How do I skip minibuffer completion in `completing-read`?

Question

In a function X that calls completing-read, I'd like to be able to call X and pass in a constant input without invoking the minibuffer. From the emacs manual, it seems I'm supposed to flet completing-read-function, but I'm not sure the correct way to do this.

Answer 1

Assuming that you want the constant input to be optional, you can declare the function as taking an optional argument and then check if the argument is actually passed: if it is, go ahead and use it; if it isn't, then call completing-read.

Something like this:

(defun X (&optional foo)
  (message (or foo (completing-read "Enter foo value: " nil)))

where I assume that the value of foo is a string.

If you call it with M-: (X), then foo will be nil and you will get the prompt. If you call it with M-: (X "bar") then no prompt will be issued and the value of foo will be the string bar.