Pattern matching on primitive types mxied with instanceof is a footgun

[Tagir Valeev]

Hello!

Well, I would say that Remi has a point. This is a silent mistake that can
be made. You don't see the declaration, as it's in another file so you may
forget about original types and assume that they are ints. The code
compiles and doesn't even throw an exception, it just works incorrectly. It
would be justified if such code pattern (converting from double to int only
when double fits the int) was common, so conditional narrowing could be
useful in many other places. But the fact is that it's highly uncommon.
Nobody does this. Many other pattern matching features are nice. E.g., I
often see the code which can be migrated to pattern switch (we are still on
Java 17) and I really want to use it. However, I never see the code which
can be migrated to a narrowing primitive pattern.

I don't agree that we should drop primitive patterns completely, but we may
reconsider floating point <-> integral conversions and disable them in
patterns for a while (enabling them back later will be always possible).

With best regards,
Tagir Valeev

On Tue, Feb 4, 2025 at 12:45 PM Brian Goetz <brian.goetz at oracle.com> wrote:

> Wow, that was pretty amazing. You went from “one student, who was never
> taught about how this works, was confused by it” to “ let’s pull the whole
> feature “ in one breath.
>
> > On Feb 4, 2025, at 11:33 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> >
> > Last week,
> > one of the TA for the course "OOP using Java" was grading student
> projects,
> > the subject of the project can be summarize to "you are alone in space
> and angry aliens want you to die".
> >
> > One of the project had a weird bug when displaying aliens.
> > Spoiling the result of bug hunting, this is due to students using the
> pattern matching on primitive without them realizing it.
> >
> > Here is a reproducer of the bug:
> >
> > // in AlienSpaceship.java
> > record AlienSpaceship(double x, double y) implements Spaceship {
> >  ...
> > }
> >
> > ...
> >
> > // in a method in another class
> >  ...
> >  for(Spaceship spacehip : ...) {
> >    ...
> >    if (spaceship instanceof AlienSpaceship(int x, int y) {
> >      // display the alien space ship at (x, y)
> >    }
> >    ...
> >  }
> >
> > The TA said she was about to give up when she found the issue and that
> the Java spec should not allow such pattern.
> > I agree with here, pattern matching on primitive types is not a feature
> people ask for and mixed with instanceof it's a footgun.
> >
> > This feature can be re-introduced later using method patterns and it
> will be easier to understand the code,
> > by example with an hyphotethical method Double.asInt
> >
> >  if (spaceship instanceof AlienSpaceship(Double.asInt(int x),
> Double.asInt(int y)) {
> >    ...
> >
> > regards,
> > Rémi
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/amber-spec-experts/attachments/20250204/06ac2ce7/attachment-0001.htm>