Hi Gavin,
Some points driven partly by the discussion with Tagir:
1. In 14.11.1, SwitchLabeledBlock should not end with a `;` -- there is
no indication in JEP 325 that a semicolon is desired after `-> {...}`
and javac in JDK 12 does not accept one there. Also,
SwitchLabeledThrowStatement should not end with a `;` because
ThrowStatement includes a `;`.
2. In 14.11.1, "This block can either be empty, or take one of two
forms:" is wrong for switch expressions. The emptiness allowed by the
grammar will be banned semantically in 15.29.1, so 14.11.1 should avoid
trouble by speaking broadly of the forms in an educational tone: "A
switch block can consist of either: - _Switch labeled rules_, which use
`->` to introduce either a _switch labeled expression_, ..." Also,
"optionally followed by switch labels." is wrong for switch expressions,
so prefer: "- _Switch labeled statement groups_, which use `:` to
introduce block statements."
3. In 15.29.1: (this is mainly driven by eyeballing against 14.11.2)
- Incorrect Markdown in section header.
- The error clause in the following bullet is redundant because the list
header already called for an error: "The switch block must be compatible
with the type of the selector expression, *****or a compile-time error
occurs*****."
- I would prefer to pull the choice of {default label, enum typed
selector expression} into a fourth bullet of the prior list, to align
how 14.11.2's list has a bullet concerning default label.
- The significant rule from 14.11.2 that "If the switch block consists
of switch labeled rules, then any switch labeled expression must be a
statement expression (14.8)." has no parallel in 15.29.1. Instead, for
switch labeled rules, 15.29.1 has a rule for switch labeled blocks. (1)
We haven't seen switch labeled blocks for ages, so a cross-ref to
14.11.1 is due. (2) A note that switch exprs allow `-> ANY_EXPRESSION`
while switch statements allow `-> NOT_ANY_EXPRESSION` is due in both
sections; grep ch.8 for "In this respect" to see what I mean. (3) The
semantic constraints on switch labeled rules+statement groups in 15.29.1
should be easily contrastable with those in 14.11.2 -- one approach is
to pull the following constraints into 15.29.1's "all conditions true,
or error" list:
-----
- If the switch block consists of switch labeled rules, then any switch
labeled block (14.11.1) MUST COMPLETE ABRUPTLY.
- If the switch block consists of switch labeled statement groups, then
the last statement in the switch block MUST COMPLETE ABRUPTLY, and the
switch block MUST NOT HAVE ANY SWITCH LABELS AFTER THE LAST SWITCH
LABELED STATEMENT GROUP.
-----
If you prefer to keep these semantic constraints standalone so that they
have negative polarity, then 14.11.2 should to the same for its
significant-but-easily-missed "must be a statement expression" constraint.
Alex
On 1/13/2019 2:53 AM, Tagir Valeev wrote:
Hello!
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.
Ah, ok, we moved away slightly from the spec draft [1]. I was not
aware, because I haven't wrote parser by myself. The draft says:
SwitchLabeledRule:
SwitchLabeledExpression
SwitchLabeledBlock
SwitchLabeledThrowStatement
SwitchLabeledExpression:
SwitchLabel -> Expression ;
SwitchLabeledBlock:
SwitchLabel -> Block ;
SwitchLabeledThrowStatement:
SwitchLabel -> ThrowStatement ;
(by the way I think that ; after block and throw should not be
present: current implementation does not require it after the block
and throw statement already includes a ; inside it).
Instead we implement it like:
SwitchLabeledRule:
SwitchLabel -> SwitchLabeledRuleStatement
SwitchLabeledRuleStatement:
ExpressionStatement
Block
ThrowStatement
So we assume that the right part of SwitchLabeledRule is always a
statement and reused ExpressionStatement to express Expression plus
semicolon, because syntactically it looks the same. Strictly following
a spec draft here looks even more ugly, because it requires more
object types in our code model and reduces the flexibility when we
need to perform code transformation. E.g. if we want to wrap
expression into block, currently we just need to replace an
ExpressionStatement with a Block not touching a SwitchLabel at all.
Had we mirrored the spec in our code model, we would need to replace
SwitchLabeledExpression with SwitchLabeledBlock which looks more
annoying.
With best regards,
Tagir Valeev
[1] http://cr.openjdk.java.net/~gbierman/switch-expressions.html#jep325-14.11
