Dmitry Soshnikov wrote:
On Tue, Sep 11, 2012 at 1:33 AM, Brendan Eich <[email protected]
<mailto:[email protected]>> wrote:
If you really want the Pythonic default parameter rules
(evaluation in definition context) then you need to address the
bad case Jason showed:
function foo(arg = []) {
arg.push('strange');
return arg;
}
foo(); // ['strange']
foo(); // ['strange', 'strange']
This is a side channel and a big footgun. ES6 default parameters
as proposed avoid it.
Sure, and as I mentioned in my first message -- ES may eval defaults
every time at activation, not only once as Python does. But -- still
in the outer scope (in the foo.[[Scope]] in your example).
No, you've solved the literal problem but not the problem of supporting
later parameters' values depending on earlier parameters.
The scope has to be unique to the activation at hand.
function foo(arg = <expression>) { ... }
evaluates the <expression> every time at activation as:
1. If expression is a literal value, create a *new* value
corresponding to literal. Assign to the parameter. Return.
Sorry, special casing like this does not work, the expression could
contain mutable (object or array) literals but not be a single literal.
2 Else, eval the <expression> in foo.[[Scope]]
The first step covers and fixes Python's issue. The second allows not
going to the activation frame and avoid complexities I noted.
That's said, we should stick either with outer, or with the inner
scope for evaluation. The outer seems more logical. Not with both,
otherwise it will be too complicated and error prone use cases (which
as I note will anyway be banned as a bad practice, so to support it?).
You say it's too error prone but you don't adduce any evidence.
Meanwhile we have lots of extant code that does things like
function foo(bar, baz) {
baz = baz || default_baz;
...
}
That's the cowpath we are paving.
/be
_______________________________________________
es-discuss mailing list
[email protected]
https://mail.mozilla.org/listinfo/es-discuss
