Handling empty function optional argument [duplicate]

Question

This function starts a new named shell buffer. How can I handle the case where bufrnm set by interactive in empty?

(defun galaxy-shell (&optional bufrnm)
  "TODO"

  (interactive "s Buffer_name: ")

  (if bufrnm
      (let ( (bufrnam (generate-new-buffer-name bufrnm)) )
         (shell bufrnam))
    (let ( (bufrnam (generate-new-buffer-name "foo")) )
       (shell bufrnam)))

Answer 1

EDIT in response to your 'extended/edited' question

I think you should think about how to simplify this yourself. But let me show you two options, then you can come up with, arguably, the most straightforward simplification, using if i.c.w. or

(shell
 (generate-new-buffer-name
  (pcase buffer-name
    ((or "" (pred null)) "galaxy")
    (name name))))

or

(when (string-blank-p buffer-name)
  (setq buffer-name nil))
(shell
 (generate-new-buffer-name
  (or buffer-name "galaxy")))

You can clean it up a little more by making the buffer-name argument mandatory, so that you do not have to check for the argument being nil.

END EDIT

I would suggest that you experiment a little using the scratch buffer

(defun galaxy-shell (&optional buffer-name)
  (interactive "sEnter buffer name: ")
  (print buffer-name))

Try calling it interactively and non-interactively (as making the argument optional suggests that you would like to support non-interactive calls also).

Now that you have checked the 'empty' value(s) of buffer-name, have a look at string-empty-p (and null).

Answer 2

Have come up with this solution. Can it be simplified?

(defun galaxy-shell (&optional bufrnm)
  "Make a console terminal subshell that works through an emacs buffer.
BUFRNM  Name used to set the title of the buffer."

  (interactive "s Buffer_name: ")

  (if bufrnm

      (if (string-blank-p bufrnm)
          (shell (generate-new-buffer-name "galaxy"))
        (shell (generate-new-buffer-name bufrnm)))

    (shell (generate-new-buffer-name "galaxy"))))

Answer 3

The real answer is that you can do algebra on lisp expressions. In algebra we learn that multiplication is distributive, meaning that ax+ay is equivalent to a(x+y). The distributive property lets us factor out the common expression, rewriting the expression so that the a is no longer repeated. We can do the same in Lisp. Starting with this code:

(if (string-blank-p bufrnm)
    (shell (generate-new-buffer-name "galaxy"))
  (shell (generate-new-buffer-name bufrnm)))

We can factor out the common call to shell, like this:

(shell
  (if (string-blank-p bufrnm)
      (generate-new-buffer-name "galaxy")
    (generate-new-buffer-name bufrnm)))

We can further factor out the call to generate-new-buffer-name:

(shell
  (generate-new-buffer-name
    (if (string-blank-p bufrnm)
        "galaxy"
      bufrnm)))

Not every programming language gives us this power. Those that do not are inferior and aren’t worth discussing here. Those that do are generally called functional programming languages.