Issue 6 - Proposal for vote

["Satish Thatte" <satisht@microsoft.com>]

Please note that this proposal also subsumes Issue 142 as the examples
show.  Also note that everyone involved in the discussion of Issue 6 has
not had time to comment on the merits of this proposal relative to
alternatives.  I am putting it up so that we can discuss it at the F2F.
Early comments are always welcome.

Proposal for resolution of both Issues 6 and 142.

Define a new activity of the form <complete scope="ScopeName"?>.  The
execution of this activity will cause premature but successful
completion of all enclosing scopes upto and including the scope named in
the optional scope attribute.  In case the attribute is omitted, the
default value of the attribute is the name of the nearest enclosing
scope.  

In case of premature but successful completion of a scope, all
concurrently running activities in that scope will be cancelled.  A
scope may optionally have a completion handler.  A completion handler is
syntactically similar to a compensation handler (no parameters).  In
case of premature but successful completion of a scope the completion
handler is run if present.  The default completion handler does nothing.
The completion handler is run in the context of the scope it is attached
to, hence may use variables and partnerLinks defined within that scope.

The behavior of the completionHandler is treated as part of the "forward
work" of the scope.  Thus all fault handlers for a scope are still
available while the completion handler for that scope is running.  In
case of a fault in a completion handler, the behavior is identical to
the occurrence of a fault in the main body of the scope.  

Question: can the <complete/> activity be used in a completionHandler?
Or perhaps only in the named (final) one in a completion process?

Rationale:  This proposal defines a "break-like" feature following the
patterns of the rest of our interrupt machinery, e.g., fault and cancel
handlers.  It is possible to use this to build declarative macros for a
variety of purposes including the various flow-based features that were
proposed.

Examples:

Break: 

Wrap a while loop with a scope S and use <complete scope="S"/> to break
from the loop.

Continue:  

Wrap the body of a while loop with a scope S and use <complete
scope="S"/> to continue with the next iteration of the loop.

At least N of M activities in a flow:

<scope name="N-of-M">
  <completionHandler>
  </completionHandler>
  <flow>
   ..
     <sequence>
       <original "activity that counts">
       <update completion count variable V>
       <switch>
         <case using V, N out of M done?>
           <complete scope="N-of-M"/>
         </case>
       </switch>
     </sequence>
    ..other activities in flow
  </flow>
</scope>

Can we do exactly N of M using an isolation scope for the switch?

Satish