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

[Tony Hosking]

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
>
>
>