[M3devel] REFANY-keyed tables?

[mika at async.caltech.edu]

Right, right.

Another way of seeing it is that if st(x) = st(y) then x and y are
pointing to the same object.  The language specification does talk about
memory, albeit abstractly.

If x and y point to the same object and you don't assign to them, they
keep pointing to the same object.  That is the language semantics,
and yes, by The Green Book I mean Systems Programming with Modula-3.

And yeah whatever the implementation does has to respect the language
semantics.  If the GC wants to change the bit representation of x then it
has to change the bit representation of y in a way such that your program
will always see x = y.  Note that it is NOT literally required that the
bit representation of x always equals the bit representation of y.  But
I do believe the GC accomplishes x = y by ensuring that their bit
representations are always equal in any state where they may be checked
by your program.

Another way of looking at this is to consider references in the language
as products of NEW.  Every reference to a particular evaluation of NEW
always points to the same thing.  If NEW has only been evaluated once
in your program, there can only be two REFANYs: NIL and the result of 
that NEW.  Those are the language semantics.  And of course the implementation
has to go to some effort to ensure this property holds even as it fiddles
with the bit representations of the two REFANYs.  (NIL's bit representation
is *probably* constant, but it doesn't have to be...)

    Mika

"Rodney M. Bates" writes:
>
>
>On 03/02/2014 09:00 AM, JC Chu wrote:
>>> You can't apply ^ to REFANY.
>>
>> That was a mistake on my part.  Let me rephrase my worry as follows, where st(r) denotes the storage location pointed to by the reference r.
>>
>>    1. x, y: REF T, where T has an equivalence relation T.Equal.
>>    2. Initially, x = y # NIL and st(x) = st(y) = s.
>>    3. Then the GC creates a copy s' of s.
>>    4. Then the GC updates x so that st(x) = s'.
>>    5. But then y still has the old value: st(y) = s.
>
>With a correctly implemented GC (which we have), step 5 can't happen.  The GC
>will prevent any use of y by a running mutator thread until it (the GC) has
>updated y as well as x, so x = y again.  My questions to Tony were about how the
>M3 GC accomplishes this, and it does.
>
>The motivatation for not giving REFANY a succeeding Hash is that, after
>the updates, x = y # <the value x and y had before the move>.  This would
>make a hash table fail, even if x = y works as defined.  Mika is right,
>as long as you stay in the safe subset of the language.  You simply can't
>write a hash function on a REFANY at all.
>
>Somebody could, however, declare the module with the Hash function UNSAFE,
>LOOPHOLE the REFANY value to a Word.T (or something else, if the sizes were
>not the same),  and return that or something derived from it.
>
>It is never feasible in any language to define everything that can happen,
>when type-unsafe techniques are used.  That inevitably gets into implementation
>details, which we often make assumptions about rather cavalierly.  Part of
>the wisdom of Modula-3 is that it clearly defines what the safe subset is,
>then equally clearly defines the semantics of that,, without getting into
>implementation.
>
>I was careless about glossing over the safe/unsafe distinction.
>
>>
>> At step 5, we have x # y, st(x) # st(y), and T.Equal(x^, y^).  If x is stored in a container for REFANY, then a membership check for x using y will fail,
> because it can only be based on Refany.Equal.  (If T.Equal is used then we won’t have this problem.)
>>
>>> REFANY behaves the way the Green Book says it does.
>>
>> You mean Systems Programming with Modula-3?
>>
>> — JC Chu
>>
>> -----Original Message-----
>> From: mika at async.caltech.edu [mailto:mika at async.caltech.edu]
>> Sent: Sunday, March 2, 2014 21:03
>> To: JC Chu; rodney_bates at lcwb.coop
>> Cc: m3devel at elegosoft.com
>> Subject: Re: [M3devel] REFANY-keyed tables?
>>
>>
>> I just want to make an observation...
>>
>> REFANY behaves the way the Green Book says it does.  There's not a word in there about concurrent garbage collectors or bit-pattern representations of RE
>FANY.
>>
>> If a, b : REFANY and a = b then a and b point to the same object.
>>
>> You can't apply ^ to REFANY.  The only thing you can do to a value beyond = is narrow it to another type.  That's why there's no Hash for REFANY.
>> (Not because the garbage collector is implemented one way or another.)
>>
>> Of course you can put REFANY in a list.  You can make as many copies in as many places as you want and = will continue working.
>>
>> The rest is implementation...
>>
>> I'm not saying there isn't a lot of implementation, or that the implementation doesn't have to get clever about some things, but you really don't need to
> think about the implementation to know what you can and can't do with REFANY.  It's all in the Green Book.
>>
>> I definitely would not suggest messing with the garbage collector because you want to get something working with REFANY in a pure Modula-3 program.
>> It defeats the purpose of REFANY.
>>
>> If you MUST hash a whole bunch of objects that have being REFANY as their only thing in common I'd suggest using TYPECODE and registering hash procedures
> for each type you're interested in.  This is a simple enough thing that Modula-3's runtime "introspection" facilities more than suffice.  (Those facilitie
>s are more limited than Java's, for mostly good reasons.)
>>
>>       Mika
>>