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(librep.info)Property Lists

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Property Lists
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   Each symbol has a property list (or "plist"), this is a structure
which associates an arbitrary Lisp object with a key (usually a
symbol). The keys in a plist may not have any duplications (so that
each property is only defined once).

   The concept of a property list is very similar to an association list
(Note: Association Lists) but there are two main differences:

  1. Structure; each element of an alist represents one key/association
     pair. In a plist each pair of elements represents an association:
     the first is the key, the second the property. For example, where
     an alist may be,

          ((one . 1) (two . 2) (three . 3))

     a property list would be,

          (one 1 two 2 three 3)

  2. Plists have their own set of functions to modify the list. This is
     done destructively, altering the property list (since the plist is
     stored in only one location, the symbol, this is quite safe).

 - Function: get symbol property
     This function searches the property list of the symbol SYMBOL for
     a property `equal' to PROPERTY. If such a property is found it is
     returned, otherwise false is returned.

          (get 'if 'lisp-indent)
              => 2
          
          (get 'set 'lisp-indent)
              => ()

 - Function: put symbol property new-value
     `put' sets the value of the property PROPERTY to NEW-VALUE in the
     property list of the symbol SYMBOL. If there is an existing value
     for this property (using `equal' to compare keys) it is
     overwritten. The value returned is NEW-VALUE.

          (put 'foo 'prop 200)
              => 200

 - Function: symbol-plist symbol
     Returns the property list of the symbol SYMBOL.

          (symbol-plist 'if)
              => (lisp-indent 2)

 - Function: setplist symbol plist
     This function sets the property list of the symbol SYMBOL to PLIST.

          (setplist 'foo '(zombie yes))
              => (zombie yes)