mapcar but return non-nil element only

Question

Is there a function that applies a transformation to a sequence and returns only the non-nil values?

Right now I am using the following (as an example):

(seq-filter #'identity (mapcar (lambda (x) 
                                 (when (< x 3) (+ x 5)))
                               '(1 2 3 4 5 6)))

Or through dolist

(defun mapcar-true (func list)
  (let (value)
    (dolist (elt list value)
      (when-let ((trans (funcall func elt)))
        (setq value (append value (list trans)))))))

But I feel there must be a more straightforward way.

Answer 1

Use remq or delq to remove nil elements from a list (to remove more complex structures, consider remove and delete, too).

(remq nil (mapcar (lambda (x) 
                    (when (< x 3) (+ x 5)))
                  '(1 2 3 4 5 6)))

The first example can be also simplified using flatten-tree as it will remove empty lists, i.e. nils.

(flatten-tree (mapcar (lambda (x) 
                        (when (< x 3) (+ x 5)))
                      '(1 2 3 4 5 6)))

Answer 2

Is there a function that applies a transformation to a sequence and returns only the non-nil values?

Since Emacs 26, you can use mapcan:

(mapcan (lambda (n)
          (and (< n 3) (list (+ n 5))))
        (number-sequence 1 6))

Alternatively, there's the age-old mapcar+delq approach like in choroba's answer (delq is more commonly used than remq, since mapcar already returns a fresh list, so there's no need to copy it first).

Which of the two approaches to use usually boils down to personal preference and/or performance profiling.

Personally, I'd submit a feature request via M-x report-emacs-bug for such a function to be added to seq.el.