Introduced by a guard? (Question on Draft Spec for JEP 440 and 441)

[David Alayachew]

Hello Amber Dev Team,

Thank you Gavin for posting the spec!

> 6.3.4 Scope for Pattern Variables in Switch Labels
>
> Pattern variables can be introduced by case labels with a
> case pattern, either by the pattern itself or by a guard,
> and are in scope for the relevant parts of the associated
> switch expression (6.3.1.6) or switch statement (6.3.2.6)
>
> The following rules apply to case labels:
>
>    * A pattern variable is introduced by a case label
>      with a case pattern p if it is declared by p.
>
>    * A pattern variable declared by the pattern of a
>      guarded case pattern is definitely matched in the
>      associated guard.
>
>        - It is a compile-time error if any pattern
>          variable declared by the pattern of a guarded
>          case pattern is already in scope at its guard.
>
>    * A pattern variable is introduced by a guarded case
>      label if it is introduced by the associated guard
>      when true (6.3.1).

Reading the bullets here (especially the final bullet point) implies to me
that the only time that the guard of a guarded case pattern can introduce
variables is when the guard contains a pattern match itself. So, something
like this.

    Number input = 4;

    String response =
        switch (input) {
            case Number n when n instanceof Integer i -> String.valueOf(i);
            default -> "idc";
        };

Apologies for the terrible example.

However, when reading the sentences in the quoted text above the bullets,
they seem a lot more loose. More specifically, reading that text makes me
think something like this is possible (obviously illegal and nonsensical
code, but trying to make a point).

    switch (someObject)
    {

        case Integer i when (int abc = method()) == i -> "???";
        default -> "idk";

    };

Apologies for the terrible example.

I guess my question is, is there another way besides patterns for a
variable to be introduced in the guard?

And if not, would it make more sense to instead say something like the
following instead?

> Pattern variables can be introduced by case labels with a
> case pattern, either by the pattern itself or by a guard
> containing a pattern, and are in scope for the relevant
> parts of the associated switch expression (6.3.1.6) or
> switch statement (6.3.2.6)

Apologies if this is just noise, but I wanted to bring it up because I
initially parsed the quoted passage to imply the second code example I
gave. It wasn't until several minutes of reading and screwing around in
jshell did I start to think the passage was actually implying the first
code example I gave.

Thank you for your time and help!
David Alayachew
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