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Re: Function that changes value of a variable

From: Stefan Monnier
Subject: Re: Function that changes value of a variable
Date: Sat, 20 Aug 2022 22:40:15 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux)

> 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.  ]

> 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, 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) ...)

Of course a simpler solution is to do something else, such as:

    (setq pingu (my-constrain pingu ...))

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


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