destructuring: as patterns?

Allen Wirfs-Brock allen at
Fri Apr 20 14:38:35 PDT 2012

On Apr 20, 2012, at 2:22 PM, Brendan Eich wrote:

> Andreas Rossberg wrote:
>> Honestly, my
>> feeling is that strictly sticking to that principle will overly
>> constrain pattern syntax anyway, if not for 'as' then for the next
>> nice feature. I dimly remember that we had it coming up before.
> Dave and I were talking about making fail-soft or "irrefutable match" opt-in:
> js> let {x} = {};
> js> x
> (SpiderMonkey shell.)
> IOW, destructuring has been conceived of as pretty thin sugar for getting a property, and if no such property, you get undefined. Of course, this makes a deeper pattern fail hard:

But also remember that we allow default value expressions in binding patterns (but not destructuring assignments, where they would be ambiguous):

let (x="default"} =  {};
print(x);  //"default"

> js> let {y:{z}} = {};
> typein:4: TypeError: (void 0) is undefined

   let {y:{z} = {z:"default"}} = {};
   let {y:{z = "default"}} = {};

> (Atrocious SpiderMonkey failure to pretty-print the blamed expression instead of its portable (void 0) value there -- my fault I think.)
> Dave suggested making the first case, let {x} = {}, throw, and requiring ? as a pattern modifier (I suggested prefix):

Then shouldn't
   let x;
be illegal?  Would you have to say:
   let ?x;

> let {?x} = {}; // x is undefined, no throw
> let {y} = {};  // throws

so, why not:

  let {x=undefined} = {};

> So there's another place the pattern language wants to diverge from object literal notation.
>> One alternative of course would be to restrict what can occur in an
>> assignment pattern, compared to bindings. The two are quite different
>> semantically, so it could perhaps be argued. But it's not the most
>> pleasant idea either.
> Turns out Allen has already done this in ES6 drafts. 11.13.1 is for destructuring assignment. 12.2.4 is destructuring binding patterns. So we can diverge patterns further.
> /be
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