Counting occurences (solution to the exercise)

What does (merge-with + {:a 12} {:b 4} {:a 3 :b 7}) return?

{:b 11, :a 15} when there are several values for a key, these values are merged (two at a time) using the specified function — here they are summed.

Can you count occurrences of each value in a collection using merge-with?

(apply merge-with + (map (fn [x] {x 1}) coll)) or, using for: (apply merge-with + (for [x coll] {x 1})).