Hyphenated keywords and switch expressions

Alex Buckley alex.buckley at oracle.com
Thu Jan 17 19:39:41 UTC 2019

Thanks Gavin. The "Jan 2019" edition looks good.

The relative shapes of switch statements and switch expressions can be 
easily discerned by reading [1] and [2] side by side.

The renumbering, which fits with my plans for the JLS, is also welcome 
in advance of the public commentary that we can expect on this spec come 
JDK 12 GA.




On 1/17/2019 1:14 AM, Gavin Bierman wrote:
> Thank you Alex and Tagir. I have uploaded a new version of the spec
> at:
> http://cr.openjdk.java.net/~gbierman/switch-expressions.html
> This contains all the changes you suggested below. In addition, there
> is a small bug fix in 5.6.3 concerning widening
> (https://bugs.openjdk.java.net/browse/JDK-8213180). I have also taken
> the opportunity to reorder chapter 15 slightly, so switch expressions
> are now section 15.28 and constant expressions are now section 15.29
> (the last section in the chapter).
> Comments welcome! Gavin
>> On 14 Jan 2019, at 21:40, Alex Buckley <alex.buckley at oracle.com>
>> wrote:
>> 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
>> 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

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