The predefined operators are shown in table 5. Operators can be redefined, unless prohibited by one of the limitations below. Applications must be careful with (re-)defining operators because changing operators may cause (other) files to be interpreted differently. Often this will lead to a syntax error. In other cases, text is read silently into a different term which may lead to subtle and difficult to track errors.

• It is not allowed to redefine the comma (',').
• The bar (|) can only be (re-)defined as infix operator with priority not less than 1001.
• It is not allowed to define the empty list ([]) or the curly-bracket pair ({}) as operators.

In SWI-Prolog, operators are local to a module (see also section 6.8). Keeping operators in modules and using controlled import/export of operators as described with the module/2 directive keep the issues manageable. The module system provides the operators from table 5 and these operators cannot be modified. Files that are loaded from the SWI-Prolog directories resolve operators and predicates from this system module rather than user, which makes the semantics of the library and development system modules independent of operator changes to the user module.