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From: shang@corp.mot.com (David L. Shang)
Subject: Re: Cows, Hogs & Farms
Reply-To: shang@corp.mot.com
Organization: MOTOROLA 
Date: Wed, 11 Jan 1995 17:25:43 GMT
Message-ID: <1995Jan11.172543.21485@schbbs.mot.com>
References: <1995Jan10.180233.23224@inca.comlab.ox.ac.uk>
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In article <1995Jan10.180233.23224@inca.comlab.ox.ac.uk>  
lady0065@sable.ox.ac.uk (David J Hopwood) writes:
> >Run time type check can also be encapsulated by a hog's method, say "try":
> >
> >	function hog::try(food: plant): plant
> >	enter
> >		when typeof(food) is corn do 
>                      ^^^^^^^^^^^^^^^^^^^^
> >		{	eat (food);
> >			return nil;
> >		}
> >		return food;
> >	end;
> 
> try should allow the run-time type of its argument to be a subtype of corn.
> It should not restrict it to being exactly corn. Ie. the language should 
> allow something like:
> 
> when typeof(food) is-subtype-of corn do ...
> 

	when typeof(food) is corn do

implies that 

	when typeof(food) is-subtype-of corn do

while

	when typeof(food) = corn do

means the exact type of corn.

I agree that is-subtype-of is clearer and more readable than just "is".  
Cluster-2 will use typecase statement similar to Sather's:

	typecase T
	when T1: ... 
	when subclassof T2: ...  // or when <:T2:
	...
	otherwise: ...

When "subclassof" is used, the linker and runtime support will load complete  
type info into the application. Therefore, "subclassof" will have a significant  
overhead (at least heavier than GC). But once the complete type info is loaded,  
applications will benefit a lot from a full dynamic typing. 

Does Sather have a light implementation on "subclassof"?

David Shang