Imagine you have the following input of a list of <code>(group value)</code> pairs:

  (setq input '(("fruit" "apple") ("veg" "cucumber") ("fruit" "banana")
                ("veg" "cabbage") ("fruit" "orange") ("fruit" "grapes")))


and you wish to transform it such that all the values are grouped together, with either ''"fruit"'' or ''"veg"'' taking the ''car'' position, to yield:

  '(("veg" "cucumber" "cabbage") ("fruit" "apple" "banana" "orange" "grapes"))


== Method 1: Build an Alist ==

Here we will iterate through each pair, appending a fruit or veg type into our outmap variable. 

  (let ((outmap nil)
        (testfn 'string=)) ;; use string equality as the test function
    (dolist (paired input outmap)
      (let ((group (first paired)) ;; "fruit" or "veg"
            (value (last paired))) ;; "apple" or "cucumber" or ...
        (if (map-contains-key outmap group testfn)
            ;; Append value to the group if the group already exists
            (let ((mapvals (map-elt input group nil testfn)))
              (map-put outmap group (append mapvals value) testfn))
          ;; Otherwise, initialise key value pair
          (map-put outmap group value testfn)))))
  ;; outmap => (("veg" "cucumber" "cabbage") ("fruit" "apple" "banana" "orange" "grapes"))


== Method 2: Group by Car ==

This more elegant solution ([https://github.com/alphapapa/emacs-package-dev-handbook/pull/11#issuecomment-579008184 courtesy of AlphaPapa]) performs an initial pass through the list, where it groups the items via the <code>seq-group-by function</code>, and then uses the <code>--map</code> function to replace each pair with its second element.

   (->> '(("fruit" "apple") ("veg" "cucumber") ("fruit" "banana")
       ("veg" "cabbage") ("fruit" "orange") ("fruit" "grapes"))
     (seq-group-by #'car))
   ;;=> (("veg" ("veg" "cucumber") ("veg" "cabbage"))
   ;;    ("fruit" ("fruit" "apple") ("fruit" "banana") ("fruit" "orange") ("fruit" "grapes"))
   (--map (cons (car it) (-map #'cadr (cdr it)))))
   ;;=> (("veg" "cucumber" "cabbage") ("fruit" "apple" "banana" "orange" "grapes"))