Futures (was: Request for JSON-LD API review)

Ron Buckton rbuckton at chronicles.org
Wed Apr 24 11:52:19 PDT 2013

Resending due to a mail error.

> -----Original Message-----
> From: es-discuss-bounces at mozilla.org [mailto:es-discuss-
> bounces at mozilla.org] On Behalf Of Tab Atkins Jr.
> Sent: Wednesday, April 24, 2013 11:18 AM
> To: Domenic Denicola
> Cc: Mark S. Miller; es-discuss
> Subject: Re: Futures (was: Request for JSON-LD API review)
> On Wed, Apr 24, 2013 at 10:51 AM, Domenic Denicola
> <domenic at domenicdenicola.com> wrote:
> > From: Andreas Rossberg [rossberg at google.com]
> >> Mark, could you summarize the rationale for this, or provide a more
> specific link to the appropriate bit of the discussion you are referring to?
> >
> > I'm not Mark, and he might have something more specific in mind, but
> > this
> summary was pretty helpful:
> >
> > https://gist.github.com/ForbesLindesay/5392612
> These aren't very good reasons, unfortunately.  :/
> The JQP... problem can be solved by a single "flatten" operation added
> to the API.  This is a totally reasonable operation, same as it would be for
> The identify function for monads is the monadic lift function -
> Future.accept() in the case of Futures.  The only reason passing the
> identity function in *at all* works is because we have special magic
> that lets authors return non-Futures from the callbacks (which I think
> is a good idea, mind you).  It's not actually an identity function, though.
> The synchronous analog objections don't make sense.  The analogue is
> returning the Error object itself, which could certainly be reasonable at
> times.
> Recursive unwrapping is like making "return new Error()"
> identical to "throw new Error()".  I don't even understand the
> attempted analogy with f() and g() - chaining takes care of unwrapping
> in all reasonable cases like that.  Similarly, I don't think the Stack
> Frame analogy has been well- thought-out - chaining Futures is the
> analogy to stack frames, and that works just fine.
> I'm not sure what the Parametricity Argument is.
> However, all of this falls before the simple fact that recursive
> unwrapping means that *no one can ever create any object with a
> .then() method on it ever again*.  If .then() is treated as the normal
> monadic operation over Futures (that is, it unwraps one layer of its
> return value), then you can safely return objects with a .then()
> method by wrapping them in a Future with a method like
> Future.accept().  (You can safely do this to *all* values unless
> you're explicitly asking for chaining to happen.)
Given the collision with possible existing uses of "then", it might be better to
either use a symbol to brand an object as a thenable, or use a symbol to
define a compatible then (or done) -like method similar to @iterator. It
would only be necessary to import and apply the symbol to interoperate with
a native Future, and subclasses of Future would implicitly have support.
Alternatively, only "recursively unwrap" native Futures or Future subclasses,
and require an explicit coercion using something like Future.of.


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