[e-lang] Bindings and guard-based auditing

[Kevin Reid]

On May 25, 2010, at 14:42, Thomas Leonard wrote:
> On 25 May 2010 17:41, Kevin Reid <kpreid at switchb.org> wrote:
>> What do you mean by "binding types"?
>
> I'm assuming I can't do:
>
> def &&x := makeMyBinding(...)

Correct.

>> Note that 'CoercedSlot' does not mean 'a (container for a) slot which
>> has been coerced'; it means 'a slot whose value *has been* coerced';
>> it is basically GuardedSlot without the mutability.
>
> I'm still confused, then. How can the value have been coerced, unless
> it's a FinalSlot. Other slots change their value over time.

Statement #1, about the ELib component known as CoercedSlot:

A CoercedSlot is “final”. CoercedSlot is precisely to FinalSlot as  
GuardedSlot is to VarSlot*: like FinalSlot, its value does not change,  
and like GuardedSlot, it remembers a guard, and guarantees that its  
value passed that guard.

Statement #2, about the use of such in guard-based auditing:

The binding for 'var x :int' is the object constructed by

   makeCoercedSlot(GuardedSlot[int],
                   makeGuardedSlot(int, specimen, ejector),
                   ejector)

The binding-guard for this binding is GuardedSlot[int], which is  
stored in the guard attribute of the binding object (which happens to  
be a CoercedSlot object).

This is arguably a type pun; we are reusing the interfaces and naming  
of slot objects because they happen to have the right properties (a  
value and a guard) for binding objects. You could say 'a binding is a  
meta-slot' in the same sense as 'metaclass'; a value is stored in a  
slot (which is a Slot) which is stored in a binding (which is also a  
Slot, and furthermore necessarily a CoercedSlot).

* See http://wiki.erights.org/wiki/Basic_slot_type_naming for issues  
with the choice of names here.

> By the way, I'm assuming we'll just audit everything for DeepFrozen
> automatically whenever possible, or my fingers will get very tired
> from typing "implements DeepFrozen" everywhere.

No. This would form an unavoidable one-bit implementation information  
leak. Feel free to suggest syntax and naming changes to reduce the  
verbosity. (You can always 'def &&DF := &&DeepFrozen', of course.)


(Random thought: This would all be much simpler if E did not have non- 
final-slots and all mutation or otherwise special slots were handled  
by user code working with explicit slot objects...)

> Well, let's add the auditor first, instead of ...:
>
> interface FooGuard guards FooAuditor {}
> def foo implements FooAuditor {}
> def bar implements Nonsklarkish { ... foo ... }
>
> In both proposals, the Nonsklarkish can't see anything about foo in
> this case, I think.
>
> To make it available, I'd assume this:
>
>  def foo :FooGuard implements FooAuditor {}
>
> Then Nonsklarkish knows that foo was coerced by FooGuard, because "def
> var :guard" exposes "guard" to auditors automatically.

This notion causes a circularity problem with the object's self- 
reference. (It also makes another lookahead mess in the parser, but  
we've got lots of those already in object/def expression heads.)

def a {
   to coerce(sp, ej) {
     return sp.this()
   }
}

def b :a {
   to this() {
     return b
   }
}

The value of b is not determined until the guard a returns, but a  
invokes b. Therefore the naming pattern of an object expression must  
not have a guard.

(Making b-inside-of-b be a promise until the guard returns makes a  
messy edge case for auditors, but might be workable...)

> Presumably for bindings the syntax would be:
>
>  def foo as FooAuditor {}
>
> But that gives Nonsklarkish the powerful FooAuditor, which seems
> wrong, so I don't think I've understood this syntax.

It is inappropriate to use the 'as' syntax for rubber-stamps, yes. You  
should really call it 'FooStamp' to indicate that it is a stamp.  
Rubber-stamps are a degenerate, but supported, case of auditors.

Instead of using the 'as' syntax, you'll have to use the separate  
guarding syntax.

def foo :Foo := { def foo implements FooStamp {} }


-- 
Kevin Reid                                  <http://switchb.org/kpreid/>