Guards redux

Brian Goetz brian.goetz at
Wed Mar 10 16:40:33 UTC 2021

> Using & between patterns is unfortunate given that if if first pattern is not total and does not match, we will not "execute" the second pattern.
> So the semantics is lazy but currently '&' means non-lazy on expressions.

While I understand the discomfort that drives you to make this 
observation, I think we should be a little more careful with our terms; 
lazy is not the right word here.  (When used this way, *ALL* patterns 
are lazy!  A pattern is not an expression; it is not "evaluated" like 
expressions are.  It is a _recipe_ for a deferred computation (like a 

When we say `x instanceof (P&Q)`, we "construct" the composite pattern 
P&Q _before we "execute" it_.  What are the semantics of such a combined 
pattern?  It is short-circuiting; when we later compute whether a target 
matches the pattern, we first try to match it to the first subpattern, 
and if that succeeds, we try to match it to the second subpattern.  This 
is not "lazy evaluation", this is just the semantics of pattern 
composition with AND. Short-circuiting is a more accurate description 
here than laziness.

As an analogy, imagine we had intersection types in instanceof (we 
already do for casts, so this is not much of a leap).  If we asked `x 
instanceof T&U`, and x was not an instanceof T, would we insist that we 
also test against U (which could throw if the JVM can't find U.class to 
load!)  Or would we accept that a test against the intersection type T&U 
can reasonably "short circuit" the test if it fails the first one?  Of 
course we would.  Why is it different for patterns, which are a 
generalization of this kind of test?

Continuing on the "like a lambda" analogy, if pattern refers to 
variables in the current scope as one of its "inputs", this is like a 
capturing lambda.  And, like a lambda, when we were looking at 
`true(e)`, we said that any locals in `e` should be effectively final.

So to propagate that constraint into the current model: in a guarded 
pattern P&&g, all locals referred to in `g` should be effectively 
final.  (We've already said this of pattern input parameters in general.)

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