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From: mbk@inls1.ucsd.edu (Matt Kennel)
Newsgroups: comp.object,comp.lang.eiffel,comp.lang.sather,comp.lang.beta,comp.lang.modula3
Subject: Re: Cows, Hogs & Farms
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Date: 6 Jan 1995 04:31:34 GMT
Organization: Institute For Nonlinear Science, UCSD
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David L. Shang (shang@corp.mot.com) wrote:

: But by Sather, run-time type check is always a neccessity. And by Eiffel's  
: covariance, a system validation is required for the above case.

: By Cluster, runtime type check is not necessary for the above cases. It is only  
: necessary when a herbivore cannot identify whether the food is its food. This  
: happens only in the case of a reverse/cross assignment:

:<deleted>

: Runtime type check is not an very important issue. The keypoint is that whether  
: the language can say the way you want!

: David Shang

You can do most of Cluster's static solution in Sather, and I guess Eiffel.

type $HERBIVORE is
	-- generic herbivore, no specific foodtype known.
	name:STR;
	eat_or_barf(food:$PLANT);

type $HERBIVORE{FOODT < $PLANT} < $HERBIVORE is
	name:STR;
	eat(food:FOODT)  -- must be able to eat
end;

class COW < $HERBIVORE{GRASS} is
	-- blah blah
end;

class HOG < $HERBIVORE{CORN} is
	-- blah blah
end;

class MONOCULTUREFARM{FOODT, ANIMALT < $HERBIVORE{FOODT} }  is
	attr livestock : LIST{ANIMALT};  -- type is known at compile time
	attr foodstock : LIST{FOODT};	 -- type is known 

	dinnertime is
	    loop
		elsie ::= livestock.elt!;
		dinner ::= foodstock.elt;

		elsie.eat(dinner);	-- sooouie! no run-time check needed.
	    end;
	end;
end;

class FREEFORMFARM is
	attr livestock : LIST{$HERBIVORE};
	attr foodstock : LIST{$PLANT};

	dinnertime is 
	    loop
		animal ::= livestock.elt!;
		dinner ::= foodstock.elt!;

		animal.eat_or_barf(dinner);  -- run-time check in there.
	    end;
	end;

end;


In Cluster I guess, you could have written MONOCULTUREFARM with
just one type parameter (the ANIMAL) and then extract its preferred
food from that.  You can't do that in Sather I don't think but
there isn't so much harm, as it's a compile time error to instantiate it
with a bad combination of herbivore and food.

cheers
Matt

--
-Matt Kennel  		mbk@inls1.ucsd.edu
-Institute for Nonlinear Science, University of California, San Diego
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