DEFSTRUCT-PRINT-FUNCTION-INHERITANCE:INCLUDE another structure type and do not specify a :PRINT-FUNCTION inherit the :PRINT-FUNCTION of the parent structure type. While it is stated on page 314 that #S syntax is used if a :PRINT-FUNCTION is not specified, the language on page 313 indicates that all operations on the parent type will also work on objects of the child type. Because of the ambiguity, existing implementations have gone both ways, and users cannot depend on either #S syntax or the parent type's :PRINT-FUNCTION being used.:INCLUDE another type but do not specify an explicit :PRINT-FUNCTION inherit the structure print function from the :INCLUDE'd type. A print function that uses the default #S syntax (overriding any print function for the parent type) can be specified by providing the :PRINT-FUNCTION option without an argument.
Supplying a :PRINT-FUNCTION in a DEFSTRUCT is equivalent to defining an appropriate method on the PRINT-OBJECT generic function.
:INCLUDEs this type will also contain the same slots; it seems more reasonable to inherit the parent's print function than to use the default #S syntax.:PRINT-FUNCTION is specified.The original version of the proposal did not provide for a way to force #S syntax to be used. This functionality was added to the current version because there seemed to be general agreement that it would be useful. Other alternatives would be to name the function that does what the #S printer does, or to provide a function that returns the #S information as a list (as suggested by Waters) so users can write their own.