4.7 Syntax Restrictions

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

1. In a term written in standard syntax, the principal functor and its following `(' must not be separated by any intervening layout-text. Thus

             point (X,Y,Z)
        

   is invalid syntax.

2. If the argument of a prefix operator starts with a `(', this `(' must be separated from the operator by at least one layout-char. Thus

             :-(p;q),r.
        

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

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

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

   In contrast, the term:

             :- (p;q),r.
        

   is valid syntax and represents the following compound term:

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