[e-lang] How to deal with eventual sending with non-local returns

[Kevin Reid]

On Nov 6, 2007, at 23:17, Rob Withers wrote:

> What does E do with this kind of expression:
>
> escape __break :any {
> def promise := bob <- foo(carol);
> when (promise p) -> done() {
>         __break.run(p)
>     }
> }
>
> ?

"promise p" isn't a valid expression, and the when/done are  
unbalanced; I'll assume you meant something like
   when (promise) -> done(p) {
or
   when (def p := promise) -> {
(which are equivalent, and the latter is the current recommended  
syntax).


Remember that 'when' is not a Kernel-E construct; it's just a  
standard object (Ref) being invoked with an ordinary function holding  
the body.

The applicable rule in this case is simple: ejectors have dynamic  
extent. Calling an ejector after its corresponding escape block has  
been exited causes the ejector to throw a complaint instead.

Since Ref.whenResolved doesn't invoke the when block's body function  
until some future turn, when this code is first executed it simply  
falls off the end of the escape block, returning what 'when' returns  
(which is a promise for the result of the body).

When the when body is run, __break will throw reporting that it is  
invalid, and (as with any other throw from a similar point) that  
exception will be in the tracelog, and in the broken reference which  
the promise for the result of the when expression resolves to.

See http://www.erights.org/elang/kernel/EscapeExpr.html for more  
information and other unusual cases.

> ----- Original Message -----
> From: "Rob Withers" <reefedjib at yahoo.com>

Please place your quotations above your responses, in the standard  
format for such (attribution line, "> " quoting), and trim them to  
only relevant text.

-- 
Kevin Reid                            <http://homepage.mac.com/kpreid/>