arrows and a proposed softCall
brendan at mozilla.com
Tue Jun 5 10:23:38 PDT 2012
Russell Leggett wrote:
> On Tue, Jun 5, 2012 at 12:36 PM, Brendan Eich <brendan at mozilla.com
> <mailto:brendan at mozilla.com>> wrote:
> What I perceive from the JSFixed effort, and from Angus who is
> good enough to post here: people have a particular concern that
> fat-arrow is too sweet and it will lure the children into the
> witch's house: passing fat arrows to dynamic-this APIs.
> We have data suggesting that fat arrows address the dominant
> use-case, thanks to Kevin Smith and others. So fat arrows are in
> ES6, well and good.
> I think the particular concern about => being an attractive
> nuisance for some APIs such as Angus's mixin combinators, which
> rely on .call overriding |this|, can be addressed by adding ->
> too. Angus agrees, but -> is not on the boards for ES6 (yet).
> We could try to revive ->, but first, we should face the
> attractive nuisance argument squarely, instead of dancing around
> it with isBound abuses that try to "catch fat arrow going into the
> witch's house".
> I think that with ->, a similar problem will still crop up -
> specifically that => will be the more common use, and then in the rare
> case that -> is needed, people may still use =>.
I agree. The problem we'll then see is anxiety over "which arrow?" --
the Paradox of Choice (Schwartz).
> Do we still need isBound to catch that error as well? Even with the
> choice of -> or =>, the person writing the code has to know which one
> to use and why. That means they have to understand the whole dynamic
> |this| problem.
Agreed, so (while we are spiraling, no worries) this helps. We cannot
remove the dynamic vs. bound |this| choice from JS. But we can avoid
adding choice when shortening, based on use-case frequency analysis.
This, we have done (many thanks to Kevin Smith again), and it is why =>
got into ES6.
> I think that the dynamic |this| behavior of jQuery is not something
> that should be encouraged. I understand it is probably mostly that way
> because of the dom event api,
JQuery goes further, I think simply due to mimesis. The DOM binds |this|
to the event target but in the context of the old DOM level 0, where the
only way to attach an event handler was as a method of the target, this
was not "wrong". JQuery goes much, much further down the path to crazy
|this| dynamic binding.
> but that doesn't change the fact that it really has a bad smell to it.
On this everyone agrees. No disrespect to JQuery, but it is important to
know what *not* to imitate. We know now, so we shouldn't be using JQuery
as a rationale for dynamic-this short forms or isBound as a general tool.
> CoffeeScript has ->, but if you look at the examples, none of them
> actually make use of dynamic |this| except for methods, and we're
> adding a nice method syntax, so it isn't really needed. Method syntax,
> and => should cover the majority of cases and lead people down the
> right path.
This is the current ES6 state and rationale, indeed.
> I know this is going around in a circle, but my point is that adding
> -> doesn't fix the problem, which is devs not knowing when to use =>
> and when to use function.
It's a good point. I hope we can wrap this thread up. It has been
helpful to disclose or emphasize the situation:
1. "soft-bind" breaks abstractions and won't fly with implementors.
2. isBound needs more discussion but it too breaks abstractions and it
must be defined (Mark's definition) to work for all this-insensitive
3. => covers the dominant use-case for functions not using |this| and
functions capturing lexical |this| via .bind or var self=this.
4. Adding -> doesn't avoid confusion over whether to use => or function,
it only adds a shorthand -- good for those who want this -- and at the
same time (the paradox of choice) creates anxiety over "which arrow do I
We could add isBound. We could add -> too. Right now ES6 has => only,
based on a weighting of the costs and benefits in 1-4.
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