This system provides a macro which acts like setf but does not change
original data-structure. It will be useful if you want to write the code
in a functional style, without side-effects:
POFTHEDAY> (defparameter *foo*
'(#(1 2 3 4 5)
#(6 7 8)
#(9)))
POFTHEDAY> (defparameter *bar*
(modf:modf (aref (second *foo*)
1)
:blah))
POFTHEDAY> *foo*
(#(1 2 3 4 5)
#(6 7 8)
#(9))
POFTHEDAY> *bar*
(#(1 2 3 4 5)
#(6 :BLAH 8)
#(9))
Resulting data structure can share some parts with the original:
POFTHEDAY> (eq *foo*
*bar*)
NIL
POFTHEDAY> (eq (first *foo*)
(first *bar*))
T
POFTHEDAY> (eq (second *foo*)
(second *bar*))
NIL
POFTHEDAY> (eq (third *foo*)
(third *bar*))
T
But this does not work for some lisp forms. For example, it is impossible
to change plist using getf:
POFTHEDAY> (defparameter *foo*
'(:name "Bob"
:points #(1 2 3 4 5)))
POFTHEDAY> (defparameter *bar*
(modf:modf (aref (getf *foo*
:points)
2)
100500))
; Debugger entered on #<SIMPLE-ERROR "How shall I invert GETF?">
Hopefuly, the library can be extended and we might define our own function to modify plists:
POFTHEDAY> (modf:define-modf-function getf 2 (new-val plist key)
(let ((new-plist (copy-list plist)))
(setf (getf new-plist key)
new-val)
new-plist))
POFTHEDAY> (defparameter *bar*
(modf:modf (aref (getf *foo*
:points)
2)
100500))
POFTHEDAY> *foo*
(:NAME "Bob" :POINTS #(1 2 3 4 5))
POFTHEDAY> *bar*
(:NAME "Bob" :POINTS #(1 2 100500 4 5))
There are a few other ways to extend modf. They are listed in the
documentation but there are no examples. But you can find them in the source code.