Call for bikeshed -- break replacement in expression switch

Alex Buckley alex.buckley at
Thu May 16 21:43:29 UTC 2019

On 5/16/2019 2:05 PM, Maurizio Cimadamore wrote:
> There are other contexts in which we limit what can be done w/r/t/
> parenthesized expressions (since these are ambiguous with cast to
> generic types). So this looks like another case where the grammar has to
> say - sorry no parens here.

If you're proposing to disallow a cast expression or a parenthesized 
expression after a `yield` token, then I think that's not right. The 
parsing of a `(` token has triggered potentially unbounded lookahead for 
some time [1][2], and everything worked out, so I don't see why the 
language should disallow any of John's examples:

yield (String)("answer is "+x);
yield ("answer is "+x).trim();
yield new String[]{ "answer is "+x }[0];
yield Arrays.asList("answer is "+x).get(0);
yield false ? 0 : ("answer is "+x).trim();


[1] See slides 9-11 from

[2] JLS 15.27 on the choice of `(...)` for lambda parameters :

The syntax has some parsing challenges. The Java programming language 
has always required arbitrary lookahead to distinguish between types and 
expressions after a '(' token: what follows may be a cast or a 
parenthesized expression. This was made worse when generics reused the 
binary operators '<' and '>' in types. Lambda expressions introduce a 
new possibility: the tokens following '(' may describe a type, an 
expression, or a lambda parameter list. Some tokens immediately indicate 
a parameter list (annotations, final); in other cases there are certain 
patterns that must be interpreted as parameter lists (two names in a 
row, a ',' not nested inside of '<' and '>'); and sometimes, the 
decision cannot be made until a '->' is encountered after a ')'. The 
simplest way to think of how this might be efficiently parsed is with a 
state machine: each state represents a subset of possible 
interpretations (type, expression, or parameters), and when the machine 
transitions to a state in which the set is a singleton, the parser knows 
which case it is. This does not map very elegantly to a fixed-lookahead 
grammar, however.

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