Variables

A variable is:

• Any sequence of characters, numbers and/or the underscore character which is preceded by an upper case character (e.g. This_IS_a_variable).
• Any sequence of characters, numbers and/or the underscore character which is preceded by an underscore (e.g. _this_IS_a_variable).
• An underscore character by itself (e.g. _). The underscore character is referred to as the anonymous variable because it cannot be referenced by its name in other parts of the clause.

The Prolog built-in predicate var/1 will tell you if a given term is or isn't a variable. It gives the following behaviour:

 
| ?- var(Foo).
     Foo=_125459 
yes
| ?- var(_).
yes
| ?- var(foo).
no