Operators

Operators in Prolog are simply a notational convenience. We could represent the English statement ``2 plus 3'' in Prolog as +(2, 3) but this isn't the way we are accustomed to seeing the expression. A more familiar notation involves making the + an INFIX operator so that we can write 2 + 3. When an operator is defined it is also allocated two descriptive attributes:

• Its precedence. This is a number between 1 and 1200 which allows the Prolog system to decide the principal functor of any nested term. For example, the precedence of + is 500 while the precedence of * (the multiplication operator) is 400. The rule is that in a nested term the operator with highest precedence value is the principal functor. For instance, the term 2 + 3 * 5 has principal functor +. If we were to write it in standard Prolog syntax this term would look like this: +(2,*(3,5)). NOTE that terms representing numerical expressions are NOT automatically evaluated, like in a functional language. There is a special built--in predicate for doing this, which we shall meet later on.
• Its type. This defines whether the operator is:
  • Prefix, in which case it is allocated type fx or fy.
  • Postfix, in which case it is allocated type xf or yf.
  • Infix, in which case it is allocated type xfx, xfy or yfx.
  Some operators may have more than one type. For example the + operator is both prefix (e.g. + 2) and infix (e.g. 2 + 3).

You can check what the current operators in your Prolog system are by using the current_op/3 built-in predicate. You can also define your own operators using the built--in op/3 predicate but we won't worry about this yet. To display a term containing operators in standard Prolog syntax, use the built-in predicate display/1.

| ?- current_op(Precedence, Type, +).

     Precedence=500
     Type=fx ;

     Precedence=500
     Type=yfx 
yes
| ?- current_op(Precedence, Type, *).

     Precedence=400
     Type=yfx
yes
| ?- display(2 + 3 + 4).
+(+(2,3),4)
yes