Scheme. Reduce & filter

(define filter
  (lambda (fn values)
    "This function is a tail recursive implementation of the filter function
    found in other Lisp dialect."
    (let loop ((v values)
               (rest '()))
      
      (let ((item (car v))
            (next (cdr v)))             ; Saves the car and cdr for efficiency.

        (if (fn item)
          ;; Add the current 'item to 'rest if 'fn returned #t, then continue
          ;; though the list. Return 'this AND 'rest revsered.
          (if (pair? next)
            (loop next (cons item rest))
            (reverse (cons item rest)))
            
          ;; Don't add the 'item to the 'rest list but continue on though the
          ;; list.
          (if (pair? next)
            (loop next rest)
            (reverse rest)))))))

(define filter
  (lambda (fn values)
    "This function is a tail recursive implementation of the filter
    function found in other Lisp dialect."
    (let loop ((v values)
               (rest '()))

      ;; If there are items in v then CAR and CDR are saved; return
      ;; rest after reversing.
      ;;
      ;;   Checks if fn  => #t  -> add item to rest & repeat.
      ;;                 => #f  -> loop without adding item.
      (if (pair? v)
        (let ((item (car v))
              (next (cdr v)))
          (if (fn item)
            (loop next (cons item rest))
            (loop next rest)))
          (reverse rest)))))