That's a good point. Neither the E language nor the Q library allow subclassing of promises. The motivating reason in both cases is the security properties that promises must provide. But you're right -- this is an additional benefit. Promises/A+, being a minimalistic codification of broader agreement, neither prohibits nor demands subclassing.
Both the E language and the Q library instead provide extension mechanisms other than subclassing[1] which are carefully designed so that such extended promises cannot violate the promise security properties. These extension mechanisms are sufficient to create lazy promises -- promises that compute their resolution of demand, and of course remote objects. This is how we keep the distributed object system out of the promise foundations itself, and instead build it as a layer on top. [1] <http://wiki.erights.org/wiki/Proxy#makeProxy> the explanation of E's makeProxy on page 10 of < http://research.google.com/pubs/pub36574.html> <https://code.google.com/p/es-lab/source/browse/trunk/src/ses/makeQ.js#410> On Thu, Apr 25, 2013 at 10:28 AM, Kevin Smith <[email protected]> wrote: > I think flattening is also tied inextricably to the fact that promises are > a featureless wrapper for values. Nobody cares about promises-as-values > because of this featureless-ness. And because they are completely > uninteresting as values, programmers can think "straight through" to the > eventual value. > > This is highly simplifying for the programmer, especially in the context > of complex asynchronous data flows. It is a valuable property and in a > sense resembles a pure, universal currency. > > This suggests a warning: if we admit promise subclassing (in which > subclasses have extra features, such as cancel-ability), then this useful > property goes away, and with it flattening. > > { Kevin } > -- Text by me above is hereby placed in the public domain Cheers, --MarkM
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