Slide 14.19: Variables and unification
Slide 14.21: Variables and unification (cont.)
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Variables and Unification (Cont.)

Variable Example I (Cont.) 
   loves( john, mary ).
   loves( fred, hobbies ).
Now let's look at some simple queries using variables 
   ?- loves( john, Who ).  /* Who does john love? */
   Who = mary              /* Yes, Who gets bound to mary */
   yes                     /*  and the query succeeds. */

   ?- loves( arnold, Who ) /* Does arnold love anybody? */
   no                      /* No, not john or fred. */

   ?- loves( fred, Who ).  /* Who does fred love? */
   Who = hobbies           /* Who is just a variable name */
   yes                     /* and it unifies with hobbies. */

Variable Example II
Here are some more difficult object/variable comparisons. Consider the following database showing a library of cassette tapes. Notice the final argument to tape, which is the name of a favourite song from the album. 
   tape( 1, van_morrison, astral_weeks, madam_george ).
   tape( 2, beatles, sgt_pepper, a_day_in_the_life ).
   tape( 3, beatles, abbey_road, something ).
   tape( 4, rolling_stones, sticky_fingers, brown_sugar ).
   tape( 5, eagles, hotel_california, new_kid_in_town ).
Let's now look at some queries. 
   ?- tape( 5, Artist, Album, Fave_Song ).
             /* What are the contents of tape 5? */
   Artist = eagles
   Album = hotel_california
   Fave_Song = new_kid_in_town
   yes