Proposal: Conditional `catch` in Promises

[Ayush Gupta]

It might be worth **explicitly** mentioning that it's not about types
either, the benefit with using functions as the filter is that we can
tackle a lot of cases. Consider this:

```js
return somePromise
    .catch((reason) => reason instanceof ValidationError, reason =>
handleValidationError(reason))
    .catch((reason) => reason.code === 'ENOENT', reason =>
handleENOENT(reason))
    .catch(reason => handleOtherErrors(reason)) // catch all others
```


On Wed, Apr 25, 2018 at 10:25 PM, Bob Myers <rtm at gol.com> wrote:

> What do you mean by "approach TypeScript"? Do you mean propose this
> feature to the TS team? TS is not about new language features (with a few
> exceptions). It's about typing. They're quite careful about not forking the
> language.
>
> >  Not sure if that supports typed errors
>
> No, it doesn't.
>
> Bob
>
> On Wed, Apr 25, 2018 at 9:49 PM, Michael J. Ryan <tracker1 at gmail.com>
> wrote:
>
>> Maybe approach typescript on this one... Not sure if that supports typed
>> errors like C# does, but would probably suit you well.
>>
>> On Wed, Apr 25, 2018, 08:31 Isiah Meadows <isiahmeadows at gmail.com> wrote:
>>
>>> I'd still prefer we wait until pattern matching [1] gets addressed
>>> first, then tackling this. Error types are represented about 50 different
>>> ways in JS, with subtyping only being one (used by the standard kind of).
>>> Node appends an `err.code`, and the DOM adds a similar type, just using a
>>> common error subclass. And in some cases where errors are planned (but
>>> exceptions are more convenient), you sometimes see non-errors thrown. So
>>> there needs to be a means of catching all of them, and `if` checks get
>>> verbose and noisy in a hurry.
>>>
>>> On Wed, Apr 25, 2018, 00:11 Ayush Gupta <ayushg3112 at gmail.com> wrote:
>>>
>>>> We could potentially provide the same functionality in `try/catch` by
>>>> extending the signature of `catch` to
>>>>
>>>> ```js
>>>> try {
>>>>
>>>> } catch(<expression_var>, <function_expression>) {
>>>>
>>>> }
>>>> ```
>>>>
>>>> If `<function_expression>` evaluates to truthy, invoke the `catch`
>>>> block, otherwise don't.
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