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(slib.info)Setters

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Setters
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  "Setters" implement "generalized locations" for objects associated
with some sort of mutable state.  A "getter" operation retrieves a
value from a generalized location and the corresponding setter
operation stores a value into the location.  Only the getter is named -
the setter is specified by a procedure call as below.  (Dylan uses
special syntax.)  Typically, but not necessarily, getters are access
operations to extract values from Yasos objects (Note: Yasos).
Several setters are predefined, corresponding to getters `car', `cdr',
`string-ref' and `vector-ref' e.g., `(setter car)' is equivalent to
`set-car!'.

  This implementation of setters is similar to that in Dylan(TM)
(`Dylan: An object-oriented dynamic language', Apple Computer Eastern
Research and Technology).  Common LISP provides similar facilities
through `setf'.

 - Function: setter getter
     Returns the setter for the procedure GETTER.  E.g., since
     `string-ref' is the getter corresponding to a setter which is
     actually `string-set!':
          (define foo "foo")
          ((setter string-ref) foo 0 #\F) ; set element 0 of foo
          foo => "Foo"

 - Syntax: set place new-value
     If PLACE is a variable name, `set' is equivalent to `set!'.
     Otherwise, PLACE must have the form of a procedure call, where the
     procedure name refers to a getter and the call indicates an
     accessible generalized location, i.e., the call would return a
     value.  The return value of `set' is usually unspecified unless
     used with a setter whose definition guarantees to return a useful
     value.
          (set (string-ref foo 2) #\O)  ; generalized location with getter
          foo => "FoO"
          (set foo "foo")               ; like set!
          foo => "foo"

 - Procedure: add-setter getter setter
     Add procedures GETTER and SETTER to the (inaccessible) list of
     valid setter/getter pairs.  SETTER implements the store operation
     corresponding to the GETTER access operation for the relevant
     state.  The return value is unspecified.

 - Procedure: remove-setter-for getter
     Removes the setter corresponding to the specified GETTER from the
     list of valid setters.  The return value is unspecified.

 - Syntax: define-access-operation getter-name
     Shorthand for a Yasos `define-operation' defining an operation
     GETTER-NAME that objects may support to return the value of some
     mutable state.  The default operation is to signal an error.  The
     return value is unspecified.