Pattern matching on primitive types mxied with instanceof is a footgun

[Kevin Bourrillion]

On Tue, Feb 4, 2025 at 8:55 AM Maurizio Cimadamore <
maurizio.cimadamore at oracle.com> wrote:

> This seems consistent. So the question is: what is the "mistake" we're
> talking about here? The user thinking it was a _unconditional_ match where
> in reality that's not the case? While in the case of instanceof
> unconditionality is harder to spot, if this was a switch, the compiler
> would have immediately asked the user to provide a default?
>
> It seems to me that the reasoning here implies that developers will use
> "if" + "instanceof" _without an "else" just for the purpose of extract some
> data, and then complain when the thing doesn't match (because they have no
> "else", and so their code will fail). It seems like our best weapon in this
> case is education, rather that introducing arbitrary restrictions and
> asymmetries in the language?
>
> (Or, perhaps, in the future have a way to perform an _unconditional_
> extraction without an "if", in which case the compiler will bark if the
> pattern is not unconditional.)
>
In some cases this will be relevant, but in *this *particular example we're
looping over Spaceship and then checking for AlienSpaceship, so, no help.

I think it is fair to acknowledge that this is a weakness of the feature
design. How to mitigate it? We can try to evangelize always preferring
`var` for nested type patterns that are expected to be unconditional.
That's an unfortunate readability trade-off, though. And a static analyzer
can't know whether you expected it to be unconditional or not, so there
might not be much it can do to help.

I am not even sure what language change could be considered here. Note that
this problem is not specific to primitive types, it's just that primitive
types are currently the only ones that support "value conversion" so we're
encountering this issue with them first. Post-Valhalla, other types might
support value conversion too.



On 04/02/2025 16:00, Tagir Valeev wrote:
>
> 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
>>
>
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