Completing read with must-match and cannot be null

Question

The function completing-read permits the user to simply press the enter key without selecting a match against the list of valid choices:

(let* ((completion-ignore-case t)
       (client (completing-read "CLIENT:  " '("a" "b" "c" "d") nil t)))
  client)

Q: Is there a similar completion function that will prevent the user from exiting by simply pressing the enter key without making a valid choice?

Answer 1

I'd do this (similar to @lawlist's answer):

(let ((completion-ignore-case t)
      (client ""))
  (while (equal client "")
    (setq client  (completing-read "CLIENT:  " '("a" "b" "c" "d") nil t)))
  client)

Answer 2

See the answer by Stefan in a similar thread on stackoverflow.com [ https://stackoverflow.com/a/12824748/2112489 ] that states in relevant part:

My guess is that you want to prevent the user from hitting RET with an
empty answer.  Indeed `completing-read' does not prevent that, even
with `require-match' set.  The way this is usually handled is by using
a non-nil value for the `default' argument, in which case this value
is returned when the user just hits RET.

If that's not good enough, then you're probably going to have to use
`minibiffer-with-setup-hook' and in the hook, setup a special keymap
you've created for this purpose where RET is bound to a new function
that signals an error if the minibuffer is empty and calls
`minibuffer-complete-and-exit' otherwise.

Another option is to use a while loop:

(let ((client (catch 'break
                (while t
                  (let* ((completion-ignore-case t)
                         (choice (completing-read "CLIENT:  " '("a" "b" "c" "d") nil t)))
                    (if (equal choice "")
                      (message "Please select a valid choice!")
                      (throw 'break choice)))))))
  client)