How to find variable in package using string and change it? - package

Question

I have package with some variables:
(in-package #:pack)
(defparameter *a* "foo")
(defparameter *b* "bar")
Variables *a* and *b* are exported.
My goal is change this variables using strings ("*a*" and "*b*")
I can find these variables with intern function
(symbol-value (intern (string-upcase "*a*") :pack)) ;; return *a* symbol
(symbol-name (intern (string-upcase "*a*") :pack)) ;; return "foo"
But if I trying to change symbol with
(setf (symbol-name (intern (string-upcase "*a*") :pack)) "baz")
I've got an error:
The function (COMMON-LISP:SETF COMMON-LISP:SYMBOL-NAME) is undefined.
[Condition of type UNDEFINED-FUNCTION]
How can I change these variables?

Answer 1

As pointed in a comment #jkiiski, there is function in common lisp used to find symbols in a package:
find_symbol:
Function FIND-SYMBOL
Syntax:
find-symbol string &optional package => symbol, status
Then the other important thing is take a look at the answer here for understanding the difference between variables and symbols in common lisp.
And the function set, setq and setf from common lisp hypersec
set changes the contents of the value cell of symbol to the given
value.
setq(setq var1 form1 var2 form2 ...) is the simple variable assignment
statement of Lisp. First form1 is evaluated and the result is stored
in the variable var1, then form2 is evaluated and the result stored in
var2, and so forth. setq may be used for assignment of both lexical
and dynamic variables.
(setf place newvalue) expands into an update form that stores the
result of evaluating newvalue into the location referred to by place.
Some place forms involve uses of accessors that take optional
arguments. Whether those optional arguments are permitted by setf, or
what their use is, is up to the setf expander function and is not
under the control of setf. The documentation for any function that
accepts &optional, &rest, or ..... key arguments and that claims to be
usable with setf must specify how those arguments are treated.
So joining all this things we can have this code:
(defpackage :pack
(:use :cl) ;; You must import this line in order to use functions from cl package like setf
(:export :*a*
:*b*))
(in-package :pack)
(defparameter *a* "foo")
(defparameter *b* "bar")
Now lets explore the variables
CL-USER> pack:*a*
"foo"
CL-USER> pack:*b*
"bar"
Then we can find the symbol a (note that common lisp is case insensitive if you don't use || for naming symbols)
CL-USER> (find-symbol "*A*" :pack)
PACK:*A*
:EXTERNAL
So here, there is nothing unexpected exists symbol a and it is external to the package pack
then let's change the value wit set we can directly change the value but with setf we need a place, a pointer in the case of common lisp a symbol have different pointers in this case the important for us is symbol-value. so for changing that we can proceed as follow:
CL-USER> (set (find-symbol "*A*" :pack) "baz")
"baz"
CL-USER> pack:*a*
"baz"
CL-USER> (setf (symbol-value (find-symbol "*A*" :pack)) "buzz" )
"buzz"
CL-USER> pack:*a*
"buzz"
Hope this helps.