ELisp: Place Expression, Generalized Variable
setf

(setf PLACE VAL PLACE VAL etc)
Set each PLACE to VAL.
Each PLACE should be a symbol, or lisp expression for a sequence's element, such as
(car xlist)
(aref xvector i)
(nth n xlist)
(elt xsequence n)
these “place forms” are called generalized variable or place expressions.
return the last VAL in the list.
;; example of place expression (setq xlist (list 3 4 5)) ;; the nth form is a place expression, aka generalized variable (setf (nth 0 xlist) 9) (eq (nth 0 xlist) 9) ;; t
What is Place Expression, Generalized Variable, in Lisp
In many programing languages, you can change a
value in a array by the normal assignment operator of equal sign=
.
For example in python:
xlist = [3, 4, 5] # here, the xlist[0] is considered a generalized variable, or place expression # it's special because it is act not as a normal variable name but act as a position in a list xlist[0] = 9 print(xlist[0] == 9) # True
in lisp syntax, this would be like this:
;; example of place expression (setq xlist (list 3 4 5)) ;; the nth form is a place expression, aka generalized variable (setf (nth 0 xlist) 9) (eq (nth 0 xlist) 9) ;; t
In the lisp syntax, the oddness became apparent. Because lisp syntax usually follows a consistant form, and arguments to a function are evaluated.
But here, we see that the argument
(nth 0 xlist)
is not evaluated, but worse, it has a special meaning,
when it is placed inside setf
.
It now has a context dependent semantics.