Emacs complaining with invalid function?

Question

When I press C-c c with the following code on a buffer, Emacs complains with Invalid function: (select-current-line). Why?

(defun select-current-line ()
  "Select the current line"
  (interactive)
  (end-of-line) ; move to end of line
  (set-mark (line-beginning-position)))

(defun my-isend ()
  (interactive)

  (if (and transient-mark-mode mark-active)
      (isend-send)

    ((select-current-line)
     (isend-send)))
)

(global-set-key (kbd "C-c c") 'my-isend)

Not that it matters, but for those interested isend-send is defined here.

Answer 1

You are missing a progn form to group statements together:

(defun my-isend ()
  (interactive)

  (if (and transient-mark-mode mark-active)
      (isend-send)

    (progn
      (select-current-line)
      (isend-send))))

---

Without the progn form, ((select-current-line) (isend-send)) is interpreted as the (select-current-line) function applied to the result of calling isend-send without arguments. But (select-current-line) is not a valid function name. In other LISPs, such a construct could be valid if the return value of select-current-line was itself a function, which would then be applied to (isend-send). But this is not the case of Emacs LISP and this would not do what you wanted to achieve anyway...