I am curious what, if anything, about this problem has to do with asynchronicity? That seems to be a key part of your point, but I don't understand why you wouldn't phrase the problem entirely in terms of synchronous operations since nothing seems unique to async.

On 5 Nov 2013, at 09:48, "Benjamin (Inglor) Gruenbaum" <inglor at gmail.com<mailto:inglor at gmail.com>> wrote:

Traditionally, JavaScript code contains some sort of asynchronous logic either on the client side or on the server side.

This makes the `try/catch` construct non-practical for many real use cases because it can not catch errors caused by callbacks and/or other asynchronous operations involved. This is why we have common idioms like the `(err,result)` of NodeJS callbacks or reject in promises implementations and so on.

NodeJS also introduces domains which let you catch all errors that originated from a piece of code in a certain way.

ES6 Generators recently implemented in NodeJS allow us to do concurrency using coroutines. This allows for very nice code that's very readable in certain cases. The async_functions proposal is also very interesting solving the problem without needing a "runner" for the generator.

However, one issue arises from using generators for such concurrency. The current way try/catch works with generators does not give us a way to differentiate things like syntax errors from logic errors.

Consider the following piece of code using Bluebird promises:

```
Promise.coroutine(function *(){
    try{
        let db = yield DBEngine.open("northwind");
        let result = yield db.query("SELECT name FROM users")
        return res;
    } catch (e){
        //code to handle exception in DB
    }
});
```
Noticed the `ReferenceError`? Not obvious at first glance. A compile time tool _might_ find it but I may have a closure variable named `res`. This might even happen later when making changes. A catch-all does not do a good job here. What I'd want to do is to catch all exceptions that are _mine_. I need a way to catch exceptions of a different type.

This is a real need in light of the new found usage for try/catch given generators.

I've looked around and found one possible solution using catch guards ( http://wiki.ecmascript.org/doku.php?id=strawman:catch_guards ) , I know Firefox supports catch guards but I was unable to find anything in the specification or in the mailing list when searching relating to it.

Petka added:

Without predicated catch clauses you would need to write something like this:

```js
Promise.coroutine(function *(){
    try{
        let db = yield DBEngine.open("northwind");
        let result = yield db.query("SELECT name FROM users")
        return res;
    } catch (e) {
        if( e instanceof RejectionError ) {
            //code to handle exception in DB
        }
        else {
            throw e;
        }

    }
});
```

The manual rethrow can easily be forgotten to be written
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I am curious what, if anything, about this problem has to do with asynchronicity? That seems to be a key part of your point, but I don't understand why you wouldn't phrase the problem entirely in terms of synchronous operations since nothing seems unique to async.