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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
Followup-To: comp.object,comp.lang.eiffel,comp.lang.sather,comp.lang.beta,comp.lang.modula3
Date: 14 Jan 1995 23:22:07 GMT
Organization: Institute For Nonlinear Science, UCSD
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Mark S. Hathaway (hathawa2@marshall.edu) wrote:

: Also, notice that if you want a heterogeneous FARM you drop back to
: using HERBIVORE, but a HERBIVORE isn't necessarily a COW or HOG.  For
: a heterogeneous farm you need LIST[COW] & LIST[HOG].  To create a
: class which could represent either COW or HOG would be interesting.
: I haven't seen any instances of that.  What I have in mind is....

:    -- HERBIVORE is the base class
:       COW         is HERBIVORE;
:       HOG         is HERBIVORE;
:       FARM_ANIMAL is COW or HOG;
:       FARM        is LIST[FARM_ANIMAL];

-- Sather

type $HERBIVORE is
       try(food:$PLANT);
end;

type $HERBIVORE{MYFOOD < $PLANT} is
	try(food:$PLANT);
	eat(food:MYFOOD);
end;


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

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

type $IOWA_FARM_ANIMAL > COW,HOG is
      try(food:$PLANT);
end;

class IOWAFARM is
	-- has hogs and cows.

	attr animals: LIST{$FARM_ANIMAL}
	attr food : LIST{$PLANT};

	feed is
	   loop
		animals.elt!.try(food.elt!);
	   end;
	end;

	feed_corn is
	    -- Cluster does this one a little cleaner
	    loop
		f ::= food.elt!;	
		a ::= animals.elt!;
		typecase f
		when CORN
		    typecase a
		    when $HERBIVORE{CORN} then a.feed(f);  end;
		end;
	    end;
	end;
end;

In general Cluster lets you get at the foodtype for animals for run-time
selection OUTSIDE their class easily, in Sather it's usually easier to put
the typecase inside the 'try'.


: Mark S. Hathaway       <hathawa2@marshall.edu>

--
-Matt Kennel  		mbk@inls1.ucsd.edu
-Institute for Nonlinear Science, University of California, San Diego
-*** AD: Archive for nonlinear dynamics papers & programs: FTP to
-***     lyapunov.ucsd.edu, username "anonymous".