[e-lang] Prize and Milestone as mutable state idioms

[Kevin Reid]

On Jun 30, 2008, at 19:20, Tyler Close wrote:

> For example, I was using a boolean variable to keep track of  
> whether or not the most recent JSON value had been fully and  
> successfully output. The update pattern here was that the variable  
> started out false and then transitioned to forever being true.  
> Verifying that this property was correctly implemented previously  
> required examining the full file for mentions of the boolean  
> variable to check that it was only ever assigned the value true.  
> The Milestone class encapsulates a boolean variable that can only  
> be updated in this way.

 From the problem description, I immediately think of a promise.

That is (in E),

def [p, r] := Ref.promise()
def yet() { return Ref.isResolved(p) }
def now() { r.resolve(null) }

In this case it's probably silly, as the update pattern is almost  
equally short (and, I would expect, shorter in ref-send/Joe-E),

def [yet, now] := {
   var v := false
   [v.get,
    def now() { v := true }]
}

but promises can be a nice primitive for more complex happens-once  
cases (and require no external encapsulation).

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