%%
%%			 Demo by Andy Long, for Mat385
%%
%% Please compile this file into SWI-Prolog with the command
%% [mat385].
%%
%% The period at the end is necessary!
%%
%% This is an implementation of Example 39, p. 64, of Gersting.
%% 
%% Note that constants are preceded by small letters, whereas variables are
%% preceded by capitals.
%% 
%% Once this file is "compiled", then the following commands will
%% give you some practice:
%%
%% ?- eat(bear,fox).
%% ?- eat(fox,bear).
%% ?- prey(bear).
%% ?- prey(fox).
%% ?- prey(little-fish).
%%
%% ?- prey(X).
%% will return the first item prolog finds which satisfies prey(X). If 
%% you type a semi-colon (;), prolog will find the next item satisfying
%% prey(X), and so on until it has found all examples.
%% 
%% You can check to see if something is in the database by using the same
%% syntax as we use to create our database below:
%%
%% ?- eat(bear,fish).
%%
%% will return "Yes", since it is in the database, whereas
%%
%% ?- eat(bear,grass).
%%
%% will return "No".
%%
%% As above, you can type 
%% ?- eat(bear,X).
%% to systematically get a printout of which things a bear eats.
%%
%% Try these:
%%
%% ?- in-food-chain(Y,grass)
%% ?- in-food-chain(bear,Y)
%%
%% NOTE: in order to negate a command, use "\+", as in 
%% \+(eat(fox,rabbit)).
%%
%%
%% NOTE: You can "trace" execution of your command by first issuing the command
%% trace. 
%%
%%		 The following commands create a database of "facts":
%%
eat(bear,fish).
eat(fish,little-fish).
eat(little-fish,algae).
eat(raccoon,fish).
eat(bear,raccoon).
eat(bear,fox).
eat(fox,rabbit).
eat(rabbit,grass).
eat(bear,deer).
eat(deer,grass).
eat(wildcat,deer).
eat(fish,bear).

animal(bear).
animal(fish).
animal(little-fish).
animal(raccoon).
animal(fox).
animal(rabbit).
animal(deer).
animal(wildcat).

plant(grass).
plant(algae).

%%
%% 	Now we'll create some interesting "rules" that we can use:
%%
%%
%% We'll give an example using a logical "or", and a logical "and".
%%
%% the comma creates a logical "and": prey(P) will only be true if animal(P)
%% and there is some Q such that Q eats P.
prey(Animal) :-
	    animal(Animal),
	    eat(Preditor,Animal).

%% by including two different "standards" for deciding if something is living,
%% we create a logical "or". isliving(P) will be true if either animal(P) or
%% plant(P) is true:
isliving(P) :-
	    animal(P).
isliving(P) :-
	    plant(P).

%% 
%% Here's the recursive definition:
%% 
%% We'd start with the base case, in the event that High actually
%% eats Low:
in-food-chain(High,Low) :-
	eat(High,Low).
%% then we move on to the case in which there is a middle level
%% of critters:
in-food-chain(High,Low) :-
	eat(High,Middle),
	in-food-chain(Middle,Low).