4.1.5.3 Syntax Restrictions

Note carefully the following syntax restrictions, which serve to remove potential ambiguities associated with prefix operators.

1. The arguments of a compound term written in standard syntax must be expressions of precedence less than 1000. Thus it is necessary to write the expression P:-Q in parentheses

             assert((P:-Q))
        

   because the precedence of the infix operator `:-', and hence of the expression P:-Q, is 1200. Enclosing the expression in parentheses reduces its precedence to 0.

2. Similarly, the elements of a list written in standard syntax must be expressions of precedence less than 1000. Thus it is necessary to write the expression P->Q in parentheses

             [(P->Q)]
        

   because the precedence of the infix operator `->', and hence of the expression P->Q, is 1050. Enclosing the expression in parentheses reduces its precedence to 0.

3. In a term written in standard syntax, the principal functor and its following `(' must not be separated by any intervening spaces, newlines, or other characters. Thus

             point (X,Y,Z)
        

   is invalid syntax.

4. If the argument of a prefix operator starts with a `(', this `(' must be separated from the operator by at least one space or other layout character. Thus

             :-(p;q),r.
        

   (where `:-' is the prefix operator) is invalid syntax. The system would try to interpret it as the structure:

                         ,
                        / \
                      :-    r
                       |
                       ;
                      / \
                     p   q
        

   That is, it would take `:-' to be a functor of arity 1. However, since the arguments of a functor are required to be expressions of precedence less than 1000, this interpretation would fail as soon as the `;' (precedence 1100) were encountered.

   In contrast, the term:

             :- (p;q),r.
        

   is valid syntax and represents the following structure:

                         :-
                          |
                          ,
                         / \
                        ;   r
                       / \
                      p   q