Awaitable operations inside try block

[rssh]

Why not support awaitable operations inside try block, by translating
try{
F
} catch {
G
} finally {
H
}
to something like: async(F).recoverWith(G).onComplete(H) ?

[soywiz]

Probably because this is incompatible with the machine state.

I did a await/async for jvm some time ago:
https://github.com/soywiz/jawaitasync

But at bytecode level. Manipulating the AST is a different story.

But that doesn't mean that supporting try...catch blocks, is impossible.

I have another proposal implementing this:
(Simplified, and don't know about the implementation, so this is pseudocode)

try {
    stm1
    val result = await(future1)
    stm2
} catch (e) {
    case e:Exception =>
}

->

class MachineState(var local1, var local2, var state, var error)

while (state.state != 5) {
    state.state match {
        case 0 => try { stm1; state.state = 1 } catch (e) { state.state = 4 }
        case 1 => try {
            future1.then(v => resume(2, v, null))).otherwise(e => resume(4, null, e))
            return
        }
        case 2 => try { stm1; state.state = 3 } catch (e) { state.state = 4 }
        case 3 => done()
        case 4 => {
            // Transform the match from the catch statement, with a normal match
            e match {
                case e:Exception => state.state = ...
            }
        }
    }

}

[phaller]

I implemented support for await within try-catch-finally a while ago, however, without a substantial rebase we couldn't integrate it: https://github.com/phaller/async/tree/topic/try-catch-finally

Contributions for porting this to the current 2.10.x branch are most welcome!

[soywiz]

The rebase is just merge manually all the commits related to try-catch-finally? Or the api has changed since then?

[phaller]

A few things internal to the async implementation were changed, but it's possible that the try-catch-finally support "just works" (at least as far as it's implemented). So, yes, I think the main part of the work is manually merging all the commits related to try-catch-finally.

Following a PR, there might be a few more issues to address, of course. Just to be clear that the implementation is probably not 100% done. :-)

[retronym]

I believe we can and should do better than generating a PartialFunction for each catch handler. Instead, these could be additional states in the state machine.