Re: Dylan Wiki FAQ question

["Peter S. Housel" <address@hidden>]

"Doug Hockin" <dough@ipeg.com> wrote:
> In the Dylan Wiki FAQ page:
>
> http://monday.sourceforge.net/wiki/index.php/DylanFrequentlyAskedQuestions
>
> is the following example of a "abstract instantiable class":
>
>   define open abstract class <abstract-foo> (<object>)
>      // slots, maybe
>    end class;
>
>    define class <concrete-foo> (<abstract-foo>)
>      // slots
>    end class;
>
>    define sealed method make(class == <abstract-foo>,
>                              #rest key-value-pairs,
>                              #key, #all-keys)
>     => (instance :: <abstract-foo>);
>      apply(make, <concrete-foo>, key-value-pairs);
>    end;
>
> How does the first argument to make (class == <abstract-foo>) work?
> Isn't it supposed to be a class? What am I missing?

The first argument to make() is a class.  The (class == <abstract-foo>) is a
shorthand; another way of writing this definition would be:

define sealed method make(class :: singleton(<abstract-foo>),
                          #rest key-value-pairs,
                          #key, #all-keys)
 => (instance :: <abstract-foo>);
  apply(make, <concrete-foo>, key-value-pairs);
end;

This defines a method on the generic function make for the case where the
first argument is the class <abstract-class>.  This implies that whenever
make(<abstract-class>) is called, this method is invoked, which then causes
make(<concrete-foo>) to be done.  Without this method definition, the
default method for make() would be called, which would signal an error due
to an attempt to instantiate an abstract class.

-Peter-