On 30 April 2013 13:30, Andy Wingo <[email protected]> wrote: > Hi Kevin, > > On Tue 30 Apr 2013 11:05, Kevin Gadd <[email protected]> writes: > >> I would definitely expect a given finally block to run if i use for-of >> or similar on the generator. This is the intent, I hope? > > Certainly they run in this situation: > > function *g1() { try { yield 1; } finally { qux(); } } > for (x of g1()) > print (x) > > Or in this one: > > function *g2() { try { yield 1; return; } finally { qux(); } } > for (x of g2()) > print (x) > > But the question is what happens here: > > function *g3() { try { yield 1; } finally { qux(); } } > for (x of g3()) > break; > > Or here: > > function *g4() { try { yield 1; } finally { qux(); } } > for (x of g4()) > throw "foo"; > > Or here: > > function *g5() { try { yield 1; } finally { qux(); } } > for (x of g5()) > call_function_that_throws_an_exception(); > > For me, it is acceptable in the last three cases to never invoke those > finally blocks. Otherwise, for-of would need to be implicitly > surrounded by a try/finally to manually "close" the generator. It > seems to me that it would have pretty negative perf implications; for > example Crankshaft doesn't currently run on functions with try/finally.
A particular Crankshaft limitation may not be the most convincing argument. :) But clearly, even without that, requiring an implicit finally wrapper for every for-of loop could still be costly -- especially because the last case cannot easily be ruled out at compile time. I'd also argue that _not_ running the finally blocks of a generator in cases where it is abandoned is consistent with coroutine semantics. In those cases, the generator basically amounts to a coroutine that is still active, but being starved because you never yield back to it again. Even if we did require a close with for-of loops, the problem would still exist if a generator is run directly through its method interface. There is no way the language can enforce a close in this situation, short of finalization. The moral is that one should simply avoid putting a yield inside a try-finally. There is no guarantee that control ever returns. /Andreas _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
