"Michael T. Richter" <mtr@ottawa.com> writes:
> Jason, you just solved a minor question I've had ever since I
> started to experiment with Dylan: What use are singletons?
Not much really since singletons are practically supported by the
language. But here's an example of a 'singleton' class as per the GOF
book as presented by Andy Armstrong a while ago:
------------------8<---------------
// An implementation of the singleton pattern - based on example
// posted to comp.lang.dylan by Andy Armstrong.
define constant *singleton-instances* = make(<table>);
define open abstract class <singleton-class> (<object>)
// each-subclass slot
// singleton-instance :: false-or(subclass(<singleton-class>)) = #f;
end class <singleton-class>;
define method make ( class :: subclass(<singleton-class>), #key )
=> (object :: <singleton-class>)
element(*singleton-instances*, class, default: #f)
| begin
*singleton-instances*[class] := next-method()
end
end method make;
------------------8<---------------
Note that 'each-subclass' wasn't used as you need an instance of a
class to modify the class slot - and you don't have one initally with
a singleton.
Example use:
define class <my-class> (<singleton-class>)
end;
begin
let a = make(<my-class>);
let b = make(<my-class>);
a == b
end; // should be #t
Chris.
--
http://www.double.co.nz/dylan