[E-Lang] when-catch-finally (was: pending revision of E in a Walnut)

[Karp, Alan]

MarkM wrote:
> 
> Now that this is control-flow oriented, there is a new 
> counter-intuitive 
> issue: An exception thrown during the evaluation of the 
> expression in the 
> "when (...) ->" head is *not* caught by these catch clauses, 
> but is rather 
> thrown in the original context.  I think it needs to be this 
> way, but people 
> will find it surprising.
> 

Well, if a broken reference is an exception, then when(...) can move into
its block whenever a reference is either resolved or broken.  That block can
contain a try-catch with an exception thrown on an attempt to use a broken
reference.  For example,

            when (nextCar <- moveTo(toX,toY)) -> done(moving) {
		    try {
                   println(nextCar name + " arrived, next car to start
moving")
                } catch BrokenReferenceException e {
                    println("car died: " + e)
		    } catch Exception e {
			  println(e + " happens.")
                } finally {
                    moveRemainingCars()
		    }
            }

which looks a lot like MarkM's example

    Ref whenResolved(xVow, def done(temp) {
        try {
            def tempProb := Ref optProblem(temp)
            if (Ref isBroken(temp)) {	
                throw(Ref optProblem(temp))  // Explicit throw not needed
(AHK)
            }
            def x := temp
            doSomething(x)
        } catch prob {			// Should be multiple catch blocks
(AHK)
            handle(prob)
        } finally {
            doLast()
        }
    })

_________________________
Alan Karp
Principal Scientist
Decision Technology Department
Hewlett-Packard Laboratories MS 1U-3
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