Info Catalog elisp: Property Lists elisp: Plist Access

elisp: Plists and Alists

 
 5.9.1 Property Lists and Association Lists
 ------------------------------------------
 
 Association lists (Association Lists) are very similar to
 property lists.  In contrast to association lists, the order of the
 pairs in the property list is not significant, since the property names
 must be distinct.
 
    Property lists are better than association lists for attaching
 information to various Lisp function names or variables.  If your
 program keeps all such information in one association list, it will
 typically need to search that entire list each time it checks for an
 association for a particular Lisp function name or variable, which could
 be slow.  By contrast, if you keep the same information in the property
 lists of the function names or variables themselves, each search will
 scan only the length of one property list, which is usually short.  This
 is why the documentation for a variable is recorded in a property named
 ‘variable-documentation’.  The byte compiler likewise uses properties to
 record those functions needing special treatment.
 
    However, association lists have their own advantages.  Depending on
 your application, it may be faster to add an association to the front of
 an association list than to update a property.  All properties for a
 symbol are stored in the same property list, so there is a possibility
 of a conflict between different uses of a property name.  (For this
 reason, it is a good idea to choose property names that are probably
 unique, such as by beginning the property name with the program’s usual
 name-prefix for variables and functions.)  An association list may be
 used like a stack where associations are pushed on the front of the list
 and later discarded; this is not possible with a property list.
 

Info Catalog elisp: Property Lists elisp: Plist Access