Hyphenated keywords and switch expressions

Alex Buckley alex.buckley at oracle.com
Sat Jan 12 00:42:47 UTC 2019

Hi Tagir,

On 1/11/2019 5:32 AM, Tagir Valeev wrote:
> On the other hand, from IDE developer point of view, having expression
> and statement with so similar syntax definitely adds a confusion to
> the parsing (and probably to users). E.g. suppose we want to parse a
> fragment which consists of a number of statements, isolated from other
> code:
> switch(0) { default -> throw new Exception(); };
> In normal context it's two statements: switch-statement followed by an
> empty statement. However inside switch expression rule it's one
> statement: an expression statement containing a switch expression:
> int x = switch(0) { default -> switch(0) { default -> throw new
> Exception(); }; };
> Normally if we take a textual representation of single statement, it
> could be parsed back into the same single statement, when isolated
> from other code (the same works for expressions). Here this rule is
> violated: the expression statement taken from switch expression rule
> could be reparsed in isolation as two statements.

I'm concerned about any claim of ambiguity in the grammar, though I'm 
not sure I'm following you correctly. I agree that your first fragment 
is parsed as two statements -- a switch statement and an empty statement 
-- but I don't know what you mean about "inside switch expression rule" 
for your second fragment. A switch expression is not an expression 
statement (JLS 14.8). In your second fragment, the leftmost default 
label is followed not by a block or a throw statement but by an 
expression (`switch (0) {...}`, a unary expression) and a semicolon. 
Yes, the phrase `switch (0) {...}` is parsed as a switch statement in 
one context and as a unary expression in another context. Is that the 
ambiguity you wished to highlight?


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