[External] : Re: Primitive type patterns

Brian Goetz brian.goetz at oracle.com
Fri Apr 8 02:54:22 UTC 2022

>     And, why would we not want duality with:
>         record R(short s) { }
>         ...
>         new R(x) 
> because new R(x) is alone while case R(...) is part of a larger set of 
> patterns/sub-pattern of the pattern matching, if for each 
> pattern/sub-pattern, we need a double-entry table to understand the 
> semantics, we are well past our complexity budget.

Again: it's the *same table* as all the other conversion contexts. To do 
something different here is what would be incremental complexity.

Let me try this one more time, from a third perspective.

What does `instanceof` do?  The only reasonable thing you would do after 
an `instanceof` is a cast.  Asking `instanceof` is asking, if I were to 
cast to this type, would I like the outcome?  Instanceof currently works 
only on reference types, and says no when (a) the cast would fail with 
CCE, and (b) if the operand is null, because, even though the cast would 
succeed, the next operation likely would not.

Instanceof may be defined only on references now, but casting primitives 
is excruciatingly defined with the same full double-entry table that you 
don't like.

So, when asking "what does a primitive type pattern mean", then, we can 
view this as the generalization of the same question: if I were to cast 
this target to this type, would I like the outcome?  The set of things 
that can go wrong (outcomes we wouldn't like) is slightly broader here; 
we can cast a long to int, and we'd get truncation, but that's a bad 
outcome, just like CCE.  If we interpret `instanceof` as the 
precondition test for a useful cast, we again get the same set of 

  - I can safely cast a non-null Integer to int (unboxing with null check)
  - I can safely cast any int to Integer (boxing)
  - I can safely cast an int in the range of -128..127 to byte 
(narrowing with value range check)
  - I can safely cast any byte to int (widening)
  - etc.

It's a third different explanation, and yet it still comes up with 
*exactly the same set of conversions* as the other two explanations 
(assignment and method invocation).  There's a reason all roads lead 
here; because the conversion sets are defined in a consistent manner.  
Doing something *different* with pattern matching would be the new 

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