'm too
old for that :)
Before implicit class, introducing field declarations without classes
is not something that was possible, so dangling fields is a new
feature. As a teacher, the main reason to not allow dangling fields is
that it creates confusion between local variables and fields which is
a real issue students wrestle with.
Also, as a side remark, implicit class/record also creates new
challenges from the teaching perspective, by example, you have to
postpone explaining static quite a bit otherwise if you explain that
main() can be a static method too soon, students will call you because
this kind of code does not work
static void main() {
var color = new Color("blue"); // oops
}
class Color(String name) {
...
}
My point is that while i agree that having an implicit container is a
good idea, it does not make necessarily teaching easier, because an
implicit container is a new feature that may interact badly with the
rest of the features you want to teach.
It’s true that we don’t want to cause harm by letting beginners
learn something that needs to later be unlearned, but that’s not
the case even for those who do learn about mutable fields. If
students begin with implicit classes and seem to think that fields
(if taught them) — or methods for that matter — are shared with
the whole world, they invariably later learn that their “whole
world” is actually a unit and that big programs are made by
composing such units. They inevitably learn that because Java
offers no other way. They *can’t* create a global variable, nor a
global method, because Java simply doesn’t have those. Even if
Java were to someday acquire package-level fields and methods (or
even, hypothetically, module-level methods and fields), it still
wouldn’t have a global namespace (not even for classes!). Learning
that basic fact doesn’t require any unlearning, just contextualising.
Everything is encapsulated in Java, but dangling fields syntactically
does not show that, that the issue.
So we're giving teachers more freedom than ever before to teach
Java in the manner each of them chooses, and I don’t think we’re
inflicting any harm in the process. I think that restricting the
abilities of implicit classes further forces a particular teaching
style — though some may consider it the only correct style — and
would also be a less natural Java construct and a less enjoyable
one for experienced programmers.
The point is to teach Java, not to have to teach yet another new
feature. Offering new freedom also implies introducing new complexity.
— Ron
Rémi
On 18 Feb 2023, at 07:11, [email protected] wrote:
yes.
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) { this.name = name; }
void hello() { System.out.println(name); }
void main() {
setName("Bob");
...
hello();
}
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 ?
