Extensible destructuring proposal

Andreas Rossberg rossberg at google.com
Tue Aug 4 12:26:45 UTC 2015

On 31 July 2015 at 20:09, Samuel Hapák <samuel.hapak at vacuumapps.com> wrote:

> So, do you still have objections against this proposal? Could we summarize
> them?
> @Andreas, do you still think that there is category error involved?

If you want to overload existing object pattern syntax, then yes,
definitely. I strongly disagree with that. It breaks regularity and
substitutability (you cannot pass a map to where a generic object is

As for a more Scala-like variant with distinguished syntax, I'm fine with
that semantically. But I still don't buy the specific motivation with maps.
Can you give a practical example where you'd want to create a map
(immutable or not) for something that is just a record of statically known
shape? And explain _why_ you need to do that? Surely it can't be
immutability, since objects can be frozen, too.

To be clear, I wouldn't reject such a feature outright. In fact, in a
language with abstract data types, "views", as they are sometimes called in
literature, can be valuable. But they are also notorious for a high
complexity cost vs minor convenience they typically provide. In particular,
it is no longer to write

  let {a: x, b: y} = unMap(map)

as you can today, then it would be to move the transformation to the

  let Map({a: x, b: y}) = map

So I encourage you to come up with more convincing examples in the
JavaScript context. Short of that, I'd argue that the complexity is not
justified, at least for the time being.

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