Patrick Wright wrote:
> The idea is interesting, but, two points
Good points...
> 1) Where does it jump to? The next line after the method invocation?
> Or a finally block, if there is on? What if I haven't defined a
> finally block in the enclosing (method-calling) scope? Does it bubble
> up to some other finally in a broader scope? What if I'm starting with
> a script and haven't defined any finally at all? I guess the compiler
> could detect that a Jump was being declared (how?) and force you to
> include a finally.
The typical way a non-local return (for example) is implemented now is
by constructing the closure containing it such that it has a unique ID
to return to. NonLocalReturn would then be constructed with this ID, and
the jump would bubble back out through the stack until someone
interested in NonLocalReturn events caught it and saw it had the
appropriate id, at which point the return value would be unwrapped and
bubbling would stop.
So it is still more of a "throw" or "raise" or "trigger event", but the
intention is that somewhere, someone will be interested in being the
"landing point" for the jump.
Now this is just the brute-force way we've been implementing it. It
could certainly be what the actual JDK7 generates as bytecode, ideally
all behind the scenes when you do a return within a closure. However I'd
wager there are specific optimizations that could be done to simplify
it. For example, exception tables could be generated such that they're
only interested in the jumps generated in that scope, both types and
IDs, allowing other jumps to transparently pass through without the cost
of catching and re-throwing.
Neil Gafter: I recall you advocating non-local returns, but I don't
remember if you also advocate other types of non-local flow control like
break or continue. Do you?
myList.each {(String element) if (element.equals("foo")) break; }
> 2) The word "jump" seems to imply a location (jump-to, jump-back-to)
> but no destination, specifically.
Perhaps it's more like "JumpAndHopeSomeoneWillCatchYouOnTheWayDown". I
think Jump is a bit nicer though.
The accurate name would probably be "BeginStackEscape" or something similar.
- Charlie
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