Standardizing conditional try/catch

Isiah Meadows isiahmeadows at gmail.com
Mon Mar 27 18:15:47 UTC 2017


Inline


On Sun, Mar 26, 2017 at 10:23 PM, Zach Lym <zachlym at indolering.com> wrote:
>> Pattern matching is to conditionals as async/await is to async tasks - it lifts the logic from fairly imperative, low level form to a high level, declarative form, with only a small loss of low-level control.
>
> Are you arguing that we should have skipped promises and gone straight
> to async/await?

No. My analogy was a bit more meta than that, and I was referring to
the difference between promise chaining and async-await control flow -
async-await abstracts over the details of task scheduling. Similarly,
instead of low-level conditionals, pattern matching just abstracts
over the details of conditional ordering.

>
> FWIW, this in itself is not a trivial change as it requires extra work
> for the debugger.

I get that it'd be difficult for the debugger to handle.

>
>
> Thank you,
> -Zach Lym
>
>
> On Sat, Mar 25, 2017 at 6:29 PM, Isiah Meadows <isiahmeadows at gmail.com> wrote:
>> I want advanced pattern matching, but not something specific to error
>> handling.
>>
>> On Thu, Mar 23, 2017, 06:01 Alexander Jones <alex at weej.com> wrote:
>>>
>>> To be clear I was talking about advanced pattern matching for exception
>>> handling. Y(probably)AGNI?
>>>
>>> On Tue, 21 Mar 2017 at 05:08, Isiah Meadows <isiahmeadows at gmail.com>
>>> wrote:
>>>>
>>>> It's possible to add a "[No LineTerminator here]" constraint when
>>>> necessary, as was done for async functions.
>>>>
>>>> As for pattern matching, if you start paying attention to features of
>>>> newer programming languages, especially those just getting past their
>>>> hype stage (like Kotlin, Rust, and Swift), that YAGNI argument is
>>>> starting to seem harder to accept.
>>>>
>>>> Pattern matching is to conditionals as async/await is to async tasks -
>>>> it lifts the logic from fairly imperative, low level form to a high
>>>> level, declarative form, with only a small loss of low-level control.
>>>> -----
>>>>
>>>> Isiah Meadows
>>>> me at isiahmeadows.com
>>>>
>>>>
>>>> On Mon, Mar 20, 2017 at 8:42 PM, Alexander Jones <alex at weej.com> wrote:
>>>> > Any future matching syntax would clearly support the special cases
>>>> > people
>>>> > want to codify now. It might be that the best possible syntax is lost,
>>>> > but
>>>> > e.g. ASI alone is probably a much bigger cause of syntax showstoppers
>>>> > to be
>>>> > worried about. IMO, let it build up, then we can start thinking about a
>>>> > syntax overhaul another day. The chances are that We Ain't Gonna Need
>>>> > It.
>>>> >
>>>> > On 19 March 2017 at 22:47, kdex <kdex at kdex.de> wrote:
>>>> >>
>>>> >> But then, we might be adding new syntax twice to solve the same
>>>> >> problem.
>>>> >> First specifically, then generally. The latter likely using an
>>>> >> entirely
>>>> >> different syntax, making the former syntax obsolete.
>>>> >> Why not start speccing out some details for an optional typing
>>>> >> proposal
>>>> >> instead, so we can get the ball rolling for true pattern matching?
>>>> >>
>>>> >> On Sunday, March 19, 2017 11:18:25 PM CET Zach Lym wrote:
>>>> >> > >
>>>> >> > > I read that TC39 agreed on adding pattern matching to language in
>>>> >> > > March
>>>> >> > > 2013. 4 years later we don't have even stage 0 proposal - so I
>>>> >> > > would
>>>> >> > > consider it to be a dead end or wishful thinking.
>>>> >> > >
>>>> >> >
>>>> >> > Exactly, this proposal has been kicking around for ~15 years but
>>>> >> > keeps
>>>> >> > getting deferred in favor of "something better."  I would be all for
>>>> >> > a
>>>> >> > special syntax using type hints or targeting easier-to-optimize
>>>> >> > subsets,
>>>> >> > but they can be added later.
>>>> >> >
>>>> >> > This proposal introduces the minimum number of features needed to
>>>> >> > handle
>>>> >> > the dynamic nature of JS.
>>>> >> >
>>>> >> > Thank you,
>>>> >> > -Zach Lym
>>>> >> >
>>>> >> > On Sun, Mar 19, 2017 at 8:23 AM, kdex <kdex at kdex.de> wrote:
>>>> >> >
>>>> >> > > Well, there has been some discussion of potentially adding
>>>> >> > > something
>>>> >> > > like
>>>> >> > > static type hints at some point in the future.
>>>> >> > > Pattern matching is a feature that inevitably requires type
>>>> >> > > information at
>>>> >> > > runtime.
>>>> >> > >
>>>> >> > > So as long as the "optional typing" story isn't dead, I would
>>>> >> > > assume
>>>> >> > > that
>>>> >> > > pattern matching isn't quite dead either, it's just not in the
>>>> >> > > currently
>>>> >> > > possible scope of things.
>>>> >> > > ECMAScript wouldn't be the only language which would have taken
>>>> >> > > years
>>>> >> > > to
>>>> >> > > come around to implementing type hinting: IIRC Python got its type
>>>> >> > > hinting
>>>> >> > > feature pretty late, too.
>>>> >> > >
>>>> >> > > On Sunday, March 19, 2017 4:22:26 PM CET Michał Wadas wrote:
>>>> >> > > > Is there a serious push to add pattern matching to language?
>>>> >> > > > Does
>>>> >> > > > any
>>>> >> > > > popular dynamically typed language have pattern matching?
>>>> >> > > > I read that TC39 agreed on adding pattern matching to language
>>>> >> > > > in
>>>> >> > > > March
>>>> >> > > > 2013. 4 years later we don't have even stage 0 proposal - so I
>>>> >> > > > would
>>>> >> > > > consider it to be a dead end or wishful thinking.
>>>> >> > > >
>>>> >> > > > On Sat, Mar 18, 2017 at 5:16 PM, kdex <kdex at kdex.de> wrote:
>>>> >> > > >
>>>> >> > > > > I'm not sure if embedding this idea into the language will
>>>> >> > > > > make
>>>> >> > > > > future
>>>> >> > > > > ideas about true pattern matching harder to implement or not.
>>>> >> > > > > Destructuring assignments are pretty slow from what I've
>>>> >> > > > > measured,
>>>> >> > > > > and
>>>> >> > > > > they still made it in, so I hardly see performance being a
>>>> >> > > > > showstopper
>>>> >> > > here.
>>>> >> > > > >
>>>> >> > > > > On Saturday, March 18, 2017 12:18:22 PM CET Michael J. Ryan
>>>> >> > > > > wrote:
>>>> >> > > > > > The if condition doesn't need to be limited to instance
>>>> >> > > > > > of...
>>>> >> > > > > >
>>>> >> > > > > > catch (err if !isNaN(err.status))
>>>> >> > > > > >
>>>> >> > > > > > Aside: entering code in a phone is hard...
>>>> >> > > > > >
>>>> >> > > > > > > `instanceof` doesn't work across realms (iframes, for
>>>> >> > > > > > > example). If
>>>> >> > > we
>>>> >> > > > > > > introduced conditional catch blocks, I'd want a more
>>>> >> > > > > > > reliable
>>>> >> > > matching
>>>> >> > > > > > > mechanism than instanceof.
>>>> >> > > > > > >
>>>> >> > > > > > > On Fri, Mar 17, 2017 at 5:01 PM, Zach Lym
>>>> >> > > > > > > <zachlym at indolering.com>
>>>> >> > > > > wrote:
>>>> >> > > > > > >
>>>> >> > > > > > >> Firefox supports the following conditional `catch`
>>>> >> > > > > > >> syntax:
>>>> >> > > > > > >>
>>>> >> > > > > > >>     try {
>>>> >> > > > > > >>         let result = await ();
>>>> >> > > > > > >>     } catch (e if e instanceof ErrorType) {
>>>> >> > > > > > >>         ...
>>>> >> > > > > > >>     }
>>>> >> > > > > > >>
>>>> >> > > > > > >>
>>>> >> > > > > > >> This was originally implemented in Spidermonkey as part
>>>> >> > > > > > >> of an
>>>> >> > > > > > >> ES
>>>> >> > > > > proposal
>>>> >> > > > > > >> around 2000, but it was rejected for unknown reasons [0].
>>>> >> > > > > > >> A
>>>> >> > > > > > >> 2012
>>>> >> > > > > email to
>>>> >> > > > > > >> this list suggesting standardization of the syntax was
>>>> >> > > > > > >> passed
>>>> >> > > over in
>>>> >> > > > > favor
>>>> >> > > > > > >> of waiting for a generic pattern matching facility
>>>> >> > > > > > >> [0][1].
>>>> >> > > > > > >> Later
>>>> >> > > > > > >> discussion suggests that the pattern matching proposal
>>>> >> > > > > > >> would
>>>> >> > > > > > >> have
>>>> >> > > > > been very
>>>> >> > > > > > >> slow [2]. A proposal for a Java-like type-based
>>>> >> > > > > > >> conditional
>>>> >> > > > > > >> was
>>>> >> > > > > proposed in
>>>> >> > > > > > >> 2016, but was criticized for lacking generality [2].
>>>> >> > > > > > >>
>>>> >> > > > > > >> If the above summary is accurate, I would like to try to
>>>> >> > > standardize
>>>> >> > > > > the
>>>> >> > > > > > >> vanilla syntax once again.  It's imperative, general, and
>>>> >> > > > > > >> doesn't
>>>> >> > > > > preclude
>>>> >> > > > > > >> the use of any hypothetical pattern matching
>>>> >> > > > > > >> functionality.
>>>> >> > > > > > >>
>>>> >> > > > > > >> Javascript's control flow has improved dramatically in
>>>> >> > > > > > >> recent
>>>> >> > > years:
>>>> >> > > > > > >> promises got rid of callbacks, `async`/`await` clipped
>>>> >> > > > > > >> promise
>>>> >> > > > > chains, and
>>>> >> > > > > > >> classes make it easy to create custom Error objects that
>>>> >> > > > > > >> preserve
>>>> >> > > > > > >> stacktraces.  Conditional catch is the last bit of syntax
>>>> >> > > > > > >> needed
>>>> >> > > to
>>>> >> > > > > make JS
>>>> >> > > > > > >> look like it was designed to handle asynchronous
>>>> >> > > > > > >> functions.
>>>> >> > > > > > >>
>>>> >> > > > > > >> Thoughts?
>>>> >> > > > > > >>
>>>> >> > > > > > >> -Zach Lym
>>>> >> > > > > > >>
>>>> >> > > > > > >> [0]:
>>>> >> > > > > > >> https://esdiscuss.org/topic/conditional-catch-clause#
>>>> >> > > content-10
>>>> >> > > > > > >> [1]: https://esdiscuss.org/topic/conditional-catch
>>>> >> > > > > > >> [2]:
>>>> >> > > > > > >>
>>>> >> > > > > > >> https://esdiscuss.org/topic/error-type-specific-try-catch-bl
>>>> >> > > > > > >> ocks#content-14
>>>> >> > > > > > >>
>>>> >> > > > > > >> _______________________________________________
>>>> >> > > > > > >> es-discuss mailing list
>>>> >> > > > > > >> es-discuss at mozilla.org
>>>> >> > > > > > >> https://mail.mozilla.org/listinfo/es-discuss
>>>> >> > > > > > >>
>>>> >> > > > > > >>
>>>> >> > > > > > >
>>>> >> > > > > > > _______________________________________________
>>>> >> > > > > > > es-discuss mailing list
>>>> >> > > > > > > es-discuss at mozilla.org
>>>> >> > > > > > > https://mail.mozilla.org/listinfo/es-discuss
>>>> >> > > > > > >
>>>> >> > > > > > >
>>>> >> > > > > >
>>>> >> > > > >
>>>> >> > > > > _______________________________________________
>>>> >> > > > > es-discuss mailing list
>>>> >> > > > > es-discuss at mozilla.org
>>>> >> > > > > https://mail.mozilla.org/listinfo/es-discuss
>>>> >> > > > >
>>>> >> > > > >
>>>> >> > > >
>>>> >> > >
>>>> >> > > _______________________________________________
>>>> >> > > es-discuss mailing list
>>>> >> > > es-discuss at mozilla.org
>>>> >> > > https://mail.mozilla.org/listinfo/es-discuss
>>>> >> > >
>>>> >> > >
>>>> >>
>>>> >> _______________________________________________
>>>> >> es-discuss mailing list
>>>> >> es-discuss at mozilla.org
>>>> >> https://mail.mozilla.org/listinfo/es-discuss
>>>> >>
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > es-discuss mailing list
>>>> > es-discuss at mozilla.org
>>>> > https://mail.mozilla.org/listinfo/es-discuss
>>>> >
>>
>>
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>>

-----

Isiah Meadows
me at isiahmeadows.com


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