isearch-forward, call-interactively, and isearch-filter-predicate

Question

Here are two use cases where I call isearch-forward with some filter predicate.

(let ((isearch-filter-predicate (lambda (a b) nil)))
  (funcall 'isearch-forward))

(let ((isearch-filter-predicate (lambda (a b) nil)))
  (call-interactively 'isearch-forward))

Why is it that only the first one works? Is there a way to have the call-interactively call take isearch-filter-predicate into account?

It seems I need call-interactively if I want commands that leave isearch (e.g., C-n) to be executed properly (that is, C-s C-n should go to the next line).

Answer 1

The answer is given in the doc string of isearch-forward:

If this function is called non-interactively with a nil NO-RECURSIVE-EDIT, it does not return to the calling function until the search is done. See the function isearch-mode for more information.

In your first form

(let ((isearch-filter-predicate (lambda (a b) nil)))
  (funcall 'isearch-forward))

you use a nil value for the optional argument no-recursive-edit of isearch-forward. In that case the incremental search runs in a recursive-edit call within isearch-forward and the control flow does not leave the local scope of your let-form while isearch-mode is active.

In the second form

(let ((isearch-filter-predicate (lambda (a b) nil)))
  (call-interactively 'isearch-forward))

you start the isearch with the interactive call of isearch-forward. The command isearch-forward just starts isearch-mode and exits. The control flow is not within the isearch-forward function form while the incremental search is running.

An alternative way to call isearch-forward with user-defined isearch-filter-predicate that uses call-interactively is given in the answer to the question about searching for bold text.