fail-fast object destructuring

Andreas Rossberg rossberg at
Mon Jul 9 09:36:43 PDT 2012

I agree with almost everything you said. But just to clarify, I think you
actually meant "refutable" everywhere you said "irrefutable".


On 9 July 2012 18:19, Russell Leggett <russell.leggett at> wrote:

> On Fri, Jul 6, 2012 at 1:37 PM, Brendan Eich <brendan at> wrote:
>> Andreas Rossberg wrote:
>>>     We should talk more at this month's TC39 meeting, but I see a
>>>     sharp divide ahead:
>>>     1. Choose (A), possibly with modification, either rejecting useful
>>>     pattern matching decisively, or else making its pattern language
>>>     differ by making prefix-! implicit. But you rejected splitting
>>>     pattern meaning across destructuring and future pattern-matching,
>>>     in the last sentence cited above.
>>>     XOR
>>>     2. Choose (B) because future-proofing for pattern matching wants
>>>     prefix-?, and impose a compatibility break from what we and others
>>>     implemented, and what most JS users might expect based on JS's
>>>     carefree imputing of undefined.
>>> Agreed.
>>>     Comments from es-discuss peanut gallery welcome.
>>>     I could go for 1/A, except for two facts. First, I believe there
>>>     is a non-trivial body of Firefox JS that depends on imputing
>>>     undefined. Second, and what is more important: we haven't
>>>     usability-tested 2/B at all.
>>> Did you mean "could go for 2/B" there?
>> Evidently!
>> I rearranged and renumbered but missed this one. Clearly I meant 2/B. I'm
>> a bit unhappy about making such a change so long after implementing 1/A
>> (without prefix-!). But never mind me -- what do others think?
> I would love to see 2/B. A lot of that is likely because I would love to
> see pattern matching, though, I'll admit. I think that the ? on refutable
> portions of the pattern would work well with the new default and
> existential operators, while I think ! will not have a good analog. If I
> were to throw out another reason based on destructuring assignment alone,
> and not pattern matching later, I would say that if parts of the
> destructuring are likely to fail, it would be better to know upfront. If
> there are optional fields, ? clearly expresses that. If there are wildly
> unknown structures, destructured assignment is probably not the best
> approach, or at least there should be some structural testing upfront
> (pattern matching would of course be ideal).
> As for usability testing, while it obviously is not done irrefutably in
> spidermonkey now, I think we can at least look to other languages. Looking
> over several languages, all of the ones I have seen are irrefutable. Some
> are strongly typed languages, but not all. Python does not have complete
> patterns, it can basically just destructure list, but even it requires that
> the number of elements be correct. Erlang, OCaml, and F# all allow full
> pattern assignments, and they will all throw errors if not correctly
> matched. Obviously, it is a little different than what it would be in ES,
> they have full on patterns not just destructuring, but it would clearly be
> all the more consistent when patterns *hopefully* get added. I know that JS
> users have come to expect a certain amount of carefree imputing, but
> destructuring is new (outside of FF), and I think it is certainly an area
> which has options. I haven't exactly done a formal survey of languages, but
> from what I've seen JS would be the first to be so lenient. Allowing ? in
> portions of a pattern, I actually think would be the perfect compromise.
> People who write very loose code will either have to put ?s on their
> patterns or keep working the same way they have been, but if there isn't a
> reasonable amount of checking by default, I think we're going to be seeing
> some more WATs in presentations. JS gets picked on for how much coercion
> and silent errors it will chug along with. Best practices of always using
> === and avoiding 'with' are just a couple of examples.
> Finally, I would like to take the approach of looking at the best way to
> handle the different use cases.
>    1. Really simple destructuring with expected results:
>        let {x,y} = getPoint();
>    In my opinion, this should throw if x or y are not available. It is
>    unlikely that the coder is checking properties here, they are just going to
>    call other functions passing through x or y. It would probably be better to
>    get the error as soon as possible. Much easier to debug that way. In many
>    cases, I think these types of destructuring assignments will lead to
>    effortless checking that would previously have been missed because of the
>    extra effort to add those checks in as if statements.
>    2. Options type objects:
>        let {url,callback,?errback} = options;
>    This lets you very clearly decompose an object and declaratively show
>    which values are optional and which ones are not. If all of them are
>    optional, the ? would just go to the left of the brace, but really this
>    should be the minority case for most situations.
>    3. A disparate list of possible types of matches. This is common in
>    "overloaded" functions where the number and type of arguments can be wildly
>    different. This is probably the strongest case against irrefutable
>    matching, because it is so likely to result in mismatches which should have
>    logic performed on them instead of having an error thrown. However, it is
>    also the strongest case for pattern matching. There is nothing wrong with
>    wanting to accept differently shaped things, but right now there is not
>    really a great facility for acting on them. Pattern matching would be
>    ideal. I think it would be such a powerful addition to the language as it
>    is used right now, that it honestly pains me that we would cut out the
>    possibility. I would love to see patterns now, but I can understand
>    waiting. Making a decision which would close it off though would be
>    extremely disappointing.
> I know that full blown patterns are out of scope (though I'm hoping maybe
> this discussion might change that), but even if we can't have it now, I
> think irrefutable destructuring would really make people see the potential.
> Even if no facility was provided for it, a hypothetical match function
> could be created which would take function and execute them in sequence in
> a try catch until one did not fail. I think we can do better than that and
> should probably try if we go with irrefutable destructuring, but just
> knowing it would be possible for the community to take and run with is nice.
> - Russ
>> /be
>> ______________________________**_________________
>> es-discuss mailing list
>> es-discuss at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the es-discuss mailing list