[Grace-core] "Object Algebras" in Grace

[James Noble]

> I am sorry to get back to you on this so late; but I have been busy for the past few weeks.
> However I've been slowly thinking about nesting, family polymorphism and object algebras. 

Good - no problem, we've been busy too, and in my case still are.
Yes I'm still planning on sending (various) versions of Grace approaches for your web page.

> I've come up with an encoding that uses object algebras to achieve the late binding typical 
> of family polymorphism approaches like virtual classes. The encoding uses Java's nested 
> classes. The code goes in annexe. 

OK, I'll have to study it... 

> I'd like to see how this encoding compares to existing virtual classes/nested inheritance approaches. 


Something we're looking at in Grace, trying a few examples, is the virtual superclasses
one finds e.g. in Newspeak and (I think) in other OOLs: 
 
class BaseFamily.new {
  class FirstClass.new { method m { "BaseFirst" } }
  class SecondClass.new {
     inherits FirstClass.new
     method m { super.m ++ " BaseSecond" } }
}

class ExtendedFamily.new  is overriding {
  inherits BaseFamily.new
  class FirstClass.new is overriding {
     inherits super.FirstClass.new 
     method m { super.m ++ " ExtendedFirst" } 
  }
}

print (ExtendedFamily.new.SecondClass.new.m) 

what we want to see is "BaseFirst ExtendedFirst BaseSecond" -
i.e. SecondClass in the ExtendedFamily inherits from FirstClass 
*in the ExtendedFamily*  - even though the only declaration is
in the BaseFamily where SecondClass inherits from FirstClass.

The point is that superclass is virtual - or dynamic as I think Gilad would put it - 
it is resolved at runtime, at the time an instance of ExtendedFamily.SecondClass 
is created.

I don't see how object algebrae allow this kind of inheritance and extension,
especially not in Java, where classes are aggressively not virtual, and 
inner classes are never overridden.   But perhaps I just haven't looked at 
your example hard enough?

> I think this may be worthwhile exploring a bit more and perhaps writing something up. Do let me  know of comments, questions or limitations. 

well I'm keen. I may not have much time but I'm keen.

James 



PS - due to bugs, the code above doesn't actually work in our prototype 
minigrace implementation http://homepages.ecs.vuw.ac.nz/~mwh/minigrace/js/

This code below is roughly similar, but doesn't trigger those bugs.

class BaseFamily.new {
  method FirstClass_new { object { method m { "BaseFirst" }  } }
  method SecondClass_new { 
    def daddy = FirstClass_new
    return object {
     inherits daddy
     method m { super.m ++ " BaseSecond" } }
} }

class ExtendedFamily.new {
  inherits BaseFamily.new
  method FirstClass_new { 
    return object { 
     inherits super.FirstClass_new 
     method m { super.m ++ " ExtendedFirst" }
  } }
}

print (ExtendedFamily.new.SecondClass_new.m)