On Jan 7, 2014, at 8:22 AM, Andreas Rossberg wrote:
> On 7 January 2014 17:19, Andreas Rossberg <[email protected]> wrote:
>> Sorry, my wording may have been ambiguous. What I meant was
>> disallowing break/continue/return inside a 'do', not giving up 'do'.
>> ;)
>
> And just to be extra-clear: by that I'm only referring to "free"
> occurrences of those, that would refer to the enclosing statement.
> Nested ones are fine, of course.
>
Unless we can identify real implementation issues, the semantics of
do { }
should simply be those of a blocks. JS programmer shouldn't have to learn
which subset of statement are invalid in a do expression block. In particular,
I see no reason why a JS programmer should be able to refactor any valid
BlockStatement in to an equivalent ExpressionStatement simply by putting a 'do'
in front of the leading '{'
The meaning of things like:
function (x) {
for (let i of x.state!==special? x : do {return bar(x)})
foo(i)
}
is clear and also easy enough to specify. Unless there are some non-obvious
implementation issues, I don't see why we would want to disallow such things.
The only place where the possible semantics isn't totally obvious is things
like:
for (x of z ? q : do {break}) ...
or
for (x of do { if (z) q; else continue}) ...
The semantics in the ES6 draft for an unlabeled break or continue completion in
the head of a for statement threats both of these as terminating the for
statement and continuing with the statement following the for.
Allen
> /Andreas
>
>
>> On 7 January 2014 17:10, Brendan Eich <[email protected]> wrote:
>>> Andreas Rossberg wrote:
>>>>
>>>> - YAGNI -- I have a hard time coming up with a use case that isn't
>>>> obfuscated code (even considering generated code).
>>>
>>>
>>> Always a good reason in the abstract, but concrete use cases have arisen,
>>> even in this thread. As you noted just last month (!),
>>>
>>>
>>> """
>>>
>>> For ES7 I would like to revive the do-expression proposal (hopefully
>>> at the next meeting), so that one can at least approximate the above
>>> with
>>>
>>> const ES6_PROXY = do { try { new Proxy({}, {}); true } catch (_) { false }
>>> };
>>>
>>> Of course, semantically the function is equivalent, and a fine
>>> solution, if a bit verbose.
>>>
>>>
>>> """
>>>
>>>> - They complicate the semantics and implementation -- for example, you
>>>> would have to roll back non-empty expression stacks (in a stack
>>>> machine implementation).
>>>
>>>
>>> This is minor in both actual effect (not many naive recursive expression
>>> parse-tree walkers) and implementation hardship (return completion types all
>>> over, respect abrupt ones in expression handlers).
>>>
>>>
>>>> - They destroy nice equivalences -- in particular, I'd like "do {...}"
>>>> to be equivalent to "(() => {...})()", e.g. to minimise refactoring
>>>> hazards.
>>>
>>>
>>> What changed your mind from 20-December?
>>>
>>> Anyway, JS has statements and expressions, but functions create new
>>> activations with their own scopes. Those create hazards when refactoring
>>> between statements and expressions.
>>>
>>> Wanting the equivalence you state here tries to deny the facts of JS and its
>>> full (ahem, perhaps disputed legitimacy) heritage.
>>>
>>>
>>>> - We can always allow them later, if the need should ever arise.
>>>
>>>
>>> ES7 is later.
>>>
>>> /be
>
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