[M3devel] Implementing the "Singleton pattern" in Modula-3

[Mika Nystrom]

Well I think the problem is with this "is known to be an object type"...

You may in fact want to override the methods of the singleton, but 
ensure that only one ever gets instantiated.

It's a minor problem, but it does sometimes bother me that it is
difficult to prevent clients from NEWing pretty much anything they
like.

By the way, the "Modula-3 Type System" paper also talks about a 
nifty way to do narrowing very quickly (page 10, last paragraph).
I remember this is occasionally problematic.  (RTType.IsSubtype
isn't great.)  They speak of keeping an array of supertypes
for each type, and recording the "depth" (distance from REFANY)
of every type in, I guess, RT0.TypeDefn.   This seems moderately
space-efficient (better than a 2D array of all the typecodes),
and very fast.

     Mika

Tony Hosking writes:
>I would exploit opaque types and branding.  If T is opaque then you  
>cannot instantiate T except in scopes where T's concrete type has been  
>revealed or is known to be an object type.  Thus, you could define an  
>interface:
>
>GENERIC INTERFACE Singleton(Rep);
>TYPE T <: REFANY;
>VAR t: T;
>END Singleton.
>
>GENERIC MODULE Singleton(Rep);
>REVEAL T = BRANDED REF Rep.T;
>BEGIN
>   t := NEW(T);
>END Singleton.
>
>where Rep defines the type T.  You could do similar for object types.
>
>
>On 7 Apr 2009, at 06:31, Mika Nystrom wrote:
>
>>
>> Tony, Jay, you two seem to be most knowledgeable about this.
>>
>> Let's say I wanted to implement the "singleton pattern" in
>> Modula-3.  At present there seems to be no way of doing this.
>>
>> What about changing RTAllocator.callback as follows (or adding
>> another callback):
>>
>> VAR callback : PROCEDURE(r : REFANY) : REFANY := NIL;
>>
>> (* If non-NIL, the allocator calls "callback(r)" just before returning
>>   a new traced reference "r" and instead returns the result of  
>> callback(r).
>>   See "RTAllocStats" for an example client.  It is a checked runtime  
>> error
>>   for the typecode of r to differ from the typecode of the return  
>> value of
>>   callback(r).  *)
>>
>>      Mika
>>
>>
>>