Changing value of a function argument

Question

Have been playing with the following function

(defun ignition (featr actm)
  "TODO."

  (when (eq 'sweep featr) (setq featr 'icomplt))
  (message "%S" featr))

Calling the following sequence of commands

(setq featr 'sweep)
(ignition featr actm)
(message "%S" featr)

gives the following result

icomplt
sweep

Meaning that inside the function, the argument featr is only changed locally, whereas getting back from the function keeps the value of featr to sweep before calling the function.

Is this the way things work in elisp? Does changing the value of an argument inside a function tolerable?

Would programmers looking at the code usually complain if someone changes argument value inside the function?

With the transition to lexical binding, would this code still be valid?

Answer 1

To understand how this works, you should read about Scoping Rules for Variable Bindings.

The paramaters of the function definition become local variables within the function body.

If you would like to set the global value of 'featr' then use:

(defun ignition (featr actm)
  "TODO."

  (when (eq 'sweep featr) (setq featr 'icomplt))
  (print featr))

(setq featr 'sweep actm 'test)
(setq featr (ignition featr actm))
(message "%S" featr)