Globalization API discussion

[Allen Wirfs-Brock]

On Nov 20, 2011, at 8:30 PM, Brendan Eich wrote:

> On Nov 20, 2011, at 1:18 PM, David Herman wrote:
> 
>>> I would not add more implicit magic to JS. E4X had junk like this in it, which only ever concealed bugs.
>> 
>> I'm of two minds about this. In the abstract, I agree with Brendan; fail-soft conceals bugs. But in reality, our destructuring logic is incredible fail-soft. Hardly anything in destructuring is treated as an error. And the syntax really *wants* to match the common pattern. So I'm torn.
> 
> 1. Failing to write that means a destructuring parameter with default values within the pattern cannot be observed via arguments[i] as undefined (or null?). If missing, the undefined will be replaced by a fresh object. This isn't consistent with any other combination of destructuring parameters and parameter default values.
> 
> 2. If a function wants to throw an error for a missing options parameter, it has no way of telling other than arguments.length checking.

function f({a,b} = (optionsMissing=true,{})) {
    var optionsMissing;
    if (optionsMissing) throw "missing options parameter";
    ...

or using Dave's do operator:

function f({a,b} = do{ throw "missing options parameter",({})}) {

> 
> 3. It's not hard to write " = {}".

I agree.  I'm a little uncomfortable with exceptions originating from formal parameter initialization, but as soon as we allowed for default value expressions that became a possibility.  Overall, I suspect that implicitly providing a "={}" is just going to mask logic errors.

Allen