rle.js

var S = ['a', 'a', 'a', 'a', 'b', 'c', 'c', 'a', 'a', 'd', 'e', 'e', 'e', 'e'];

var pack = L.partial1(L.partitionBy, _.identity);

pack(S);
//=> [['a', 'a', 'a', 'a'],['b'],['c', 'c'],['a', 'a'],['d'],['e', 'e', 'e', 'e']]

var rle = _.compose(
  L.partial1(L.flip(_.map), 
             L.juxt(L.plucker('length'), _.first)), 
  pack);

rle(S);
//=> [[4, 'a'], [1, 'b'], [2, 'c'], [2, 'a'], [1, 'd'], [4, 'e']]

var rld = _.compose(_.flatten, L.partial1(L.flip(_.map), L.splat(L.repeat)));

L.pipeline(S, rle, rld);

//=> ["a", "a", "a", "a", "b", "c", "c", "a", "a", "d", "e", "e", "e", "e"]