Elisp function return value

Question

I'm having a (probably) dumb problem with Elisp. I want a function to return t or nil depending on a when condition. This is the code:

(defun tmr-active-timer-p
  "Returns t or nil depending of if there's an active timer"
  (progn
    (if (not (file-exists-p tmr-file))
        nil
      ; (... more code)
      )
    )
)

But I'm having an error. I'm not sure how to make a function return a value... I've read a function return the last expression result value but in this case, I wan't to make something like (PHP mess warning):

// code

if ($condition) {
  return false;
}

// more code...

Maybe I'm missing the point and functional programming doesn't allow this approach?

Ok the problem was actually the lack of parenthesis before tmr-active-timer-p. — hhaamm
– hhaamm, Commented May 14, 2013 at 16:19

sds · Accepted Answer · 2017-08-22 13:44:14Z

18

First, you need an argument list after tmr-active-timer-p; the defun syntax is

(defun function-name (arg1 arg2 ...) code...)

Second, you do not need to wrap the body in progn.

Third, the return value is the last form evaluated. If your case you can just write

(defun tmr-active-timer-p ()
  "Returns t or nil depending of if there's an active timer."
  (when (file-exists-p tmr-file)
      ; (... more code)
    ))

then it will return nil if the file does not exist (because (when foo bar) is the same as (if foo (progn bar) nil)).

Finally, hanging parentheses are considered to be a poor code formatting style in lisp.

PS. Emacs Lisp does not have return, but it does have Nonlocal Exits. I urge you to avoid them unless you really know what you are doing.

edited Aug 22, 2017 at 13:44

answered May 14, 2013 at 16:47

sds

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1 Comment

hhaamm Over a year ago

Well, your answer was correct. Basically there is no return sintaxis. You cannot return easily from a middle of a function in Elisp and probably is wouldn't be a good idea because of the functional paradigm. Thanks for the advice with hanging parenthesis.

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