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Re: Reconciling handling of optional arguments and handling of default values across ES and webidl

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Wed, 8 May 2013 11:29:02 -0700
Message-ID: <CAAWBYDAbW6oDwpzUFiRRp-pN0yLgD-RV-asmqPp7qed1cUbwTg@mail.gmail.com>
To: Boris Zbarsky <bzbarsky@mit.edu>
Cc: "public-script-coord@w3.org" <public-script-coord@w3.org>
On Wed, May 8, 2013 at 11:18 AM, Boris Zbarsky <bzbarsky@mit.edu> wrote:
> This keeps coming up, so here comes a proposal....  I believe what I'm
> proposing does not represent the status quo for either ES6 or WebIDL, but it
> seems like the sanest available behavior to me, and I'd like to see what the
> objections are to it.  I am aware that there has been a good bit of
> discussion about this in ES6, and I've read all of it that I could find, for
> what it's worth.  And I think that people are undecided about how exactly
> "optional" arguments and "default values" should work, which is leading to
> some wackiness.
>
> WebIDL behavior:
>
> 1)  Allow any argument to be marked as optional in WebIDL, even if followed
> by non-optional ones.
>
> 2)  Allow any argument that is optional to have a default value provided in
> WebIDL, even if followed or preceded by optional arguments without default
> values.
>
> 3)  Treat undefined passed for an optional argument as "not passed".  If
> there is a default value, the default value takes effect.
>
> ES behavior:
>
> 1)  Allow specifying a default value for any function argument, even if
> followed or preceded by arguments without default values, with the default
> taking effect if undefined is passed.
>
> 2)  Have arguments.length not include trailing undefined values.
>
> I believe this will completely align the observable behavior of the two and
> the capabilities available to both JS functions and C++ implementations of
> WebIDL functions in terms introspecting the arguments.  The mental model is
> "undefined should act just like not passed".
>
> In trailing position that means you can in fact omit the arguments, or pass
> undefined, and you get the same arguments.length either way, and the same
> default-value handling.
>
> In non-trailing position you have to explicitly pass undefined to get the
> default, unless we grow named-argument passing a la python.  If we do, it
> would still be compatible with the above proposal, I believe, though at that
> point arguments.length and indexing into arguments might need a bit more
> discussion.  ;)
>
> Thoughts?

+1.  We already know that we need to treat undefined as "missing", to
allow for pass-through function combinators that arent' insane.  Given
that, I see nothing wrong with allowing "optional" arguments anywhere
in the list.

~TJ
Received on Wednesday, 8 May 2013 18:29:53 UTC

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