Precedence of yield operator

Mark S. Miller erights at google.com
Sat Jun 15 19:46:33 PDT 2013


I hadn't thought of the "urgent" implication -- interesting. I agree that
this has the contrary connotations which is unfortunate.

E and AmbientTalk use "<-" which I always thought read well. In E, the
"then" concept is named "when" and is also supported as a syntactic form
using the "-> syntax. These work well together, as both indicate
asynchrony, with the direction of the arrow indicating what is eventually
being delivered to what.

In JS, "a <- b" already parses as "a < -b" so introducing a "<-" token is a
non-starter. Process calculi languages like Pict (based on Pi Calculus),
and languages inspired by process calculi notions like Erlang and Scala use
an infix "!" for message sending, with the receiver on the left and the
payload on the right. This started with Tony Hoare's CSP (Communicating
Sequential Processes) and was picked up by Occam, where it was synchronous.
Technically it is synchronous in the Pi Calculus too. But in Pict, Erlang,
and Scala it is asynchronous. (Pict is based on the asynchronous subset of
the Pi Calculus.)

More importantly for our audience, for JS (and similarly for Scala),
there's also a nice mnemonic rule: Wherever you can write an infix "." you
can write an infix "!" instead, changing the meaning of the construct from
synchronous to asynchronous. The "!" symbol has a "." at the bottom of it,
so you can think of it as "an enhanced dot" or even "a dot with a trail" if
you'd like.

In any case, once the eye gets used to it the need for these mnemonics
rapidly drops away. Having lived with both "<-" and "!", I've come to
prefer the "!". I'm not really sure why.




On Sat, Jun 15, 2013 at 7:23 PM, Rick Waldron <waldron.rick at gmail.com>wrote:

>
>
> On Saturday, June 15, 2013, Mark S. Miller wrote:
>
>> If we do provide such special syntax, then I suggest (a variant on some
>> else's suggestion -- I forget who) that "yield" is to "function* as "await"
>> is to "function!", amplifying the association of "!" with asynchrony.
>> Rather than say "async function" or whatever, we'd just say "function!".
>> Since "function!" would be new in ES7, there's no reason to reserve
>> anything in ES6 to prepare for this.
>>
>
> Mark, is there a particular rationale for the reuse of "!" as the eventual
> send operator and here for similar semantics? I recall this also appeared
> in you Distributed Electronic Rights paper. Putting its strong existing
> meaning aside (negation), when I see "!" I think of "urgent" things—quite
> the opposite of the behavior described in the paper and above.
>
> Rick
>
>
>
>
>>
>>
>>
>>
>> On Sat, Jun 15, 2013 at 2:40 AM, Bruno Jouhier <bjouhier at gmail.com>wrote:
>>
>> If this is on the agenda for ES7 maybe ES6 should at least reserve
>> "await" as a keyword in function* scopes.
>>
>>
>> 2013/6/15 François REMY <francois.remy.dev at outlook.com>
>>
>> I'm maybe biased, but I would love to consider "yield" as a function.
>> Indeed, it calls another code, which can return you a value. That looks
>> very similar to a function call to me. If we do this, the predecence
>> becomes intuitive again:
>>
>>    var x = yield(a) + yield(b);
>>    yield(a+b);
>>
>> I think there was a proposal to allow parenthesis-free function call at
>> some point at the root of a statement. When it lands, you'll be able to do
>> things like
>>
>>    yield a+b;
>>
>> as a normal statement again. In the mean time we can just use
>> parentheses, that's not a huge issue and it helps clarity.
>>
>>
>>
>> But maybe this is too late to change ES6, it's just an idea I had while
>> reading this thread, not a strong call for change.
>>
>>
>>
>>
>>
>>
>> -----Message d'origine----- From: Brendan Eich
>> Sent: Saturday, June 15, 2013 6:17 AM
>> To: Dean Tribble
>> Cc: Bruno Jouhier ; es-discuss at mozilla.org
>> Subject: Re: Precedence of yield operator
>>
>>
>> This is all on the agenda for ES7. It cleanly missed ES6 in May 2011(!).
>>
>> https://mail.mozilla.org/**pipermail/es-discuss/2011-May/**014748.html<https://mail.mozilla.org/pipermail/es-discuss/2011-May/014748.html>
>>
>> /be
>>
>> Dean Tribble wrote:
>>
>> This is a familiar discussion from C#. I forwarded it to the mediator of
>> that convresation and got a nice summary, pasted here:
>>
>> ---------- Forwarded message ----------
>> From: *Mads Torgersen* <Mads.Torgersen at microsoft.com <mailto:
>> Mads.Torgersen@**microsoft.com>>
>> Date: Fri, Jun 14, 2013 at 2:11 PM
>> Subject: RE: Precedence of yield operator
>> To: Dean Tribble <tribble at e-dean.com <mailto:tribble at e-dean.com>>
>>
>>
>> I’m not on the mailing list. Feel free to forward to it.
>>
>> In C# we have separate keywords too, and indeed the precedence differs as
>> described below. For “yield return” (our yield) the lower precendence falls
>> out naturally since it engenders a statement, not an expression.
>>
>> “await” is not a reserved keyword in C# either, but we managed to wedge
>> it in all the same. Just adding await as an operator would lead to all
>> kinds of ambiguity; e.g. “await (x)” could be a function call or an await
>> expression, and the statement “await x;” could be a variable declaration or
>> an await statement.
>>
>> However, in C# “await” is only allowed inside methods marked “async”, and
>> since there weren’t any of those around before the feature was introduced,
>> it is not a breaking change. Inside non-async methods, therefore, “await”
>> continues to be just an identifier.
>>
>> I don’t know if a similar thing is possible in EcmaScript. But I believe
>> that a low-precedence yield as a substitute for a high-precedence await is
>> problematic: you never want “yield a + yield b” to mean “yield (a + (yield
>> b))”: the things you await – Task, Promises, Futures, whatever you call
>> them – just don’t have operators defined on them, and it would be silly to
>> parse them as if they might and then give errors (at runtime in EcmaScript,
>> at compile time in e.g. TypeScript).
>>
>> Mads
>>
>>
>> On Fri, Jun 14, 2013 at 11:07 AM, Brendan Eich <brendan at mozilla.com<mailto:
>> brendan at mozilla.com>> wrote:
>>
>>     Bruno Jouhier wrote:
>>
>>         While playing with my little async/await library, I noticed
>>
>> --
>>     Cheers,
>>     --MarkM
>>
>
>



-- 
    Cheers,
    --MarkM
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