On Tue, Jun 9, 2015 at 12:38 PM, Mark S. Miller <erig...@google.com> wrote:
> On Tue, Jun 9, 2015 at 9:29 AM, C. Scott Ananian <ecmascr...@cscott.net>
> wrote:
>
>> Mark: The `prfun` library in fact uses `Promise#timeout(n)` instead of a
>> `TimeoutPromise` subclass. But this is really a language-design question.
>> You might as well ask why we have `WeakMap()` as a constructor instead of
>> using `Map#weak()` or `weakmapify(map)`. The fundamental reason is "so you
>> can name (and thus test) the type of the object".
>>
>
> Do you ever test that the object returned by `Promise#timeout(n)` is
> something other than a plain promise?
>
As I said, I use `Promise#timeout` myself in my implementation, not a
subclass. So I haven't written tests for TimeoutPromise myself.
But the smalltalk community has a number of places where it is worth
testing whether an argument is a WeakArray. Those examples would translate
directly to tests for WeakPromise.
And for TimeoutPromise you could imagine something like:
```
var protect = function(x) {
return (x instanceof TimeoutPromise) ? x : TimeoutPromise.resolve(x);
};
var doSomethingImportant = function(x) {
// ensure that we're not going to hang forever waiting for x
return protect(x).catch(....).then(....);
};
```
It is rather convenient that the `protect` function already has a clever
name: `TimeoutPromise.resolve`.
--scott
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