[e-lang] Reentrancy via Recursion

[Kevin Reid]

On Feb 26, 2009, at 7:18, Toby Murray wrote:

> However, this kind of nondeterminism can already be created in E.
> Suppose we have a variable 'InUse' and Bob is now
>
> def Bob() {
>   if (InUse) {
>       throw "in use"
>   }
> }
>
> Bob is nondeterministic in that Bob() will fail sometimes if called  
> in a
> context during which InUse is true.
>
> I see that the first case does reveal information that the second
> doesn't -- about the call-stack. However, I don't see how it adds any
> more nondeterminism than is already present.


Okay, my revised statement:

Checking the call stack *for occurrences of yourself* is safe, and  
equivalent to maintaining state. An object which uses this "I call  
you, don't call me" operator cannot be considered DeepFrozen.

Furthermore, it can be implemented user-level by a library using an in- 
use flag:

def [Bob, protectedCall] := prohibitMarkedReentrance(def Bob0() {
   protectedCall(Carol).hello(Bob) # instead of Carol.hello(Bob)
}}

Here Bob is a wrapper around Bob0 which checks the flag which is set  
by protectedCall.

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