Re: Function that changes value of a variable

[wilnerthomas]

Aug 21, 2022, 02:40 by help-gnu-emacs@gnu.org:

>> I am trying to change the value a a variable with the following function,
>> but is not working as intended.
>>
>> (defun constrain (var min-n max-n)  "Ensure that value of var is between
>> MIN-N and MAX-N inclusive by constraining."  (cond   ((< var min-n)  (setq
>> var min-n))   ((> var max-n)  (setq var max-n))   ((eq var t)     (setq var
>> 1))   ((eq var nil)   (setq var 0))))
>>
>
> [ Written this way, it's rather illegible.  It was probably manged by your
>  MUA because it wasn't labeled as "code" or "preformatted text"
>  or somesuch.  ]
>
This time it should show properly

(defvar pingu 21)

(defun constrain (variable min-n max-n)
  "Ensure that value of variable is between MIN-N and MAX-N inclusive by 
constraining."

  (cond
   ((< (symbol-value variable) min-n)  (set variable min-n))
   ((> (symbol-value variable) max-n)  (set variable max-n))
   ((eq (symbol-value variable) t)     (set variable 1))
   ((eq (symbol-value variable) nil)   (set variable 0))))

(constrain 'pingu 0 8)



>> Have done the following test, but the value stays 21.
>>
>
> Of course, because the call only passes the value of the variable, not
> the variable itself [ and according your your comment in
> https://emacs.stackexchange.com/questions/73192, you know that
> already.  ]
>
> Variables aren't first class objects, so you need to use a reference to
> the variable:
>
>  (defun my-constrain (varef ...)
>  ... (setf (gv-deref varref) ...)
>  ...)
>
> You can then pass a reference to your variable with something like:
>
>  (my-constrain (gv-ref pingu) ...)
>

Not so great with reference


> Of course a simpler solution is to do something else, such as:
>
>  (setq pingu (my-constrain pingu ...))
>
This looks standard, which is good.


> so `my-constrain` doesn't need to set any variable any more and can be
> a pure function instead.
>
>  Stefan
>
Have done 

(defun constrain (variable min-n max-n)
  "Returns value between MIN-N and MAX-N inclusive by constraining."

  (cond
     ((< (symbol-value variable) min-n)  min-n)
     ((> (symbol-value variable) max-n)  max-n)
     ((eq (symbol-value variable) t)  1)
     ((eq (symbol-value variable) nil)  0) ))

(defvar pingu 21)
(setq pingu (constrain pingu 0 8))

Which gives me 

Lisp error: (wrong-type-argument number-or-marker-p nil)
  <(nil 0)
(cond ((< (symbol-value variable) min-n) min-n) ((> (symbol-value variable) 
max-n) max-n) ((eq (symbol-value variable) t) 1) ((eq (symbol-value variable) 
nil) 0))
  constrain(nil 0 8)
  (setq pingu (constrain pingu 0 8))
  (progn (setq pingu (constrain pingu 0 8)))
  eval((progn (setq pingu (constrain pingu 0 8))) t)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)