Thanks everyone for the discussion. I have updated the JEP: http://openjdk.java.net/jeps/8213076
It proposes *guarded patterns* of the form `p&&e`, as well as parenthesized patterns, written `(p)`. I have left out AND and OR patterns, at least for now. Thanks to Guy we now know how to add them elegantly to the grammar when the time comes :-) When people come to play with this, I’d be especially interested to hear about the need for AND and OR patterns. (I have a feeling that OR is more important, but that’s another email...) Comments on the JEP very welcome. Thanks, Gavin > On 11 Mar 2021, at 13:51, Gavin Bierman <gavin.bier...@oracle.com> wrote: > > Very clever, John. Yes, let’s not add any more puzzlers to the language! > >> On 10 Mar 2021, at 21:59, John Rose <john.r.r...@oracle.com> wrote: >> >> Good. PrimaryPattern is a good concept. >> >> On Mar 10, 2021, at 6:47 AM, Gavin Bierman <gavin.bier...@oracle.com> wrote: >>> >>> Guard: >>> : AssignmentExpression >> >> I think it should be this instead: >> >> Guard: >> : ConditionalAndExpression >> >> The effects of this narrower definition of a guard expression >> are two: >> >> First, any guard of the form (a || b), (a ? b : c), or (a = b) >> will require parentheses. >> >> Second, as a result, the following puzzlers will not be legal >> inputs: >> >> case P && a || b: // compare x instanceof P && a || b >> case P && a ? b : c: // compare x instanceof P && a ? b : c >> case P && a = b: // compare x instanceof P && a = b >> >> In all of these puzzlers, the “extremely fortuitous” >> correspondence between the syntaxes of guarded >> cases and instanceof’s with following boolean logic >> are broken. >> >> The fix to align the meanings of the cases with the >> instanceof’s is to add parentheses: >> >> case P && (a || b): // compare x instanceof P && (a || b) >> case P && (a ? b : c): // compare x instanceof P && (a ? b : c) >> case P && (a = b): // compare x instanceof P && (a = b) >> >> Using the modified grammar rule above forces the >> coder to write the parentheses, I think. >> >> I think we should aim for “perfectly fortuitous” >> here, not just “well, look how often the syntaxes >> seem to mean the same thing although sometimes >> they don’t”. Indeed, this is my main reason for >> plumping for P && g in the first place. >> >> — John >> >> >
