Pattern Matching for instanceof (Preview 2)

Brian Goetz brian.goetz at
Tue Feb 18 15:14:57 UTC 2020

>> I don’t know what you mean here. There are two patterns, a type test pattern and
>> a deconstruction pattern. In v2 we propose to support deconstruction patterns
>> over record types *only*. A deconstruction pattern looks like this: Point(var
>> a, var b), i.e. all the components are either (recursively) deconstruction
>> patterns, or `var` <identifier>, i.e. with no type needed. I added a note to
>> the JEP page pointing out that this is a starting point, and eventually we will
>> support other patterns in the argument position, specifically <type>
>> <identifier>; hopefully in this release.
> Currently we don't support mixing var and non var in lambda parameters.
> So my question is: does this pattern Point(var x, int y) that mix a 'var' and an explicit type allowed or not ?

Unequivocal "yes".   There is a universe of patterns.  Some might be 
disallowed in certain contexts (e.g., `instanceof var x` seems kind of 
silly), but once we decide on the set of patterns allowable in which 
contexts, any pattern can be nested inside a deconstruction pattern.

The `var x` pattern can equally be thought of as inference for a total 
type pattern, or an "any" pattern; they are semantically equivalent.

If you're asking "but why can I not mix them in a lambda", the answer 
is: because we don't support partial inference in lambdas at this time.  
We could, and we might someday.  (If you're suggesting that the 
consistency between the two superficially-related forms is the most 
important consideration here, I would disagree.)

>> Absolutely not! Note this does mean that you can write confusing code:
>> record Point(int x, int y) { }
>> if (o instanceof Point(var y, var x)) {
>> … // y refers to x component, y refers to x component
>> }
>> (I’ll get my coat…)
> yes, that's why i ask, it leads to very confusing code, some languages don't allow this kind of code,
> by example destructuring in JavaScript as a special syntax if you want to use different names.

Preventing this kind of confusing code (which is just a special case of 
"picking stupid names") is much more of a job for IDEs than language 

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