On 14 February 2014 00:32, C. Scott Ananian <ecmascr...@cscott.net> wrote:
> For your consideration, here is an implementation of Monadic Promises,
> as a Promise subclass:
Since promises are expressible in JS you can always create a new class
that does what you want. It's also well-understood that you can do
that using the existing class by wrapping all resolution values.
The fundamental problem, however, is that existing producers will not
give you instances of the new class, which makes it rather useless in
most cases.
Whether the new class is a subclass or not is mostly immaterial
regarding that problem. In fact, I would argue that it is incorrect to
implement it via subclassing, since you are effectively using a
different internal representation (a wrapped value) that the base
class methods do not recognize. If, for example, somebody was to add a
method like
value() {
if (this.status != resolved) throw TypeError;
return this.value;
}
to the base class then it would not work properly with subclass
instances. In other words, this form of subclassing violates
substitutability.
/Andreas
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