Operators

Before looking at other examples of recursion, it will be helpful to look at operators, because these are used in Prolog arithmetic. The basic arithmetic operations of addition, subtraction, multiplication etc., could be treated as predicates within the kind of Prolog style you have encountered so far. Addition, for example, might look like this:

+(X, Y, Z)

where X and Y are the arguments to be added, and Z is the result of the addition. Indeed, some Prologs allow you to do addition like this. Many more Prologs, however, define arithmetical operations in a way which is much more like the standard conventions, and write the above equation as

Z is X + Y

This is a novel format, because it writes the predicate symbols is and + between their arguments, without brackets. This is because these two symbols have been defined within Prolog as infix operators. In working with operators, we need to pay attention to two factors: precedence and associativity.