Finalising the on-ramp feature

[Clement Cherlin]

On Tue, Jan 21, 2025, 2:49 PM Remi Forax <forax at univ-mlv.fr> wrote:

>
>
> ----- Original Message -----
> > From: "Gavin Bierman" <gavin.bierman at oracle.com>
> > To: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> > Cc: "amber-dev" <amber-dev at openjdk.org>
> > Sent: Tuesday, January 21, 2025 8:06:46 PM
> > Subject: Re: Finalising the on-ramp feature
>
> > [Resending with correction]
> >
> > # Finalising the on-ramp feature
> >
> > With JDK 24 preparations complete, we are now planning our work for JDK
> 25 and
> > in particular the on-ramp JEP. It is our intention to *finalise* the
> on-ramp
> > feature in JDK 25. We are grateful for the feedback that we have
> received from
> > our expert and developer communities - thank you! - and we have been
> performing
> > our own extensive in-house experiments with the preview feature. Given
> this, we
> > propose that we make the following changes/simplifications to the
> feature when
> > it finalizes in JDK 25.
> >
> > ## 1. Remove the auto-static-import feature and move the `IO` class
> >
> > We had proposed that the static members of a new class `IO` would
> automatically
> > be imported by simple compilation units. This allows the use of the
> simple names
> > `println`, `print`, and `readln`. Whilst this is very convenient, it
> creates a
> > bump in the on-ramp experience when migrating a simple compilation unit
> to an
> > ordinary compilation unit, which does not implicitly import these static
> > methods. This means that when migrating we either add an explicit static
> import,
> > or rewrite all the `println`, `print`, and `readln` method calls to
> qualified
> > calls.
> >
> > We have come to the conclusion that the graceful on-ramp experience is
> the more
> > important goal. So, we propose in JDK 25 that (1) we drop the automatic
> static
> > import of the `IO` class by simple compilation units, and (2) We move
> the `IO`
> > class from package `java.io` to `java.lang`.
> >
> > This means that in simple compilation units calls to the `IO` methods
> should be
> > qualified, i.e. `IO.println`, `IO.print`, and `IO.readln`. However,
> > when migrating from a simple compilation unit to an ordinary compilation
> unit,
> > no static import declaration will need to be added, nor will any of
> these calls
> > need to be rewritten; the on-ramp experience is simpler. For example:
> >
> > ```
> > // Simple.java
> > void main() {
> >    IO.println("Hello, world.");
> > }
> > ```
> >
> > is migrated to:
> >
> > ```
> > // Ordinary.java
> > class Ordinary {
> >    void main() {
> >        IO.println("Hello, world.”);
> >    }
> > }
> > ```
>
> I am a little disapointed by this proposal,
> As a teacher, it means that you have to explain the dot syntax early on,
> which is not necessary when discovering basic things like literals, control
> flow, arrays, user defined functions (yes, functions not methods, because
> the compact class is not visible, so everything is a function).
>
> Yes, later on, when you explain the concept of IO, the dot syntax ('.'),
> the "println" inside "IO", the "parseInt" inside "Integer".
> But for the first lectures, there is no need to introduce the dot syntax.
>
> More fundamentally, this proposal is weird to me, we want to introduce
> compact classes so there no need to declare a container class but to print
> something, you have to us a syntax that says go into the container class
> "IO" to find the method "println". So compact class => no container class,
> IO.println => container class ??
>
> I understand that it means that the migration from a compact class to an
> ordinary class can be seen as more complex, but it's because you are
> skiping an intermediary step, which is moving from the world of functions
> to the world of methods. I think that the world of functions is important
> enough so we should support it, by allowing "println" to be an alias of
> "IO.println"
>
> regards,
> Rémi
>

I agree. The goal is to make it extremely simple for beginners to write
basic Java programs. Writing HelloWorld.java should be as easy as writing
hello-world.sh or hello-world.pl. I think an implicit `import static
java.lang.IO.*;` is reasonable to provide a smoother ramp. There's a simple
migration path once dot notation is intoduced from `println()` to
`IO.println`, without forcing it up front.

If we're going to require beginners to learn dot notation right out of the
gate, it would be simpler to statically import System.in and System.out in
compact source files and dispense with the IO class entirely.

Cheers,
Clement Cherlin

>
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