I have written the following function for determining if two strings differ by exactly differing-char-no character positions.
(defn differ-by
[id1 id2 differing-char-no]
(let [zipped-ids (map vector (seq id1) (seq id2))
differing-char-positions (filter #(not= %1 %2) zipped-ids)]
(= differing-char-no (count differing-char-positions))))
The function, however, produces an ArityException when called. The exception is from the count function call, and says Wrong number of args (1) passed to: ....
By debugging the code I observed that both differing-char-positions and zipped-ids are lazy sequences which have not been realized. However, evaluating (count zipped-ids) works fine right away, but (count differring-char-positions) only works after having called (count zipped-ids) once, i.e. after zipped-ids has been realized. Could someone explain this behaviour? Why do these lazy sequences behave differently?
zipped-ids produces a sequence of vectors, so the filter function must accept a single parameter:
(filter (fn [[a b]] (not= a b)) (map vector (seq [1 2 3]) (seq '(:a :b :c))))
But there is a better way:
(filter (partial apply not=) (map vector (seq [1 2 3]) (seq '(1 :b :c))))