Slide 14.18: Facts with arguments (cont.)
Slide 14.20: Variables and unification (cont.)
Home

Variables and Unification

How do we say something like “What does Fred eat?” Suppose we had the following fact in our database: 
   eats( fred, mangoes ).
How do we ask what fred eats? We could type in something like 
   ?- eats( fred, what ).
However Prolog will say no. The reason for this is that what does not match with mangoes. In order to match arguments in this way we must use a variable. The process of matching items with variables is known as unification. Variables are starting with a capital letter, for example, 
   X             /* a capital letter */
   VaRiAbLe      /* a word——either case of letters */
   My_name       /* linking words together via '_' */
Thus returning to our first question we can find out what fred eats by typing 
   ?- eats( fred, What ).
   What = mangoes
   yes
As a result of this query, the variable What has matched (or unified) with mangoes. We say that the variable What now has the binding mangoes. When we pose a query, if the query is successful, Prolog prints both the variable and the variable name, as we see above.

Variable Example I
Let's consider some examples using facts. First consider the following database. 
   loves( john, mary ).
   loves( fred, hobbies ).